Lake Hennessey: toxic algae advisories
Lake Hennessey, Napa County. See all Napa County waters.
What the state's records say
Caution advisory, last posted Apr 4, 2024. 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 Lake Hennessey
The state has an advisory on file for Lake Hennessey (Caution advisory), last posted Apr 4, 2024, 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 Jun 6, 2026: about 7,400 cells/mL of cyanobacteria near the center of Lake Hennessey.
A satellite estimate from EPA CyAN for a 3.1 km² lake. It does not measure toxins and does not change the advisory status.
| Week ending | Estimate |
|---|---|
| Jun 6, 2026 | about 7,400 cells/mL |
| May 16, 2026 | about 13,000 cells/mL |
| Mar 21, 2026 | about 270,000 cells/mL |
| Feb 7, 2026 | about 440,000 cells/mL |
| Jan 31, 2026 | about 320,000 cells/mL |
| Jan 24, 2026 | about 1,200,000 cells/mL |
| Jan 17, 2026 | about 590,000 cells/mL |
| Jan 10, 2026 | about 1,400,000 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 |
|---|---|
| Lake Hennessey boat launch | The state's latest record, from Aug 28, 2026, recommends no advisory. |
| Lake Hennessey along Sage Canyon Road | Caution advisory, last posted Apr 4, 2024. No all-clear recorded. |
Advisory history
Every advisory record the state has published for Lake Hennessey, 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 |
|---|---|---|---|
| Aug 28, 2026 | Lake Hennessey boat launch | None | Updates from routine monitoring program-organization |
| Jul 2, 2026 | Lake Hennessey boat launch | None | |
| Apr 4, 2024 | Lake Hennessey along Sage Canyon Road | Caution | Suspected bloom, investigation ongoing |
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.