Nicasio Reservoir: toxic algae advisories
Nicasio Reservoir, Marin County. See all Marin County waters.
What the state's records say
Caution advisory, last posted May 19, 2021. No all-clear recorded.
Reporting is voluntary, so the state may not have recorded an end even if conditions changed. Check posted signs and the official map.
What this level asks: Algae that can make toxins may be present. Keep people, children and pets away from algae and scum, and do not let pets drink the water.
Before you go, check the state's official algae page and any signs posted at the water.
Dogs at Nicasio Reservoir
The state has an advisory on file for Nicasio Reservoir (Caution advisory), last posted May 19, 2021, and no all-clear recorded. Keep your dog out of the water and away from scum on the shore until the state records an end.
Satellite estimate (not an advisory)
Week ending Oct 17, 2026: about 550,000 cells/mL of cyanobacteria near the center of Nicasio Reservoir.
A satellite estimate from EPA CyAN for a 3.5 km² lake. It does not measure toxins and does not change the advisory status.
| Week ending | Estimate |
|---|---|
| Oct 17, 2026 | about 550,000 cells/mL |
| Oct 10, 2026 | about 670,000 cells/mL |
| Oct 3, 2026 | about 600,000 cells/mL |
| Sep 19, 2026 | about 220,000 cells/mL |
| Sep 12, 2026 | about 130,000 cells/mL |
| May 2, 2026 | about 910,000 cells/mL |
| Jan 17, 2026 | about 47,000 cells/mL |
| Jan 10, 2026 | about 260,000 cells/mL |
This is a satellite estimate from the U.S. EPA's Cyanobacteria Assessment Network, not an advisory. It estimates how much cyanobacteria is in the water near the lake's center, not whether toxins are present: a high number does not mean toxins, and a low one does not mean there are none. It covers lakes of about 1 square kilometer and larger, so small ponds, dog parks and most rivers are not covered, and clouds can hide a lake for weeks. It never changes the advisory status above.
Satellite estimates: U.S. EPA Cyanobacteria Assessment Network (CyAN), from Copernicus Sentinel-3 data. Provisional, research-level estimates; EPA does not endorse this site. Checked Oct 22, 2026.
Reported spots
The state records each report at a spot, such as a beach or a boat ramp. Each spot's latest deciding record:
| Spot | Status |
|---|---|
| Spot not described | Caution advisory, last posted May 19, 2021. No all-clear recorded. |
Advisory history
Every advisory record the state has published for Nicasio Reservoir, newest first. "None" is the state's record that no advisory was recommended on that date. "NA" records do not change a status.
| Date | Spot | State's level | Notes |
|---|---|---|---|
| May 19, 2021 | Spot not described | Caution | Advisory based on visual and toxins |
Source
Advisory data: California State Water Resources Control Board, Freshwater Harmful Algal Bloom Program, via data.ca.gov (public domain). Advisories are recommendations; reporting is voluntary and incomplete. The state's data. An advisory ends only when the state records a later "None" (no advisory recommended) at the same spot, or an end date. A blank end date is unknown.