Is Discovery Bay safe? Toxic algae status
Contra Costa County, California. Harmful algal bloom (blue-green algae) reports from the California State Water Resources Control Board.
Data checked 24 October 2026. California data is updated daily.
74
bloom reports since 2016
11 of 11
years with a bloom reported (2016–2026)
34
advisories recommended: 21 Caution, 4 Warning, 9 Danger
Year by year
Each box shows bloom reports logged that year, coloured by the highest advisory recommended.
Reports at Discovery Bay are most often logged in February, June and July.
Recent reports
| Date | Location | Result |
|---|---|---|
| 8 July 2026 | Carillo Bay | Caution |
| 31 May 2026 | Willow Lake | Bloom reported |
| 14 September 2025 | near Willow Lake Ct | Bloom reported |
| 6 September 2025 | Discovery Bay general | Bloom reported |
| 18 July 2025 | Willow Lake | Caution |
| 24 June 2025 | Marlin Dr. near the marina | Bloom reported |
| 25 July 2024 | Willow Lake - Town Dock | Danger |
| 11 July 2024 | Windward Point and driftwood bay | Caution |
| 10 July 2024 | Lighthouse Bay | Visual observation |
| 4 July 2024 | Lighthouse Pl & Newport | Caution |
| 28 June 2024 | Seal Way near gas station and Cypress Channel | Caution |
| 28 June 2024 | Lighthouse Place and Cypress Channel | Bloom reported |
What the results mean
Algae may be present. Stay away from scum and discoloured water, and keep pets and livestock from drinking it.
Toxins at higher levels. No swimming, and keep pets and children well away from the water.
High toxin levels. Stay out of the water and off the shoreline.
Dogs and toxic algae
Dogs face the greatest risk because they swallow water and lick algae off their fur. If a bloom has been reported, keep dogs out of the water and away from scum on the shore. If your dog swims in water that might have algae, rinse it with clean water straight away and call a vet if it seems unwell.
Common questions
Is Discovery Bay safe to swim in?
The most recent entry in California's published data is from 8 July 2026: caution (Carillo Bay). Blooms have been recorded at Discovery Bay in 11 different years, so treat green, scummy or discoloured water with caution. For current conditions, check the California HABs Portal map and posted signs.
When does Discovery Bay get algae blooms?
In the state's records, reports are most often logged in February, June and July. Blooms are most likely in warm, calm weather, but they can appear at other times of year.
Source: California State Water Resources Control Board, official data. Discovery Bay spans Contra Costa County and San Joaquin County; reports from all of them are combined here. This page is independent and not affiliated with the State of California.