PlainLakes

Some of these decisions are EPA's. 6 of these 607 records carry a Clean Water Act cleanup plan EPA established rather than California and the Big Valley Band of Pomo Indians of the Big Valley Rancheria. They are counted in every figure on this page, and each record page names which.

State water-quality picture · #17 by reported lake-type waterbodies

California lakes and reservoirs

Every figure on this page is computed from the assessment decisions California and the Big Valley Band of Pomo Indians of the Big Valley Rancheria reported to the U.S. Environmental Protection Agency under Clean Water Act sections 303(d) and 305(b) (reporting cycles 2009–2026), plus daily water-level observations from the U.S. Geological Survey.

607
Reported waterbodies
202
Not supporting a use
146
Fish use not supporting
3
Gauged lakes

California in one line

607 lake-type waterbodies in California are reported to EPA ATTAINS across reporting cycles 2009–2026: 202 assessed as not supporting at least one designated use, 22 fully supporting, and 383 without a definitive overall verdict. The most-listed impairment cause is Mercury, on 136 waterbodies.

607
reported waterbodies
202
not supporting a use
22
fully supporting
90%
of definitive verdicts impaired

An unassessed waterbody is neither confirmed clean nor confirmed impaired; states assess on multi-year cycles.

California's mid-size register with a heavier not-supporting share

California sits in the middle third by inventory size (607 units) and in the upper third for ruled not-supporting share (90%).

Data updated 2026-08-17

According to the U.S. Environmental Protection Agency's ATTAINS program, California and the Big Valley Band of Pomo Indians of the Big Valley Rancheria reported 607 lake-type waterbodies in California in the August 2026 extract. See our methodology for filer attribution and multi-year reporting cycles.

Overall-status mix on California's mid-size inventory

Every reported lake-type unit in California falls into exactly one overall-status bucket. This bar is the inventory partition; the cause chart and national gauge below answer different questions.

Most-listed impairment causes in California

Waterbodies carrying each listed cause, most first

waterbodies

What this shows Mercury is the most-listed impairment cause in California, on 136 of the lake-type waterbodies reported there.

Source EPA ATTAINS As of Reporting cycles 1987–2026
90 N

Computed share from EPA ATTAINS filed records, not an official EPA rating or safety grade.

Share of California waterbodies with a verdict on fish, recreation, aquatic life or drinking water assessed as not supporting a use

California: 90% of ruled verdicts are not-supporting, against a national share of 53%, 1.7× the national rate.

Reporting practice

A definitive verdict is still missing for most of California's inventory

Each reporting authority decides how much of the water it files to EPA it actually assesses in a cycle. That decision sets the denominator behind every count on this page.

California holds the 17th-largest reported lake-type inventory in the country, 607 assessment units, carrying a definitive verdict on 36.9% of it. The counts above describe the 224 units carrying a verdict, not all 607 reported.

The states reporting at the nearest rates are Idaho (37.6%) and New York (28.8%).

Assessing more of an inventory is a reporting choice, set by cycle length, monitoring budget and how finely a state splits its waters into units. It is not a measure of water quality, and it is why a raw count from one state does not sit beside a raw count from another. How assessment coverage varies.

Not every unit counted here was assessed by California. 27 of them were filed by a sovereign tribal nation reporting directly to EPA under Clean Water Act section 518, which lets a tribe set and apply its own water quality standards in the same manner as a state: Big Valley Band of Pomo Indians of the Big Valley Rancheria (27). Those decisions are theirs, not California's.

California against every comparable state

Share of reported lake-type waterbodies carrying a definitive verdict

36.9% 40th of 47 higher than 7 of 47 states

the 47 comparable states, by definitive-verdict share

Each bar is a band; taller bars hold more states. The dashed line + filled bar mark this entry. Hover or tap any bar for its full count and share, and where it sits relative to this entry.

Source EPA ATTAINS · Reporting cycles 1987–2026

Each bar counts states whose definitive-verdict share falls in that 10-point band. The 47 states shown are those reporting at least 25 lake-type units; a share off a smaller inventory is precision the data does not have.

By size

Largest assessed lakes in California

By assessed acreage as reported in ATTAINS. Acreage is the assessed unit's size, which can differ from a lake's full surface area.

# Waterbody Assessed acres
1 Salton Sea Not Supporting 233,045
2 Tahoe, Lake Not Supporting 84,092
3 Goose Lake (Modoc County) Not reported 62,385
4 Honey Lake Not Supporting 55,526
5 Klamath River HU, Tule Lake and Lower Klamath Lake National Wildlife Refuge Not Supporting 48,791
6 Mono Lake Not Supporting 45,299
7 Clear Lake Not Supporting 39,582
8 Searles Lake Not Supporting 29,632
9 Klamath River HU, Lost River HA, Clear Lake, Boles HSAs Not reported 29,220
10 Almanor Lake Not Supporting 24,897
11 Eagle Lake (Lassen County) Not Supporting 24,377
12 Berryessa, Lake Not Assessed 18,677

By cause count

Most-listed waterbodies in California

Waterbodies carrying the most listed impairment causes among the lake-type units reported by California and by the Big Valley Band of Pomo Indians of the Big Valley Rancheria.

# Waterbody Listed causes
1 Salton Sea Not Supporting 11
2 Eagle Lake (Lassen County) Not Supporting 9
3 Otay Reservoir, Lower Not Supporting 8
4 Lake Hodges Not Supporting 7
4 Lake Lindero Not Supporting 7
4 Pyramid Lake Not Supporting 7
4 Sutherland Reservoir Not Supporting 7
8 Barrett Lake Not Supporting 6
8 Big Bear Lake Not Supporting 6
8 El Capitan Lake Not Supporting 6

7 of the 10 rows in this table tie on the ranked value: 4 at 7 (rank 4), 3 at 6 (rank 8). Tied rows share a rank and are listed in no meaningful order.

Gauge network

Lakes with a daily level record in California

Lakes with a linked USGS gauge and a daily water-level record. Readings are daily-batch snapshots, not real-time.

Where California sits nationally on scale vs. impaired share

Assessed-corpus size does not predict impaired share: a state with a large register can still post a low not-supporting rate, and a small register can post a high one.

Source: EPA ATTAINS, states above the sample floor 2×2 strategic matrix plotting 36 entities by Definitive verdicts (assessed corpus) (X) and Impaired share (%) (Y), with a crosshair dividing the plot into four quadrants. Large corpus, high shareSmall corpus, high shareLarge corpus, low shareSmall corpus, low share 01,0002,0003,0004,0005,000 0%20%40%60%80%100%120% Definitive verdicts (assessed corpus) Impaired share (%) New Hampshire · 2,783, 100%Washington · 393, 100%New Jersey · 604, 96.5%Massachusetts · 718, 94.4%Oregon · 178, 92.7%California · 224, 90.2%CaliforniaNew York · 642, 87.7%Illinois · 497, 87.3%Rhode Island · 140, 85%Oklahoma · 195, 78.5%Virginia · 231, 78.4%South Dakota · 186, 78%Kansas · 316, 76.3%Iowa · 203, 67.5%Florida · 1,003, 63.4%Nebraska · 352, 62.2%Indiana · 241, 60.2%North Carolina · 152, 59.2%Utah · 134, 57.5%Missouri · 267, 56.6%Kentucky · 129, 55.8%Colorado · 163, 54.6%Minnesota · 3,364, 53.4%Arkansas · 117, 53%Alabama · 273, 52.4%Georgia · 119, 39.5%South Carolina · 540, 39.3%Michigan · 2,339, 34.5%Connecticut · 139, 31.7%North Dakota · 176, 29.5%Texas · 433, 28.9%Pennsylvania · 516, 26.2%Vermont · 712, 21.6%Idaho · 237, 21.5%Wisconsin · 4,718, 10.4%Maine · 303, 10.2%
Source: EPA ATTAINS, states above the sample floor
NameDefinitive verdicts (assessed corpus)Impaired share (%)
New Hampshire2,783100%
Washington393100%
New Jersey60496.5%
Massachusetts71894.4%
Oregon17892.7%
California (this record)22490.2%
New York64287.7%
Illinois49787.3%
Rhode Island14085%
Oklahoma19578.5%
Virginia23178.4%
South Dakota18678%
Kansas31676.3%
Iowa20367.5%
Florida1,00363.4%
Nebraska35262.2%
Indiana24160.2%
North Carolina15259.2%
Utah13457.5%
Missouri26756.6%
Kentucky12955.8%
Colorado16354.6%
Minnesota3,36453.4%
Arkansas11753%
Alabama27352.4%
Georgia11939.5%
South Carolina54039.3%
Michigan2,33934.5%
Connecticut13931.7%
North Dakota17629.5%
Texas43328.9%
Pennsylvania51626.2%
Vermont71221.6%
Idaho23721.5%
Wisconsin4,71810.4%
Maine30310.2%
Source: EPA ATTAINS, states above the sample floor

The crosshair sits at the median across the 36 qualifying states on each axis, not the range midpoint, so it splits the group evenly. California is highlighted.

States near California's heavier not-supporting load

Two EPA ATTAINS peer sets outside California. The reporting-practice chart above stays on definitive-verdict share; the neighborhoods below emphasize cross-state nearest neighbors when the not-supporting count is in the upper third.

Comparable register size

Nearest states by lake-type units on the register (607 here).

Similar not-supporting load

Nearest states by units assessed not supporting a use (202 here). Inventory peers above are omitted.

Peers are nearest-neighbor matches on EPA ATTAINS register size and not-supporting counts among states with at least 25 reported units. Impairment peers skip states already listed in the inventory column so the two link sets stay distinct.

Using this state picture

  • Comparisons across states hold on shares, not counts: 36.9% of the inventory reported from California carries a definitive verdict, 40th of 47 comparable states. National rankings
  • Fish-consumption verdicts here are assessment decisions. Species-level meal advice comes from the state fish-advisory program. Consumption guide
  • Browse every named California waterbody with a populated record. Browse waterbodies

PlainLakes renders state assessment decisions and federal gauge observations without modification.

Figures on this page are computed from the assessment decisions California and the Big Valley Band of Pomo Indians of the Big Valley Rancheria reported to EPA's ATTAINS system and from USGS National Water Information System daily values, no number is typed in by an editor. State counts aggregate every lake-type assessment unit reported by California and by the Big Valley Band of Pomo Indians of the Big Valley Rancheria, including unnamed units that never receive their own page. See our editorial standards & corrections policy, the methodology behind these numbers, or report a data error. Data current as of 2026-08-17. Primary sources: EPA ATTAINS and USGS National Water Information System.