Nicasio Reservoir
Marin County, California677 acresView mapDirections
SWRCB publishes the water-quality measurements for Nicasio Reservoir. Here is what has been measured.
- Eutrophic
- Impaired · Mercury
- Limited data
Nicasio Reservoir in Marin County, California has water-quality measurements published by SWRCB, shown here as reported. Chlorophyll-a (algae): 14.2 µg/L. Surface area: 677 acres.
Nicasio Reservoir is a 677-acre productive, nutrient-rich eutrophic lake in Marin County, California.
Source: EPA Water Quality Portal sampling records, your state environmental agency, last sampled 2017-08-07. Methodology: how these readings are compared.
Swimming conditions
Readings onlyLimited data
Based on clarity and algae readings from past sampling — there is no live water temperature and a current cyanobacteria reading for this lake. Check your state or county health department before you swim.
Nutrient and algae readings are not a swimming clearance. Nicasio Reservoir is listed impaired under Clean Water Act §303(d) (IR 5) for mercury. Check local advisories before you swim.
Whether Nicasio Reservoir is safe to swim in today is a call for the California State Water Resources Control Board and the county health department, who post current advisories. LakeQuality does not issue swimming clearances.
Water quality measurements for Nicasio Reservoir
These are the measurements SWRCB publishes for Nicasio Reservoir, shown as reported. The condition class beside each reading is the EPA National Lakes Assessment class for this lake’s ecoregion.
No clarity data. Chlorophyll-a 14.2 µg/L.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | No reading How deep a black-and-white disk stays visible from the surface. Higher is clearer. | |
| Total phosphorus | No reading The nutrient that most limits algae growth in freshwater. More phosphorus, more algae potential. | |
| Chlorophyll-a (algae) | 14.2µg/L | Poor GoodFairPoor |
High nutrients, frequent algae, reduced clarity
Ask about Nicasio Reservoir
Answers come straight from the public measurements and survey records on this page.
Lake Details
Surface area
677 acres
No bathymetric survey has been published for Nicasio Reservoir, so its depth is not documented here.
Stays via Stay22
Stays near Nicasio Reservoir
Compare lakeside cabins, hotels, and vacation rentals near Nicasio Reservoir in Marin County, California — live availability and prices for your trip. Switch the map to Satellite to see the lake and how close each stay is to the water. The water-quality readings cover nutrients and algae, not lodging. Cabins and rentals around Nicasio Reservoir are listed below.
Top-rated places to stay near Nicasio Reservoir
Prices for Oct 9–11, 4 adults. Change dates on the map below.
Availability and prices from Stay22 partners (Booking.com, Vrbo, Expedia). We may earn a commission from bookings, at no extra cost to you.
Follow Nicasio Reservoir’s readings
Email me when new measurements post for this lake. Updates stay free.
Or email
Fishing Nicasio Reservoir
EPA Impairment Status
Nicasio Reservoir is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 5).
Causes of impairment
Designated uses not supporting: Fish consumption, Other.
Source: EPA ATTAINS assessment unit CAL2011301220050519182548 · Official waterbody report
Nearby Lakes in Marin County
Frequently asked questions about Nicasio Reservoir
Where is Nicasio Reservoir?
- Nicasio Reservoir is a lake in Marin County, California. Its water-quality measurements are published by SWRCB and shown on this page as reported.
What is the water quality of Nicasio Reservoir?
- Nicasio Reservoir in Marin County, California has water-quality measurements published by SWRCB, shown here as reported. Chlorophyll-a (algae): 14.2 µg/L. Surface area: 677 acres.
Is Nicasio Reservoir safe to swim in?
- Nutrient and algae readings are not a swimming clearance. Nicasio Reservoir is listed impaired under Clean Water Act §303(d) (IR 5) for mercury. Check local advisories before you swim.
Explore Nicasio Reservoir across LakeQuality
More lakes in California
Other Lakes in California
Eutrophic Lakes in California
Where this data comes from
Measurements for Nicasio Reservoir come from the EPA Water Quality Portal. Per-sample source details are not recorded for this lake.
EPA condition classes from the National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See where the data comes from and data sources.
Other records on this page
Impairment status from EPA ATTAINS 303(d) database