                   FOR PUBLICATION
  UNITED STATES COURT OF APPEALS
       FOR THE NINTH CIRCUIT

NATURAL RESOURCES DEFENSE                
COUNCIL, INC.; SANTA MONICA
BAYKEEPER,
                Plaintiffs-Appellants,
                  v.
COUNTY OF LOS ANGELES; LOS
ANGELES COUNTY FLOOD CONTROL
DISTRICT; MICHAEL ANTONOVICH, in              No. 10-56017
his official capacity as Supervisor;            D.C. No.
YVONNE BURKE, in her official
capacity as Supervisor; GLORIA              2:08-cv-01467-
                                               AHM-PLA
MOLINA, in her official capacity as
Supervisor; ZEV YAROSLAVSKY, in               ORDER and
his official capacity as Supervisor;           OPINION
DEAN D. EFSTATHIOU, in his
official capacity as Acting
Director of Los Angeles County
Department of Public Works; DON
KNABE, in his official capacity as
Supervisor,
               Defendants-Appellees.
                                         
        Appeal from the United States District Court
            for the Central District of California
         A. Howard Matz, District Judge, Presiding

                 Argued and Submitted
         December 10, 2010—Pasadena, California

                      Filed July 13, 2011



                              9425
9426             NRDC v. COUNTY OF LOS ANGELES
                              ORDER

   This Court’s Opinion, filed March 10, 2011, and published
at 636 F.3d 1235 (9th Cir. 2011), is withdrawn and replaced
by the attached Opinion.

  With this filing, the panel has voted unanimously to deny
Appellees’ petition for panel rehearing. Judge Pregerson and
Judge M. Smith have voted to deny Appellees’ petition for
rehearing en banc, and Judge Holland so recommends.

   The full court has been advised of the Opinion and petition
for rehearing en banc, and no active judge has requested a
vote on whether to rehear the matter en banc. Fed. R. App. P.
35.

  Accordingly, Appellees’ petition for rehearing or for
rehearing en banc is DENIED.

  No further petitions for rehearing or rehearing en banc will
be entertained in this case.


                             OPINION

  Before: Harry Pregerson, and Milan D. Smith, Jr., Circuit
   Judges, and H. Russel Holland, Senior District Judge.*

             Opinion by Judge Milan D. Smith, Jr.




   *The Honorable H. Russel Holland, Senior United States District Judge
for the District of Alaska, sitting by designation.
             NRDC v. COUNTY OF LOS ANGELES          9429




                      COUNSEL

Aaron Colangelo, Natural Resources Defense Council, Wash-
ington, D.C.; Daniel Cooper, Lawyers for Clean Water, San
9430           NRDC v. COUNTY OF LOS ANGELES
Francisco, California, for plaintiffs-appellants Natural
Resources Defense Council, Inc. and Santa Monica
Baykeeper.

Andrea Sheridan Ordin, Judith A. Fries, Laurie Dods, Los
Angeles County Department of County Counsel, Los Ange-
les, California, Howard Gest, David W. Burhenn, Burhenn &
Gest LLP, Los Angeles, California, for defendants-appellees
County of Los Angeles, et al.

Gregory Thomas Broderick, Downey Brand, LLP, Sacra-
mento, California, for amicus curiae California State Associa-
tion of Counties.

Theresa Ann Dunham, Somach Simmons & Dunn, Sacra-
mento, California, for amicus curiae California Stormwater
Quality Association.

Kamala D. Harris, Attorney General of California; Kathleen
A. Kenealy, Senior Assistant Attorney General; James R. Pot-
ter, Jennifer Novak, Deputy Attorneys General, Office of the
California Attorney General, Los Angeles, California, for
amicus curiae California Regional Water Quality Control
Board, Los Angeles Region.


                         OPINION

M. SMITH, Circuit Judge:

   Plaintiffs-Appellants Natural Resources Defense Council
and Santa Monica Baykeeper appeal the district court’s grant
of summary judgment in favor of two municipal entities that
Plaintiffs allege are discharging polluted stormwater in viola-
tion of the Federal Water Pollution Control Act (the Clean
Water Act, Act, or CWA), 86 Stat. 816, codified as amended
at 33 U.S.C. § 1251 et seq. Plaintiffs contend that Defendants-
               NRDC v. COUNTY OF LOS ANGELES                9431
Appellees County of Los Angeles (County) and Los Angeles
County Flood Control District (District) are discharging pol-
luted urban stormwater runoff collected by municipal separate
storm sewer systems (ms4) into navigable waters in Southern
California. The levels of pollutants detected in four rivers—
the Santa Clara River, the Los Angeles River, the San Gabriel
River, and Malibu Creek (collectively, the Watershed Rivers)
—exceed the limits allowed in a National Pollutant Discharge
Elimination System (NPDES) permit which governs munici-
pal stormwater discharges in the County. Although all parties
agree that numerous water-quality standards have been
exceeded in the Watershed Rivers, Defendants contend that
there is no evidence establishing their responsibility for, or
discharge of, stormwater carrying pollutants to the rivers. The
district court agreed with Defendants and entered a partial
final judgment.

   We conclude that the district court erred with respect to the
evidence of discharges by the District into two of the Water-
shed Rivers—the Los Angeles River and San Gabriel River.
Specifically, Plaintiffs provided evidence that the monitoring
stations for the Los Angeles and San Gabriel Rivers are
located in a section of ms4 owned and operated by the District
and, after stormwater known to contain standards-exceeding
pollutants passes through these monitoring stations, this pol-
luted stormwater is discharged into the two rivers. Accord-
ingly, Plaintiffs were entitled to summary judgment on the
District’s liability for discharges into the Los Angeles River
and San Gabriel River, and therefore we reverse the district
court’s grant of summary judgment in favor of the District on
these claims.

   Plaintiffs, however, failed to meet their evidentiary burden
with respect to discharges by the District into the Santa Clara
River and Malibu Creek. Plaintiffs did not provide evidence
sufficient for the district court to determine if stormwater dis-
charged from an ms4 controlled by the District caused or con-
tributed to pollution exceedances located in these two rivers.
9432           NRDC v. COUNTY OF LOS ANGELES
Similarly, Plaintiffs did not delineate how stormwater from
ms4s controlled by the County caused or contributed to
exceedances in any of the Watershed Rivers. Accordingly, we
affirm the district court’s grant of summary judgment in favor
of the Defendants on these claims.

     FACTUAL AND PROCEDURAL BACKGROUND

I.    Stormwater Runoff in Los Angeles County

     A.   The MS4

   Stormwater runoff is surface water generated by precipita-
tion events, such as rainstorms, which flows over streets,
parking lots, commercial sites, and other developed parcels of
land. Whereas natural, vegetated soil can absorb rainwater
and capture pollutants, paved surfaces and developed land can
do neither. When stormwater flows over urban environs, it
collects “suspended metals, sediments, algae-promoting nutri-
ents (nitrogen and phosphorus), floatable trash, used motor
oil, raw sewage, pesticides, and other toxic contaminants[.]”
Envtl. Def. Ctr., Inc. v. EPA, 344 F.3d 832, 840 (9th Cir.
2003). This runoff is a major contributor to water pollution in
Southern California rivers and the Pacific Ocean and contrib-
utes to the sickening of many ocean users each year.

   The County is a sprawling 4,500 square-mile amalgam of
populous incorporated cities and significant swaths of unin-
corporated land. The District is a public entity governed by
the Los Angeles County Board of Supervisors and the Depart-
ment of Public Works. The District is comprised of 84 cities
and some unincorporated areas of the County. The County
and the District are separate legal entities.

   In the District, stormwater runoff is collected by thousands
of storm drains located in each municipality and channeled to
a storm sewer system. The municipalities in the District oper-
                    NRDC v. COUNTY OF LOS ANGELES                          9433
ate ms4s1 to collect and channel stormwater. The County also
operates an ms4 for certain unincorporated areas. Unlike a
sanitary sewer system, which transports municipal sewage for
treatment at a wastewater facility, or a combined sewer sys-
tem, which transports sewage and stormwater for treatment,
ms4s contain and convey only untreated stormwater. See 40
C.F.R. § 122.26(a)(7), (b)(8). In the County, municipal ms4s
are “highly interconnected” because the District allows each
municipality to connect its storm drains to the District’s
extensive flood-control and storm-sewer infrastructure (the
MS4).2 That infrastructure includes 500 miles of open chan-
nels and 2,800 miles of storm drains. The length of the MS4
system and the locations of all storm drain connections are not
known exactly because a comprehensive map of the storm
drain system does not exist. While the number and location of
storm drains are too numerous to catalogue, it is undisputed
that the MS4 collects and channels stormwater runoff from
  1
   Under Federal Regulations, an ms4 is:
      a conveyance or system of conveyances (including roads with
      drainage systems, municipal streets, catch basins, curbs, gutters,
      ditches, man-made channels, or storm drains):
      (i) Owned or operated by a State, city, town, borough, county,
      parish, district, association, or other public body . . . having juris-
      diction over disposal of sewage, industrial wastes, storm water,
      or other wastes, including special districts under State law such
      as a sewer district, flood control district or drainage district, or
      similar entity . . .
      (ii) Designed or used for collecting or conveying storm water;
      (iii) Which is not a combined sewer; and
      (iv) Which is not part of a Publicly Owned Treatment Works
      (POTW) . . . .
40 C.F.R. § 122.26(b)(8).
  2
    Throughout this Opinion, reference is made to both “ms4” and “the
MS4.” The former is a generic reference to municipal separate storm
sewer systems without regard to their particular location, while the latter
specifically refers to the flood control and storm-sewer infrastructure
described supra that exists in the County and is controlled by the District.
9434           NRDC v. COUNTY OF LOS ANGELES
across the County. That stormwater is channeled in the MS4
to various watercourses including the four Watershed Rivers
at the heart of this litigation: the Los Angeles River, the San
Gabriel River, the Santa Clara River, and Malibu Creek. The
Watershed Rivers drain into the Pacific Ocean at Santa Mon-
ica Bay, Los Angeles Harbor, and Long Beach Harbor.

   The gravamen of Plaintiffs’ action is that by allowing
untreated and heavily-polluted stormwater to flow unabated
from the MS4 into the Watershed Rivers, and eventually into
the Pacific Ocean, Defendants have violated the Clean Water
Act.

  B.   The Clean Water Act and NPDES Permit

   The Clean Water Act is the nation’s primary water-
pollution-control law. The Act’s purpose is “to restore and
maintain the chemical, physical, and biological integrity of
the Nation’s waters.” 33 U.S.C. § 1251(a). “To serve those
ends, the Act prohibits ‘the discharge of any pollutant by any
person’ unless done in compliance with some provision of the
Act.” S. Fl. Water Mgmt. Dist. v. Miccosukee Tribe of Indi-
ans, 541 U.S. 95, 102 (2004) (quoting 33 U.S.C. § 1311(a)).
“Discharge of a pollutant” is defined as “any addition of any
pollutant to navigable waters from any point source[.]” 33
U.S.C. § 1362(12); see Comm. to Save Mokelumne River v.
East Bay Mun. Util. Dist., 13 F.3d 305, 308 (9th Cir. 1993)
(characterizing “discharge” as “ ‘add[ing]’ pollutants from the
outside world to navigable water”).

   Under the Clean Water Act, ms4s fall under the definition
of “point sources.” 33 U.S.C. § 1362(14). A point source is
“any discernible, confined and discrete conveyance, including
but not limited to any pipe, ditch, channel, tunnel, conduit,
well, discrete fissure, container, rolling stock, concentrated
animal feeding operation, or vessel or other floating craft,
from which pollutants are or may be discharged.” 33 U.S.C.
§ 1362(14).
               NRDC v. COUNTY OF LOS ANGELES                9435
   A person or entity wishing to add pollutants to navigable
waters must comply with the NPDES, which “requires dis-
chargers to obtain permits that place limits on the type and
quantity of pollutants that can be released into the Nation’s
waters.” Miccosukee Tribe, 541 U.S. at 102; 33 U.S.C.
§ 1342(a), (p). The Act “generally prohibits the ‘discharge of
any pollutant’ . . . from a ‘point source’ into the navigable
waters of the United States’ ” unless the point source is cov-
ered by an NPDES permit. Defenders of Wildlife v. Browner,
191 F.3d 1159, 1163 (9th Cir. 1999) (quoting 33 U.S.C.
§§ 1311(a), 1362(12)(A)) (emphasis added); see also Arkan-
sas v. Oklahoma, 503 U.S. 91, 101-02 (1992) (describing
NPDES permitting system). An NPDES permit requires its
holder—the “permittee”—to follow the requirements of
numerous Clean Water Act provisions, see 33 U.S.C.
§ 1342(a), which include effluent limitations, water-quality
standards, water monitoring obligations, public reporting
mechanisms, and certain discharge requirements. See id.
§§ 1311, 1312, 1314, 1316, 1317, 1318, 1343.

   The Act uses two water-quality-performance standards, by
which a discharger of water may be evaluated—“effluent lim-
itations” and “water quality standards.” Arkansas v. Okla-
homa, 503 U.S. at 101 (citing 33 U.S.C. §§ 1311, 1313,
1314); see also Sierra Club v. Union Oil Co. of Calif., 813
F.2d 1480, 1483 (9th Cir. 1987), vacated on other grounds,
485 U.S. 931 (1988), reinstated, 853 F.2d 667 (9th Cir. 1988).
An effluent limitation is “any restriction established by a State
or the [Environmental Protection Agency (EPA)] Administra-
tor on quantities, rates, and concentrations of chemical, physi-
cal, biological, and other constituents which are discharged
from point sources into navigable waters. . . .” 33 U.S.C.
§ 1362(11). An effluent-limitation guideline is determined in
light of “ ‘the best practicable control technology currently
available.’ ” Union Oil, 813 F.2d at 1483 (quoting 33 U.S.C.
§ 1311(b)(1)(A)).

   Water-quality standards “are used as a supplementary basis
for effluent limitations, so that numerous dischargers, despite
9436           NRDC v. COUNTY OF LOS ANGELES
their individual compliance with technology-based limita-
tions, can be regulated to prevent water quality from falling
below acceptable levels.” Union Oil, 813 F.2d at 1483 (citing
EPA v. Calif. ex rel. State Water Res. Control Bd., 426 U.S.
200, 205 n.12 (1976) (hereafter EPA v. Calif.)). Water-quality
standards are developed in a two-step process. First, the EPA,
or state water authorities establish a waterway’s “beneficial
use.” Natural Res. Def. Council, Inc. v. EPA, 16 F.3d 1395,
1400 (4th Cir. 1993); see also Cal. Water Code § 13050(f)
(“‘Beneficial uses’ of the waters of the state that may be pro-
tected against quality degradation include, but are not limited
to, domestic, municipal, agricultural and industrial supply;
power generation; recreation; aesthetic enjoyment; naviga-
tion; and preservation and enhancement of fish, wildlife, and
other aquatic resources or preserves.”). Once the beneficial
use is determined, water quality criteria that will yield the
desired water conditions are formulated and implemented. See
NRDC v. EPA, 16 F.3d at 1400; see also 33 U.S.C. § 1313(a),
(c)(2)(A); 40 C.F.R. § 131.3(i) (“Water quality standards are
provisions of State or Federal law which consist of a desig-
nated use or uses for the waters of the United States and water
quality criteria for such waters based upon such uses.”).

   Unlike effluent limitations, which are promulgated by the
EPA to achieve a certain level of pollution reduction in light
of available technology, water-quality standards emanate
from the state boards charged with managing their domestic
water resources. See Arkansas v. Oklahoma, 503 U.S. at 101.
The EPA gives the states guidance in drafting water-quality
standards and “state authorities periodically review water
quality standards and secure the EPA’s approval of any revi-
sions in the standards.” Id.

  The EPA has authorized the State of California to develop
water-quality standards and issue NPDES permits. Under the
Porter-Cologne Water Quality Control Act, California state
law designates the State Water Resources Control Board and
nine regional boards as the principal state agencies for enforc-
                 NRDC v. COUNTY OF LOS ANGELES                       9437
ing federal and state water pollution law and for issuing per-
mits. See Cal. Water Code §§ 13000, 13001, 13140, 13240,
13370, 13377. Beginning in 1990, the California State Water
Resources Control Board for the Los Angeles Region (the
Regional Board) issued an NPDES permit (the Permit) to
cover stormwater discharges by the County, the District, and
84 incorporated municipalities in the County (collectively the
Permittees or Co-Permittees).3 See City of Arcadia v. State
Water Res. Control Bd., 119 Cal. Rptr. 3d 232, 240-41 (Cal.
Ct. App. 2010). The Permit was renewed in 1996, 2001, 2006,
and 2007.

   The Permit is divided into two broad sections: findings by
the Regional Board and an order authorizing and governing
the Permittees’ discharges (Order). The findings cover many
introductory and background subjects, including a history of
NPDES permitting in the County; applicable state and federal
laws governing stormwater discharges; studies conducted by
the County and researchers about the deleterious effects of
polluted stormwater; coverage and implementation provi-
sions; and guidelines for administrative review of Permit pro-
visions. The Permit covers “all areas within the boundaries of
the Permittee municipalities . . . over which they have regula-
tory jurisdiction as well as unincorporated areas in Los Ange-
les County within the jurisdiction of the Regional Board.” In
total, the Permit governs municipal stormwater discharge
across more than 3,100 square miles of land in the County.

   The Permit relates the many federal and state regulations
governing stormwater discharges to Southern California’s
watercourses. Among these regulations is the Water Quality
Control Plan for the Los Angeles Region (the Basin Plan).
Under California law, the regional boards’ “water quality
plans, called ‘basin plans,’ must address the beneficial uses to
  3
   “Co-permittee means a permittee to a NPDES permit that is only
responsible for permit conditions relating to the discharge for which it is
operator.” 40 C.F.R. § 122.26(b)(1).
9438           NRDC v. COUNTY OF LOS ANGELES
be protected as well as water quality objectives, and they must
establish a program of implementation.” City of Arcadia, 119
Cal. Rptr. 3d at 240 (quoting City of Burbank v. State Water
Res. Control Bd., 108 P.3d 862, 865 (Cal. 2005) (citing Cal.
Water Code § 13050(j))). The Permit provides that “[t]he
Basin Plan designates beneficial uses of receiving waters and
specifies both narrative and numerical water quality objec-
tives for the receiving water in Los Angeles County.” “Re-
ceiving waters” are defined as all surface water bodies in the
Los Angeles Region that are identified in the Basin Plan. Per-
mittees are to assure that storm water discharges from the
MS4 shall neither cause nor contribute to the exceedance of
water quality standards and objectives nor create conditions of
nuisance in the receiving waters, and that the discharge of
non-storm water to the MS4 has been effectively prohibited.
The Permit incorporates and adopts the Basin Plan, which sets
limits on bacteria and contaminants for the receiving waters
of Southern California. The water-quality standards limit,
among other pollutants, the levels of ammonia, fecal coliform
bacteria, arsenic, mercury, and cyanide in Southern Califor-
nia’s inland rivers.

   The Permit contains myriad prohibitions and conditions
regarding discharges into and from the MS4. Under Part 1, the
Permittees are directed to “effectively prohibit non-storm
water discharges into the MS4 and watercourses” unless
allowed by an NPDES permit. Under Part 2, titled “Receiving
Water Limitations,” “discharges from the MS4 that cause or
contribute to the violation of the Water Quality Standards or
water quality objectives are prohibited.” The “Water Quality
Standards and Water Quality Objectives” are defined in the
Permit as “water quality criteria contained in the Basin Plan,
the California Ocean Plan, the National Toxics Rule, the Cali-
fornia Toxics Rule, and other state or federal approved sur-
face water quality plans. Such plans are used by the Regional
Board to regulate all discharges, including storm water dis-
charges.”
              NRDC v. COUNTY OF LOS ANGELES                 9439
   The Permit, in Part 2.3, provides that Permittees “shall
comply” with the MS4 discharge prohibitions, set forth in
Parts 2.1 and 2.2, “through timely implementation of control
measures and other actions to reduce pollutants in the dis-
charges in accordance with [the Los Angeles Stormwater
Quality Management Program (SQMP)] and its components
and other requirements of this Order. . . .” The SQMP
includes “descriptions of programs, collectively developed by
the Permittees in accordance with provisions of the NPDES
Permit, to comply with applicable federal and state law.” Part
2.3 further provides that “[i]f exceedances of Water Quality
Objectives or Water Quality Standards [ ] persist, notwith-
standing implementation of the SQMP and its components
and other requirements of this permit,” Permittees “shall
assure compliance with discharge prohibitions and receiving
water limitations” by engaging in an “iterative process” pro-
cedure:

    a) Upon a determination by either the Permittee or
    the Regional Board that discharges are causing or
    contributing to an exceedance of an applicable Water
    Quality Standard, the Permittee shall promptly notify
    and thereafter submit a Receiving Water Limitations
    (RWL) Compliance Report . . . to the Regional
    Board that describes [Best Management Practices
    (BMPs)] that are currently being implemented and
    additional BMPs that will be implemented to prevent
    or reduce any pollutants that are causing or contrib-
    uting to the exceedances of Water Quality Standards.

    ...

    c) Within 30 days following the approval of the
    RWL Compliance Report, the Permittee shall revise
    the SQMP and its components and monitoring pro-
    gram to incorporate the approved modified BMPs
    that have been and will be implemented, an imple-
9440           NRDC v. COUNTY OF LOS ANGELES
    mentation schedule, and any additional monitoring
    required.

    d) Implement the revised SQMP and its components
    and monitoring program according to the approved
    schedule.

    [Part 2.4] So long as the Permittee has complied with
    the procedures set forth above and is implementing
    the revised SQMP and its components, the Permittee
    does not have to repeat the same procedure for con-
    tinuing or recurring exceedances of the same receiv-
    ing water limitations unless directed by the Regional
    Board to develop additional BMPs.

When a violation arises, a Permittee must adhere to the proce-
dures in its Compliance Report until the exceedances abate.

   Part 3 of the Permit, titled “Storm Water Quality Manage-
ment Program (SQMP) Implementation,” provides that
“[e]ach Permittee shall, at a minimum, implement the
SQMP.” Part 3.A.3 requires Permittees to “implement addi-
tional controls, where necessary, to reduce the discharge of
pollutants in storm water to the [Maximum Extent Practicable
(MEP)].” Part 3.B requires the implementation of BMPs by
the Permittees. Part 3.G specifies that each Permittee is vested
with the “necessary legal authority” to prohibit discharges to
the MS4, and the Permittees are directed to develop storm-
water and urban runoff ordinances for its jurisdiction.

   The Permit has both self-monitoring and public-reporting
requirements, which include: (1) monitoring of “mass emis-
sions” at seven mass emission monitoring stations; (2) Water
Column Toxicity Monitoring; (3) Tributary Monitoring; (4)
Shoreline Monitoring; (5) Trash Monitoring; (6) Estuary
Sampling; (7) Bioassessment; and (8) Special Studies.

  This case concerns high levels of pollutants, particularly
heavy metals and fecal bacteria, identified by mass-emissions
                NRDC v. COUNTY OF LOS ANGELES                   9441
monitoring stations for the four Watershed Rivers (the Moni-
toring Stations). Mass-emissions monitoring measures all
constituents present in water, and the readings give a cumula-
tive picture of the pollutant load in a waterbody. According
to the Permit, the purpose of mass-emissions monitoring is to
(1) estimate the mass emissions from the MS4, (2) assess
trends in the mass emissions over time, and (3) determine if
the MS4 is contributing to exceedances of Water Quality
Standards by comparing results to the applicable standards in
the Basin Plan. The Permit establishes that the Principal Per-
mittee, which is the District, shall monitor the mass-emissions
stations. The Permit requires that mass-emission readings be
taken five times per year for the Watershed Rivers.

   The Los Angeles River and San Gabriel River Monitoring
Stations are located in a channelized portion of the MS4 that
is owned and operated by the District. See Excerpts of Record
at 11; see also Dist. Ct. Docket No. 101: Declaration of Aaron
Colangelo Ex. N: Deposition of Mark Pestrella at 476-78. The
Los Angeles River Monitoring Station is located in the City
of Long Beach in “a concrete lined trapezoidal channel.”4 The
Los Angeles River Monitoring Station measures “total
upstream tributary drainage” of 825 square miles, as the Los
Angeles River is the largest watershed outlet in the County.
The San Gabriel River Monitoring Station is located in Pico
Rivera and measures an upstream tributary watershed of 450
square miles.

  The Malibu Creek Monitoring Station is not located within
a channelized portion of the MS4 but at an “existing stream
gage station” near Malibu Canyon Road. It measures 105
  4
    “Section Two: Site Descriptions,” Los Angeles Cnty. Dept. of Pub.
Works, available at http://dpw.lacounty.gov/wmd/npdes/9899_report/
SiteDesc.pdf (last accessed July 6, 2011); see also “Section Two: Site
Descriptions,” Los Angeles Cnty. Dept. of Pub. Works, available at
http://dpw.lacounty.gov/wmd/NPDES/2006-07_report%5CSection
%202.pdf (last accessed July 6, 2011).
9442            NRDC v. COUNTY OF LOS ANGELES
miles of tributary watershed. The Santa Clara River Monitor-
ing Station is located in the City of Santa Clara and measures
an upstream tributary area of 411 square miles.5

  C.    Water-Quality Exceedances in the Watershed
        Rivers

   Between 2002 and 2008, the four Monitoring Stations iden-
tified hundreds of exceedances of the Permit’s water-quality
standards. These water-quality exceedances are not disputed.
For instance, monitoring for the Los Angeles and San Gabriel
Rivers showed 140 separate exceedances. These included
high levels of aluminum, copper, cyanide, fecal coliform bac-
teria, and zinc in the rivers. Further, ocean monitoring at Surf-
rider Beach showed that there were 126 separate bacteria
exceedances on 79 days, including 29 days where the fecal
coliform bacteria limit was exceeded.

   The District admits that it conveys pollutants via the MS4,
but contends that its infrastructure alone does not generate or
discharge pollutants. According to Defendants, the District
conveys the collective discharges of the numerous “up-sewer”
municipalities. Moreover, Defendants identify thousands of
permitted dischargers whose pollutants are reaching the
Watershed Rivers:

      (1) Los Angeles River watershed: (a) at least 1,344
      NPDES-permitted industrial and 488 construction
      stormwater dischargers allowed to discharge during
      the time period relevant to the case; (b) three waste-
      water treatment plants; and (c) 42 separate incorpo-
      rated cities within the Los Angeles River watershed
      discharging into the river upstream of the mass emis-
      sion station.
  5
   “Section Two: Site Descriptions,” Los Angeles Cnty. Dept. of Pub.
Works, available at http://dpw.lacounty.gov/wmd/NPDES/2006-07_
report%5CSection%202.pdf (last accessed July 6, 2011).
                NRDC v. COUNTY OF LOS ANGELES                    9443
      (2) San Gabriel River watershed: (a) at least 276
      industrial and 232 construction stormwater discharg-
      ers during the relevant time period; (b) at least 20
      other industrial dischargers that were specifically
      permitted to discharge pollutants in excess of the
      water quality standards at issue in this action; (c) two
      wastewater treatment plants; and (d) 21 separate
      incorporated cities discharging into the watershed
      upstream of the mass emission station.

      (3) Santa Clara River watershed: (a) eight discharg-
      ers permitted by industrial wastewater discharge per-
      mits where the limits in the permit allowed
      discharges of pollutants at concentrations higher than
      the water quality standards which plaintiffs contend
      were exceeded; (b) approximately 26 industrial and
      187 construction stormwater dischargers; and (c) the
      Saugus Wastewater Reclamation Plant.

      (4) Malibu Creek watershed: (a) seven industrial
      wastewater dischargers; and (b) at least five permit-
      ted discharges under the general industrial storm-
      water permit and at least 16 construction sites
      permitted to discharge under the general construction
      stormwater permit.

II.   Proceedings before the District Court

   Based on data self-reported by Defendants, Plaintiffs
catalogued the water-quality exceedances in the Watershed
Rivers. Beginning on May 31, 2007, Plaintiffs sent a series of
notice letters to Defendants concerning these exceedances. On
March 3, 2008, based on these purported violations, Plaintiffs
commenced this citizen-enforcement action. After the district
court dismissed certain elements of Plaintiffs’ initial com-
plaint because notice of the Permit violations was defective,
Plaintiffs sent Defendants an adequate notice letter on July 3,
2008.
9444           NRDC v. COUNTY OF LOS ANGELES
   Plaintiffs filed the First Amended Complaint (Complaint)
on September 18, 2008. In the Complaint, Plaintiffs assert six
causes of action under the Clean Water Act. Only the first
four of Plaintiffs’ claims, which relate to the exceedances in
the Watershed Rivers, and which the district court designated
the “Watershed Claims,” are before us. The first three Water-
shed Claims allege that, beginning in 2002 or 2003, the Dis-
trict and the County caused or contributed to exceedances of
water-quality standards in the Santa Clara River (Claim 1),
the Los Angeles River (Claim 2), and the San Gabriel River
(Claim 3), in violation of 33 U.S.C. §§ 1311(a), 1342(p). The
fourth Watershed Claim alleges that, beginning in 2002,
Defendants caused or contributed to exceedances of the water
quality standards and violated the Total Maximum Daily Load
(TMDL) limits in Malibu Creek. Plaintiffs’ four Watershed
Claims each rest on the same premise: (1) the Permit sets
water-quality limits for each of the four rivers; (2) the mass-
emissions stations have recorded exceedances of those stan-
dards; (3) an exceedance is non-compliance with the Permit
and, thereby, the Clean Water Act; and (4) Defendants, as
holders of the Permit and operators of the MS4, are liable
under the Act.

  Before the district court, Plaintiffs moved for partial sum-
mary judgment on two of the Watershed Claims: the Los
Angeles River and San Gabriel River exceedances. Defen-
dants cross-moved for summary judgment on all four Water-
shed Claims.

   In a March 2, 2010 Order, the district court denied each
cross-motion for summary judgment on the Watershed
Claims. NRDC v. County of Los Angeles, No. 08 Civ. 1467
(AHM), 2010 WL 761287 (C.D. Cal. Mar. 2, 2010), amended
on other grounds, 2011 WL 666875 (C.D. Cal. Jan. 27, 2011).
Although the district court accepted Plaintiffs’ arguments that
the Permit “clearly prohibits ‘discharges from the MS4 that
cause or contribute to the violation of Water Quality Stan-
dards or water quality objectives,’ ” 2010 WL 761287, at *6,
               NRDC v. COUNTY OF LOS ANGELES                9445
and that mass-monitoring stations “are the proper monitoring
locations to determine if the MS4 is contributing to excee-
dances [of the Water Quality Standards or water quality
objectives,]” id., the district court held that Plaintiffs were
attempting to establish liability without presenting evidence
of who was responsible for the stormwater discharge. The dis-
trict court observed that although “the District is responsible
for the pollutants in the MS4” at the time they pass the mass-
emissions stations, “that does not necessarily determine the
question of whether the water passing by these points is a
‘discharge’ within the meaning of the Permit and the Clean
Water Act.” Id. at *7. Unable to decipher from the record
where the MS4 ended and the Watershed Rivers begin, or
whether any upstream outflows were contributing stormwater
to the MS4, the district court stated that “Plaintiffs would
need to present some evidence (monitoring data or an admis-
sion) that some amount of a standards-exceeding pollutant is
being discharged though at least one District outlet.” Id. at *8.

   Following supplemental briefing, the district court again
determined that “Plaintiffs failed to present evidence that the
standards-exceeding pollutants passed through the Defen-
dants’ MS4 outflows at or near the time the exceedances were
observed. Nor did Plaintiffs provide any evidence that the
mass emissions stations themselves are located at or near a
Defendant’s outflow.” The district court thereupon entered
summary judgment for Defendants on all four Watershed
Claims.

   Under Fed. R. Civ. P. 54(b), the district court entered a par-
tial final judgment on the Watershed Claims because they
were “factually and legally severable” from the other claims
and “[t]he parties and the Court would benefit from appellate
resolution of the central legal question underlying the water-
shed claims: what level of proof is necessary to establish
defendants’ liability.” Plaintiffs timely appeal.
9446            NRDC v. COUNTY OF LOS ANGELES
      JURISDICTION AND STANDARD OF REVIEW

     We have jurisdiction under 28 U.S.C. § 1291.

   We review the district court’s grant of summary judgment
in a Clean Water Act enforcement action de novo. Assoc. to
Protect Hammersley, Eld, and Totten Inlets v. Taylor Res.,
Inc., 299 F.3d 1007, 1009 (9th Cir. 2002) (citing Waste Action
Project v. Dawn Mining Corp., 137 F.3d 1426, 1428 (9th Cir.
1998)).

                        DISCUSSION

   Determining whether the County or the District violated the
Permit’s conditions, and thereby the Clean Water Act,
requires us to examine whether an exceedance at a mass-
emission monitoring station is a Permit violation, and, if so,
whether it is beyond dispute that Defendants discharged pol-
lutants that caused or contributed to water-quality excee-
dances.

I.    Whether Exceedances at Mass-Emission Stations Con-
      stitute Permit Violations

   [1] “The Clean Water Act regulates the discharge of pollu-
tants into navigable waters, prohibiting their discharge unless
certain statutory exceptions apply.” Russian River Watershed
Protection Comm. v. City of Santa Rosa, 142 F.3d 1136, 1138
(9th Cir. 1998) (citing 33 U.S.C. § 1311(a)). One such excep-
tion is for discharges by entities or individuals who hold
NPDES permits. Id. The NPDES permitting program is the
“centerpiece” of the Clean Water Act and the primary method
for enforcing the effluent and water-quality standards estab-
lished by the EPA and state governments. Am. Iron & Steel
Inst. v. EPA, 115 F.3d 979, 990 (D.C. Cir. 1997); see also Nw.
Envtl. Advocates v. City of Portland, 56 F.3d 979, 986-90 (9th
Cir. 1995) (“Citizen suits to enforce water quality standards
effectuate complementary provisions of the CWA and the
               NRDC v. COUNTY OF LOS ANGELES              9447
underlying purpose of the statute as a whole.”); Friends of the
Everglades v. S. Fla. Water Mgmt. Dist., 570 F.3d 1210, 1225
(11th Cir. 2009) (citing Nat’l Wildlife Fed’n v. Gorsuch, 693
F.2d 156,175-76 (D.C. Cir. 1982) (“There is indeed some
basis in the legislative history for the position that Congress
viewed the NPDES program as its most effective weapon
against pollution.”)).

   To decipher the meaning and enforceability of NPDES per-
mit terms, we interpret the unambiguous language contained
in the permit. Russian River, 142 F.3d at 1141. We review a
permit’s provisions and meaning as we would any contract or
legal document. See Nw. Envtl. Advocates, 56 F.3d at 982. As
described supra, the Permit prohibits MS4 discharges into
receiving waters that exceed the Water Quality Standards
established in the Basin Plan and elsewhere. Specifically, Sec-
tion 2.1 provides: “[D]ischarges from the MS4 that cause or
contribute to the violation of Water Quality Standards or
water quality objectives are prohibited.” Section 2.2 of the
Permit reads: “Discharges from the MS4 of storm water, or
non-storm water, for which a Permittee is responsible for,
shall not cause or contribute to a condition of nuisance.”

   Nevertheless, Defendants contend that exceedances
observed at mass-emissions stations cannot establish liability
on behalf of any individual Permittee. Their argument in this
respect, as we discuss more thoroughly infra, relies heavily on
their belief that the record is bereft of evidence connecting
Defendants to the water-quality exceedances. Defendants also
assert that the mass-emissions stations are “neither designed
nor intended” to measure the compliance of any Permittee
and, therefore, cannot form the basis for a Permit violation.
Defendants also argue that municipal compliance with an
NPDES stormwater permit cannot be reviewed under the
same regulatory framework as a private entity or an individ-
ual. In support of this contention, Defendants cite to a 1990
EPA rule:
9448           NRDC v. COUNTY OF LOS ANGELES
    When enacting this provision, Congress was aware
    of the difficulties in regulating discharges from
    municipal separate storm sewers solely through tra-
    ditional end-of-pipe treatment and intended for EPA
    and NPDES States to develop permit requirements
    that were much broader in nature than requirements
    which are traditionally found in NPDES permits for
    industrial process discharges or POTWs. The legisla-
    tive history indicates, municipal storm sewer system
    “permits will not necessarily be like industrial dis-
    charge permits.” Often, an end-of-the-pipe treatment
    technology is not appropriate for this type of dis-
    charge.

Brief of Appellees 33 (quoting “National Pollutant Discharge
Elimination System Permit Application Regulations for Storm
Water Discharges,” 55 Fed. Reg. 47,990, 48,037-38 (Nov. 16,
1990)).

   As we detail infra, neither the statutory development of the
Clean Water Act nor the plain language of EPA regulations
supports Defendants’ arguments that NPDES permit viola-
tions are less enforceable or unenforceable in the municipal-
stormwater context. In fact, since the inception of the NPDES,
Congress has expanded NPDES permitting to bring municipal
dischargers within the Clean Water Act’s coverage.

  A.   Regulating MS4 Operators

  The NPDES permitting program originated in the 1972
amendments to the Clean Water Act. Pub. L. 92-500, § 2, 86
Stat. 88, reprinted in 1972 U.S.C.C.A.N. 3668 (codified as
amended at 33 U.S.C. § 1342). At the time, the NPDES pro-
gram was viewed “as the primary means of enforcing the
Act’s effluent limitations.” Natural Res. Def. Council v.
Costle, 568 F.2d 1369, 1371 (D.C. Cir. 1977); see also Natu-
ral Res. Def. Council, Inc. v. EPA, 966 F.2d 1292, 1295 (9th
Cir. 1992) (examining statutory history of 1972 amendments
               NRDC v. COUNTY OF LOS ANGELES                 9449
to the Clean Water Act) (hereafter NRDC v. EPA). The per-
mitting program is codified at Section 402 of the Clean Water
Act. 33 U.S.C. § 1342. In 1973, the EPA promulgated regula-
tions categorically exempting “discharges from a number of
classes of point sources . . . including . . . separate storm sew-
ers containing only storm runoff uncontaminated by any
industrial or commercial activity.” Costle, 568 F.2d at 1372
(citing 40 C.F.R. § 125.4 (1975)). The EPA’s exemption of
certain point sources, including ms4s, from Section 402’s
blanket requirement was invalidated by the United States
Court of Appeals for the District of Columbia Circuit in
Costle. Id. at 1376-77. The Costle court highlighted that “[t]he
wording of the [CWA], legislative history, and precedents are
clear: the EPA Administrator does not have authority to
exempt categories of point sources from the permit require-
ments of § 402.” Id. at 1377.

   In the ten-year period following the Costle decision, the
EPA did not promulgate regulations addressing discharges by
ms4 operators. See NRDC v. EPA, 966 F.2d at 1296 (citing
“National Pollutant Discharge Elimination System Permit
Application Regulations for Storm Water Discharges; Appli-
cation Deadlines,” 56 Fed. Reg. 56,548 (1991)). In 1987, after
continued nonfeasance by the EPA, Congress enacted the
Water Quality Act amendments to the Clean Water Act to
regulate stormwater discharges from, inter alia, ms4s. See
Defenders of Wildlife, 191 F.3d at 1163 (“Ultimately, in 1987,
Congress enacted the Water Quality Act amendments to the
CWA.”); NRDC v. EPA, 966 F.2d at 1296 (“Recognizing both
the environmental threat posed by storm water runoff and
EPA’s problems in implementing regulations, Congress
passed the Water Quality Act of 1987[.]”) (internal citations
omitted); see also 55 Fed. Reg. 47,994 (“[P]ermits for dis-
charges from municipal separate storm sewer systems must
require controls to reduce the discharge of pollutants to the
maximum extent practicable, and where necessary water
quality-based controls, and must include a requirement to
effectively prohibit non-storm water discharges into the storm
9450            NRDC v. COUNTY OF LOS ANGELES
sewers. Furthermore, EPA in consultation with State and local
officials must develop a comprehensive program to designate
and regulate other storm water discharges to protect water
quality.”).

   [2] The principal effect of the 1987 amendments was to
expand the coverage of Section 402’s permitting require-
ments. NRDC v. EPA, 966 F.2d at 1296. Section 402(p) estab-
lished a “phased and tiered approach” for NPDES permitting.
Nw. Envtl. Def. Ctr. v. Brown, 640 F.3d 1063, 1081-82 (9th
Cir. 2011) (citing 33 U.S. § 1342(p)). “The purpose of this
approach was to allow EPA and the states to focus their atten-
tion on the most serious problems first.” NRDC v. EPA, 966
F.2d at 1296. “Phase I” included “five categories of storm-
water discharges,” deemed “the most significant sources of
stormwater pollution,” who were required to obtain an
NPDES permit for their stormwater discharge by 1990.
Brown, 640 F.3d at 1082 (citing 33 U.S. § 1342(p)(2)). The
five categories of the most serious discharge were:

    (p) Municipal and industrial stormwater discharges
    ...

         (2) . . .

         ...

         (A) A discharge with respect to which a
         permit has been issued under this section
         before February 4, 1987.

         (B) A discharge associated with industrial
         activity.

         (C) A discharge from a municipal separate
         storm sewer system serving a population of
         250,000 or more.
                 NRDC v. COUNTY OF LOS ANGELES                     9451
          (D) A discharge from a municipal separate
          storm sewer system serving a population of
          100,000 or more but less than 250,000.

          (E) A discharge for which the Administra-
          tor or the State, as the case may be, deter-
          mines that the stormwater discharge
          contributes to a violation of a water quality
          standard or is a significant contributor of
          pollutants to waters of the United States.

33 U.S.C. § 1342(p)(2) (emphases added). Of the five catego-
ries of Phase I dischargers required to obtain the first permits,
two are ms4 operators: municipalities with populations over
250,000, and municipalities with populations between
100,000 and 250,000. Id. § 1342(p)(2)(C)-(D). Indeed, as
noted supra, the Permit at issue here was first authorized in
1990 pursuant to the 1987 amendments.

   Rather than regulate individual sources of runoff, such as
churches, schools and residential property (which one Con-
gressman described as a potential “nightmare”),6 and as regu-
lations prior to 1987 theoretically required, Congress put the
NPDES permitting requirement at the municipal level to ease
the burden of administering the program. Brown, 640 F.3d at
1085-86. That assumption of municipal control is found in the
Permit at issue here—Part 3.G.2 of the Permit states that “Per-
mittees shall possess adequate legal authority to . . . [r]equire
persons within their jurisdiction to comply with conditions in
Permittee’s ordinances, permits, contracts, model programs,
or orders (i.e. hold dischargers to its MS4 accountable for
their contributions of pollutants and flows.)[.]”
  6
    See 131 Cong. Rec. 15616, 15657 (Jun. 13, 1985) (Statement of Sen.
Wallop) (“[The regulations] can be interpreted to require everyone who
has a device to divert, gather, or collect stormwater runoff and snowmelt
to get a permit from EPA as a point source. . . . Requiring a permit for
these kinds of stormwater runoff conveyance systems would be an admin-
istrative nightmare.”).
9452           NRDC v. COUNTY OF LOS ANGELES
   [3] Defendants’ position that they are subject to a less rig-
orous or unenforceable regulatory scheme for their storm-
water discharges cannot be reconciled with the significant
legislative history showing Congress’s intent to bring ms4
operators under the NPDES-permitting system. Even the
selectively excerpted regulatory language Defendants present
to us—“Congress was aware of the difficulties in regulating
discharges from municipal separate storm sewers . . . [and]
intended for EPA and NPDES States to develop permit
requirements that were much broader in nature than require-
ments which are traditionally found in NPDES permits”—
does not support Defendants’ view. Indeed, this excerpt is but
one paragraph from a longer section titled, “Site-Specific
Storm Water Quality Management Programs for Municipal
Systems.” 55 Fed. Reg. 48,037-38. The quoted language fol-
lows a paragraph which reads:

    Section 402(p)(3)(iii) of the CWA mandates that
    permits for discharges from municipal separate
    storm sewers shall require controls to reduce the
    discharge of pollutants to the maximum extent prac-
    ticable (MEP), including management practices,
    control techniques and systems, design and engineer-
    ing methods, and such other provisions as the Direc-
    tor determines appropriate for the control of such
    pollutants.

55 Fed. Reg. 48,038 (emphasis added). The use of such
language—employing “mandates” and commands to regulate
—hardly supports Defendants’ notion that NPDES permits
are unenforceable against municipalities for their stormwater
discharges. Moreover, the paragraphs that follow the excerpt
explain why developing system-wide controls to manage
municipal stormwater is preferable to controlling pollution
through end-of-pipe effluent technologies. Id. The regulations
highlight that “Congress recognized that permit requirements
for municipal separate storm sewer systems should be devel-
oped in a flexible manner to allow site-specific permit condi-
               NRDC v. COUNTY OF LOS ANGELES                9453
tions to reflect the wide range of impacts that can be
associated with these discharges.” Id. Rather than evincing
any intent to treat permitting “differently” for municipalities,
the EPA merely explains why state authorities that issue per-
mits should draft site-specific rules, as the Regional Board did
here, and why water-quality standards may be preferable over
more-difficult-to-enforce effluent limitations. Avoiding
wooden permitting requirements and granting states flexibility
in setting forth requirements is not equivalent to immunizing
municipalities for stormwater discharges that violate the pro-
visions of a permit.

  B.   Enforcement of Mass-Emissions Violations

   Part and parcel with Defendants’ argument that they are
subject to a relaxed regulatory structure is their view that the
Permit’s language indicates that mass-emissions monitoring is
not intended to be enforcement mechanism against municipal
dischargers. Defendants claim that measuring water-quality
serves only an hortatory purpose—as Defendants state, “the
mass emission monitoring program . . . neither measures nor
was designed to measure any individual permittee’s compli-
ance with the Permit.” This proposition, which if accepted
would emasculate the Permit, is unsupported by either our
case law or the plain language of the Permit conditions.

   [4] “The plain language of CWA § 505 authorizes citizens
to enforce all permit conditions.” Nw. Envtl. Advocates, 56
F.3d at 986 (emphasis in original). We used these words and
emphasized all permit conditions because the language of the
Clean Water Act is clear in its intent to guard against all
sources and superintendents of water pollution and “clearly
contemplates citizen suits to enforce ‘a permit or condition
thereof.’ ” Id. (citing 33 U.S.C. § 1365(f)(2), (f)(6)); see also
W. Va. Highlands Conservancy, Inc. v. Huffman, 625 F.3d
159, 167 (4th Cir. 2010) (“In other words, the statute takes the
water’s point of view: water is indifferent about who initially
polluted it so long as pollution continues to occur.”).
9454           NRDC v. COUNTY OF LOS ANGELES
   We have previously addressed, and rejected, municipal
attempts to avoid NPDES permit enforcement. In Northwest
Environmental Advocates, we considered a citizen-suit chal-
lenging the City of Portland’s operation of a combined sewer
system which periodically overflowed and discharged raw
sewage into two rivers. 56 F.3d at 981-82. The plaintiffs
brought suit on the basis of an NPDES permit condition
which “prohibit[ed] any discharges that would violate Oregon
water quality standards.” Id. at 985. Reviewing the history of
the 1972 amendments and the Supreme Court’s decision in
PUD No.1 of Jefferson County v. Washington Department of
Ecology, 511 U.S. 700 (1994), we recognized that Congress
had authorized enforcement of state water-quality standards,
lest municipalities be immunized on the technicality that not
all water standards can be expressed as effluent limitations.
Id. at 988-89. The overflows from the Portland sewer system
were “caused primarily by uncontrollable events—i.e., the
amount of stormwater entering the system[.]” Id. at 989.
Because the total amount of water entering and leaving the
sewer system was unknown, it was impossible to articulate
effluent standards which would “ensure that the gross amount
of pollution discharged [would] not violate water quality stan-
dards.” Id. Only by enforcing the water-quality standards
themselves as the limits could the purpose of the CWA and
the NPDES system be effectuated. Id. at 988-90. Indeed, we
noted that prior to the 1972 incorporation of effluent limita-
tions, the Clean Water Act depended entirely on enforcement
based on water-quality standards. Id. at 986. However, trou-
bled by the “ ‘almost total lack of enforcement’ ” under the
old system, Congress added the effluent limitation standards
“not to supplant the old system” but to “improve enforce-
ment.” Id. at 986 (quoting S. Rep. No. 414, 92d Cong., 2d
Sess. 2 (1972), reprinted in 1972 U.S.C.C.A.N. 3668, 3671).

  Our prior case law emphasizes that NPDES permit enforce-
ment is not scattershot—each permit term is simply enforced
as written. See Union Oil, 813 F.2d at 1491 (“It is unclear
whether the court intended to excuse these violations under
               NRDC v. COUNTY OF LOS ANGELES              9455
the upset defense or under a de minimis theory. In either
event, the district court erred. The Clean Water Act and the
regulations promulgated under it make no provision for ‘rare’
violations.”); see also United States v. CPS Chem. Co., 779
F. Supp. 437, 442 (D. Ark. 1991) (“For enforcement pur-
poses, a permittee’s [Discharge Monitoring Reports] consti-
tute admissions regarding the levels of effluents that the
permittee has discharged.”). As we explained in Union Oil,
Congress structured the CWA to function by self-monitoring
and self-reporting of violations to “ ‘avoid the necessity of
lengthy fact finding, investigations, and negotiations at the
time of enforcement.’ ” 813 F.2d at 1492 (quoting S. Rep. No.
414, 92d Cong., 1st Sess. 64, reprinted in 1972 U.S.C.C.A.N.
3668, 3730). When self-reported exceedances of an NPDES
permit occur, the Clean Water Act allows citizens to bring
suit to enforce the terms of the Permit.

   [5] The plain language of the Permit countenances
enforcement of the water-quality standards when exceedances
are detected by the various compliance mechanisms, includ-
ing mass-emissions monitoring. First, the Permit incorporates
and adopts the Basin Plan, which sets the water-quality stan-
dards for bacteria and contaminants for the receiving waters
of Southern California, including the Watershed Rivers. The
Permit then sets out a multi-part monitoring program for those
standards, the goals of which explicitly include “[a]ssessing
compliance with this Order[.]” “Compliance” under the Clean
Water Act primarily means adhering to the terms and condi-
tions of an NPDES permit. EPA v. Calif., 426 U.S. at 223
(“Thus, the principal means of enforcing the pollution control
and abatement provisions of the Amendments is to enforce
compliance with a permit.”). The first monitoring program
listed in the Permit is “Mass Emissions.” While Defendants
are correct to note that mass-emissions monitoring has as one
of its goals “estimat[ing] the mass emissions from the MS4,”
Defendants fail to mention that another goal, listed just below
“estimating,” is “[d]etermin[ing] if the MS4 is contributing to
exceedances of Water Quality Standards.”
9456              NRDC v. COUNTY OF LOS ANGELES
   Although Defendants argue that compliance with other Per-
mit provisions, in particular Part 2.3’s iterative process, for-
gives violations of the discharge prohibitions in Parts 2.1 and
2.2, no such “safe harbor” is present in this Permit.7 Rather,
Part 2.3 first provides that Permittees shall comply with the
Water Quality Standards “through timely implementation of
control measures and other actions. . . in accordance with the
SQMP and its components.” Part 2.3 clarifies that Parts 2 and
3 of the Permit interact, but it offers no textual support for the
proposition that compliance with certain provisions shall for-
give non-compliance with the discharge prohibitions. As
opposed to absolving noncompliance or exclusively adopting
the MEP standard, the iterative process ensures that if water
quality exceedances “persist,” despite prior abatement efforts,
a process will commence whereby a responsible Permittee
amends its SQMP. Given that Part 3 of the Permit states that
SQMP implementation is the “minimum” required of each
Permittee, the discharge prohibitions serve as additional
requirements that operate as enforceable water-quality-based
performance standards required by the Regional Board. See
e.g., Bldg. Indus. Ass’n of San Diego Cnty. v. State Water Res.
Control Bd., 22 Cal. Rptr. 3d 128, 141 (Cal. Ct. App. 2004)
(rejecting arguments that “under federal law the ‘maximum
extent practicable’ standard is the ‘exclusive’ measure that
may be applied to municipal storm sewer discharges and
[that] a regulatory agency may not require a Municipality to
comply with a state water quality standard if the required con-
trols exceed a ‘maximum extent practicable’ standard”).
  7
    We also note, as did the district court, that when the validity of this
Permit was challenged in California state court by various municipal enti-
ties, including the District, the argument that the Permit’s discharge prohi-
bitions were invalid for not containing a “safe harbor” was rejected. See
In re L.A. Cnty. Mun. Storm Water Permit Litig., No. BS 080548, at 4-5,
7 (L.A. Super. Ct. Mar. 24, 2005) (“In sum, the Regional Board acted
within its authority when it included Parts 2.1 and 2.2. in the Permit with-
out a ‘safe harbor,’ whether or not compliance therewith requires efforts
that exceed the ‘MEP’ standard.”).
               NRDC v. COUNTY OF LOS ANGELES                 9457
   Part 6.D of the Permit, titled “Duty to Comply,” lays any
doubts about municipal compliance to rest: “Each Permittee
must comply with all terms, requirements, and conditions of
this Order. Any violation of this order constitutes a violation
of the Clean Water Act . . . and is grounds for enforcement
action, Order termination, Order revocation and reissuance,
denial of an application for reissuance; or a combination
thereof[.]” This unequivocal language is unsurprising given
that all NPDES permits must include monitoring provisions
ensuring that permit conditions are satisfied. See 33 U.S.C.
§ 1318(a)(A) (“[T]he Administrator [of the EPA] shall require
the owner or operator of any point source to (i) establish and
maintain such records, (ii) make such reports, (iii) install, use,
and maintain such monitoring equipment or methods (includ-
ing where appropriate, biological monitoring methods), [and]
(iv) sample such effluents (in accordance with such methods,
at such locations, at such intervals, and in such manner as the
Administrator shall prescribe)[.]”); 40 C.F.R. § 122.44(i)(1)
(specifying the monitoring requirements for compliance,
“mass . . . for each pollutant limited in the permit,” and vol-
ume of effluent discharged); Ackels v. EPA, 7 F.3d 862, 866
(9th Cir. 1993) (“[T]he Act grants EPA broad authority to
require NPDES permitees to monitor, at such intervals as the
Administrator shall prescribe, whenever it is required to carry
out the objectives of the Act.”).

   [6] In sum, the Permit’s provisions plainly specify that the
mass-emissions monitoring is intended to measure compli-
ance and that “[a]ny violation of this Order” is a Clean Water
Act violation. The Permit is available for public inspection to
aid this purpose. Accordingly, we agree with the district
court’s determination that an exceedance detected through
mass-emissions monitoring is a Permit violation that gives
rise to liability for contributing dischargers.

II.   Evidence of Discharge

  We next turn to the factual issue on which the district court
granted summary judgement in favor of Defendants—whether
9458           NRDC v. COUNTY OF LOS ANGELES
any evidence in the record shows Defendants discharged
stormwater that caused or contributed to water-quality viola-
tions. The district court determined that a factual basis was
lacking:

    Plaintiffs failed to present evidence that the
    standards-exceeding pollutants passed through the
    Defendants’ MS4 outflows at or near the time the
    exceedances were observed. Nor did Plaintiffs pro-
    vide any evidence that the mass emissions stations
    themselves are located at or near a Defendant’s out-
    flow. Plaintiffs do represent in their supplemental
    briefing that their monitoring data reflects sampling
    conducted at or near Defendants’ outflows. . . . How-
    ever, the declarations on which Plaintiffs rely do not
    clearly indicate that the sampling in question was
    conducted at an outflow (as opposed to in-stream).

    ...

    In short, Plaintiffs have failed to follow the Court’s
    instructions and present data which could establish
    that “standards-exceeding pollutants . . . passed
    through Defendants’ MS4 outflows at or near the
    time the exceedances were observed.” That the pol-
    lutants must have passed through an outflow is key
    because, as the Court found in the March 2 Order,
    standards-exceeding pollutants must have passed
    through a County or District outflow in order to con-
    stitute a discharge under the Clean Water Act and
    the Permit.

   [7] Plaintiffs have argued throughout this litigation that the
measured exceedances in the Watershed Rivers ipso facto
establish Permit violations by Defendants. Because these
points are designated in the Permit for purposes of assessing
“compliance,” this argument is facially appealing. But the
Clean Water Act does not prohibit “undisputed” exceedances;
               NRDC v. COUNTY OF LOS ANGELES                 9459
it prohibits “discharges” that are not in compliance with the
Act, which means in compliance with the NPDES. See 33
U.S.C. § 1311(a); see also Miccosukee Tribe, 541 U.S. at 102.
While it may be undisputed that exceedances have been
detected, responsibility for those exceedances requires proof
that some entity discharged a pollutant. Indeed, the Permit
specifically states that “discharges from the MS4 that cause
or contribute to the violation of the Water Quality Standards
or water quality objectives are prohibited.”

   “[D]ischarge of pollutant” is defined as “any addition of
any pollutant to navigable waters from any point source[.]” 33
U.S.C. § 1362(12). Under the Clean Water Act, the MS4 is a
“Point Source.” See 33 U.S.C. § 1342(p)(2), 1362(14). “Navi-
gable waters” is used interchangeably with “waters of the
United States.” See Headwaters, Inc. v. Talent Irrigation
Dist., 243 F.3d 526, 532 (9th Cir. 2001). Those terms mean,
inter alia, “[a]ll waters which are currently used, were used
in the past, or may be susceptible to use in interstate or for-
eign commerce, including all waters which are subject to the
ebb and flow of the tide[.]” 40 C.F.R. § 122.2. The Watershed
Rivers are all navigable waters.

   Thus, the primary factual dispute between the parties is
whether the evidence shows any addition of pollutants by
Defendants to the Watershed Rivers. Defendants contend that
the “District does not generate any of the pollutants in the sys-
tem, but only transports them from other permitted and non-
permitted sources.” Moreover, Defendants contend that by
measuring mass-emissions downstream from where the pollu-
tants entered the sewer system, it is not possible to pinpoint
which entity, if any, is responsible for adding them to the riv-
ers. In the words of the district court, there is no evidence that
“standards-exceeding pollutants . . . passed through Defen-
dants’ MS4 outflows at or near the time the exceedances were
observed.” Plaintiffs counter that the Monitoring Stations are
downstream from hundreds of miles of storm drains which
have generated the pollutants being detected. To Plaintiffs, it
9460               NRDC v. COUNTY OF LOS ANGELES
is irrelevant which of the thousands of storm drains were the
source of polluted stormwater—as holders of the Permit,
Defendants bear responsibility for the detected exceedances.

   Resolving this dispute over whether Defendants added pol-
lutants depends heavily on the level of generality at which the
facts are viewed. At the broadest level, all sides agree with
basic hydrology—upland water becomes polluted as it runs
over urbanized land and begins a downhill flow, first through
municipal storm drains, then into the MS4 which carries the
water (and everything in it) to the Watershed Rivers, which
flow into the Pacific Ocean. More narrowly, it is, as Plaintiffs
concede, impossible to identify the particular storm drains
that had, for instance, some fecal bacteria which contributed
to a water-quality violation. Ultimately, each side fails to
rebut the other’s arguments. Defendants ignore their role as
controllers of thousands of miles of MS4 and the stormwater
it conveys8 by demanding that Plaintiffs engage in the Sisy-
phean task of testing particular storm drains in the County for
the source of each pollutant. Likewise, Plaintiffs did not
enlighten the district court with sufficient evidence for certain
claims and assumed it was obvious to anyone how stormwater
makes its way from a parking lot in Pasadena into the MS4,
through a mass-emissions station, and then to a Watershed
River.

   [8] Despite shortcomings in each side’s arguments, there is
evidence in the record showing that polluted stormwater from
the MS4 was added to two of the Watershed Rivers: the Los
  8
    Defendants’ untenable position about their responsibility for discharges
is confirmed by the testimony of their Rule 30(b)(6) witness:
      Question: What if those flows [which exceeded water-quality
      standards] were so polluted with oil and grease that they were on
      fire as they came out of the system? Would your view be the
      same, that the District is not contributing to exceedances?
      Answer: That the system the District maintains is not contributing
      to, yes.
                 NRDC v. COUNTY OF LOS ANGELES                       9461
Angeles River and San Gabriel River. Because the mass-
emissions stations, as the appropriate locations to measure
compliance, for these two rivers are located in a section of the
MS4 owned and operated by the District, when pollutants
were detected, they had not yet exited the point source into
navigable waters. As such, there is no question over who con-
trolled the polluted stormwater at the time it was measured or
who caused or contributed to the exceedances when that water
was again discharged to the rivers—in both cases, the District.
As a matter of law and fact, the MS4 is distinct from the two
navigable rivers; the MS4 is an intra-state man-made
construction—not a naturally occurring Watershed River. See
Headwaters, 243 F.3d at 533 (“The EPA has interpreted
‘waters of the United States’ to include ‘intrastate lakes, riv-
ers, streams (including intermittent streams) . . . the use, deg-
radation, or destruction of which would affect or could affect
interstate or foreign commerce’ and ‘tributaries of [those]
waters.’ ” (quoting 40 C.F.R. § 122.2(c), (e)). At least some
outfalls for the MS4 were downstream from the mass-
emissions stations. See 40 C.F.R. § 122.26(9) (“Outfall means
a point source . . . at the point where a municipal separate
storm sewer discharges to waters of the United States . . . .”).
The discharge from a point source occurred when the still-
polluted stormwater flowed out of the concrete channels
where the Monitoring Stations are located, through an outfall,
and into the navigable waterways. We agree with Plaintiffs
that the precise location of each outfall is ultimately irrelevant
because there is no dispute that MS4 eventually adds storm-
water to the Los Angeles and San Gabriel Rivers downstream
from the Monitoring Stations.

   Although the District argues that merely channeling pollu-
tants created by other municipalities or industrial NPDES per-
mittees should not create liability because the District is not
an instrument of “addition” or “generation,”9 the Clean Water
  9
    This issue does not usually arise in Clean Water Act litigation because
it is generally assumed that ms4s “discharge” stormwater. See, e.g., Miss.
9462             NRDC v. COUNTY OF LOS ANGELES
Act does not distinguish between those who add and those
who convey what is added by others—the Act is indifferent
to the originator of water pollution. As Judge Wilkinson of
the Fourth Circuit cogently framed it: “[The Act] bans ‘the
discharge of any pollutant by any person’ regardless of
whether that ‘person’ was the root cause or merely the current
superintendent of the discharge.” Huffman, 625 F.3d at 167
(emphasis added). “Point sources” include instruments that
channel water, such as “any pipe, ditch, channel, tunnel, con-
duit, well, discrete fissure, container, rolling stock, concen-
trated animal feeding operation, or vessel or other floating
craft, from which pollutants are or may be discharged.” 33
U.S.C. § 1362(14) (emphasis added). The EPA’s regulations
further specify that ms4 operators require permits for channel-
ing: “Discharge of a pollutant . . . includes additions of pollu-
tants into waters of the United States from: surface runoff
which is collected or channelled by man; discharges through
pipes, sewers, or other conveyances owned by a State [or]
municipality.” 40 C.F.R. § 122.2 (emphasis added). “[M]ost
urban runoff is discharged through conveyances such as sepa-
rate storm sewers or other conveyances which are point
sources under the CWA. These discharges are subject to the
NPDES program.” 55 Fed. Reg. 47,991. Finally, the Supreme
Court stated in Miccosukee Tribe that “the definition of ‘dis-
charge of a pollutant’ contained in § 1362(12) . . . includes
within its reach point sources that do not themselves generate
pollutants.” 541 U.S. at 105 (emphasis added).

  [9] Accordingly, the district court erred in stating that
“Plaintiffs have not provided the Court with the necessary
evidence to establish that the Los Angeles River and the San
Gabriel River below the mass emissions monitoring stations

River Revival v. Adm’r, E.P.A., 107 F. Supp. 2d 1008, 1009 (D. Minn.
2000) (“These lawsuits involve the discharge of storm water into the Mis-
sissippi River through the Cities’ storm sewers. Thus, and this is not in
dispute, the storm water discharge is subject to the NPDES permitting
requirements.”).
                NRDC v. COUNTY OF LOS ANGELES                  9463
are bodies of water that are distinct from the MS4 above these
monitoring stations.” In light of the evidence that the Los
Angeles River and San Gabriel River mass-emission stations
are in concrete portions of the MS4 controlled by the District,
it is beyond dispute that the District is discharging pollutants
from the MS4 to the Los Angeles River and San Gabriel
River in violation of the Permit. Thus, Plaintiffs are entitled
to summary judgment on Claims 2 and 3.

   [10] However, we agree with the district court that, as the
record is currently constituted, it is not possible to mete out
responsibility for exceedances detected in the Santa Clara
River and Malibu Creek (Claims 1 and 4). Like the district
court, we are unable to identify the relationship between the
MS4 and these mass-emissions stations. From the record, it
appears that both monitoring stations are located within the
rivers themselves. Plaintiffs have not endeavored to provide
the Court with a map or cogent explanation of the inter-
workings or connections of this complicated drainage system.
We recognize that both the Santa Clara and Malibu Creek
Monitoring Stations are downstream from hundreds or thou-
sands of storm drains and MS4 channels. It is highly likely,
but on this record nothing more than assumption, that polluted
stormwater exits the MS4 controlled by the District and the
County, and flows downstream in these rivers past the mass-
emissions stations. To establish a violation, Plaintiffs were
obligated to spell out this process for the district court’s con-
sideration and to spotlight how the flow of water from an ms4
“contributed” to a water-quality exceedance detected at the
Monitoring Stations. See, e.g., Nicholas Acoustics & Specialty
Co. v. H & M Constr. Co., 695 F.2d 839, 846-47 (5th Cir.
1983) (“We wish to emphasize most strongly that it is fool-
hardy for counsel to rely on a court to find disputed issues of
material fact not highlighted by counsel’s paperwork; a party
that has suffered the consequences of summary judgment
below has a definite and specific duty to point out the thwart-
ing facts . . . . Judges are not ferrets!”). Contrary to Plaintiffs’
contention, this would not require independent sampling of
9464           NRDC v. COUNTY OF LOS ANGELES
the District’s outfalls. Indeed, simply ruling out the other con-
tributors of stormwater to these two rivers or following up to
vague answers given by Defendants’ witnesses could have
satisfied Plaintiffs’ evidentiary obligation. In the alternative,
prior to commencing actions such as this one, Plaintiffs could
heed the district court’s sensible observation and, for purposes
of their evidentiary burden, “sample from at least one outflow
that included a standards-exceeding pollutant[.]”

   Finally, for all four Watershed Rivers, the record is silent
regarding the path stormwater takes from the unincorporated
land controlled by the County to the Monitoring Stations. The
district court correctly demanded evidence for the County’s
liability, which Plaintiffs did not proffer.

   [11] In sum, Plaintiffs were entitled to summary judgment
on Claims 2 and 3 against the District for the Los Angeles
River and San Gabriel River because (1) the Monitoring Sta-
tions for these two rivers are located in a portion of the MS4
owned and operated by the District, (2) these Monitoring Sta-
tions detected pollutants in excess of the amount authorized
by the NPDES permit, and (3) this polluted water “dis-
charged” into the Los Angeles River and San Gabriel River.
The Plaintiffs, however, have not met their burden on sum-
mary judgment for their other claims because they did not
provide the district court with evidence that the MS4 con-
trolled by the District “discharged” pollutants that passed
through the Monitoring Stations in the Santa Clara River and
Malibu Creek, or that ms4s controlled by the County “dis-
charged” pollutants that passed through the Monitoring Sta-
tions in any of the four rivers in question.

                       CONCLUSION

  The district court’s judgment for Defendant District on
Claims 2 and 3 of the First Amended Complaint is
REVERSED, and this matter is REMANDED to the district
court for further proceedings consistent with this opinion. The
              NRDC v. COUNTY OF LOS ANGELES             9465
district court’s grant of summary judgment for Defendant Dis-
trict on Claims 1 and 4 and for Defendant County on all
Watershed Claims is AFFIRMED.

 AFFIRMED IN PART, REVERSED IN PART, and
REMANDED.

  Each side shall bear its own costs.
