Marin County Lake Quality
California, 4 lakes, average grade F (Algae-prone)
Only 4 lakes in Marin County carry an active grade. The county-level summary is therefore representative of those specific lakes, not necessarily of every water body in the county. The county average is F — the most-stressed bracket in our rubric. Most monitored lakes here show persistent nutrient loading and algal pressure.
The county's lakes lean shallow, which makes the grades sensitive to wind, sediment-resuspension, and any nutrient inputs from the watershed. The cleanest lake on the books is Rodeo Lagoon W Bottom Lake (D); the most-stressed is Rodeo Lagoon E Bottom Lake (F).
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Quick Answers for Marin County
Planning a trip? Check special fishing regulations for Rodeo Lagoon W Bottom Lake — or check the official California fishing regulations.
All Lakes in Marin County
4 lakes · 3 graded, 1 not rated3 graded lakes · 1 not rated, hidden
| # | Lake | Grade | Clarity | Phosphorus | Algae | Trophic | Access |
|---|---|---|---|---|---|---|---|
| 1 | DRodeo Lagoon W Bottom LakeMarin | D | No recent samples | No recent samples | 29.2 µg/L | No recent samples | Not recorded |
| 2 | FRodeo Lagoon E Bottom LakeMarin | F | No recent samples | No recent samples | 40.5 µg/L | No recent samples | Not recorded |
| 3 | FRodeo Lagoon E Top LakeMarin | F | No recent samples | No recent samples | 58.3 µg/L | No recent samples | Not recorded |
| NRRodeo Lagoon W Top LakeMarin | NR | No recent samples | No recent samples | 66.7 µg/L | No recent samples | Not recorded |
NR = not rated: the monitoring record does not meet the evidence bar for a grade. That is EPA Integrated Reporting Category 3 (insufficient information), not a failing grade. How grading works.
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Other Counties in California
See how Marin County stacks up in the national county water-quality rankings, or browse the full directory of graded lakes across all 49 states and 2 Canadian provinces.
Source: EPA National Aquatic Resource Surveys, 2026.