Oso Flaco Lake: toxic algae advisories
Oso Flaco Lake, San Luis Obispo County. See all San Luis Obispo County waters.
What the state's records say
Caution advisory, last posted Sep 4, 2026. No all-clear recorded.
2 reported spots here have an advisory with 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 Oso Flaco Lake
The state has an advisory on file for Oso Flaco Lake (Caution advisory), last posted Sep 4, 2026, 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.
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 |
|---|---|
| Oso Flaco Lake boardwark at first alcove | Caution advisory, last posted Sep 4, 2026. No all-clear recorded. |
| boardwalk at first alcove | Caution advisory, last posted Aug 26, 2021. No all-clear recorded. |
Advisory history
Every advisory record the state has published for Oso Flaco Lake, 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 |
|---|---|---|---|
| Sep 4, 2026 | Oso Flaco Lake boardwark at first alcove | Caution | Floating/planktonic bloom observed; Lake-wide advisory; Updates from routine monitoring program-organization |
| Sep 4, 2026 | Oso Flaco Lake boardwark at first alcove | Caution | Floating/planktonic bloom observed; Lake-wide advisory; Updates from routine monitoring program-organization |
| Jun 25, 2026 | Oso Flaco Lake boardwark at first alcove | Caution | Floating/planktonic bloom observed; Lake-wide advisory; Updates from routine monitoring program-organization |
| May 21, 2026 | Oso Flaco Lake boardwark at first alcove | Caution | Floating/planktonic bloom observed; Lake-wide advisory; Updates from routine monitoring program-organization |
| Aug 28, 2025 | Oso Flaco Lake boardwark at first alcove | Caution | Floating/planktonic bloom observed; Lake-wide advisory; Updates from routine monitoring program-organization |
| Jun 30, 2025 | Oso Flaco Lake boardwark at first alcove | Caution | Floating/planktonic bloom observed; Lake-wide advisory; Updates from routine monitoring program-organization |
| May 15, 2025 | Oso Flaco Lake boardwark at first alcove | Caution | Floating/planktonic bloom observed; Lake-wide advisory; Updates from routine monitoring program-organization |
| May 15, 2025 | Oso Flaco Lake boardwark at first alcove | None | Floating/planktonic bloom observed; Lake-wide advisory; Updates from routine monitoring program-organization |
| Aug 26, 2024 | Oso Flaco Lake boardwark at first alcove | Caution | Floating/planktonic bloom observed; Lake-wide advisory; Updates from routine monitoring program-organization |
| Jun 28, 2024 | Oso Flaco Lake boardwark at first alcove | Caution | Floating/planktonic bloom observed; Lake-wide advisory; Updates from routine monitoring program-organization |
| May 21, 2024 | Oso Flaco Lake boardwark at first alcove | Caution | Floating/planktonic bloom observed; Lake-wide advisory; Updates from routine monitoring program-organization |
| Aug 31, 2023 | Oso Flaco Lake boardwark at first alcove | Caution | Floating/planktonic bloom observed; Lake-wide advisory; Updates from routine monitoring program-organization |
| Aug 16, 2023 | Oso Flaco Lake boardwark at first alcove | Caution | Floating/planktonic bloom observed; Lake-wide advisory; Updates from routine monitoring program-organization |
| Jul 14, 2023 | Oso Flaco Lake boardwark at first alcove | Caution | Floating/planktonic bloom observed; Lake-wide advisory; Updates from routine monitoring program-organization |
| Jun 29, 2023 | Oso Flaco Lake boardwark at first alcove | Caution | Floating/planktonic bloom observed; Lake-wide advisory; Updates from routine monitoring program-organization |
| May 24, 2023 | Oso Flaco Lake boardwark at first alcove | Caution | Lake-wide advisory; Updates from routine monitoring program-organization |
| May 24, 2023 | Oso Flaco Lake boardwark at first alcove | Caution | Updates from routine monitoring program-organization |
| Dec 9, 2022 | Oso Flaco Lake boardwark at first alcove | Caution | Updates from routine monitoring program-organization |
| Dec 9, 2022 | Oso Flaco Lake boardwark at first alcove | General awareness | Updates from routine monitoring program-organization |
| Nov 28, 2022 | Oso Flaco Lake boardwark at first alcove | General awareness | Updates from routine monitoring program-organization |
| Nov 10, 2022 | Oso Flaco Lake boardwark at first alcove | Caution | Updates from routine monitoring program-organization |
| Oct 19, 2022 | Oso Flaco Lake boardwark at first alcove | Caution | Updates from routine monitoring program-organization |
| Sep 1, 2022 | Oso Flaco Lake boardwark at first alcove | Caution | Updates from routine monitoring program-organization |
| Jun 30, 2022 | Oso Flaco Lake boardwark at first alcove | Caution | Updates from routine monitoring program-organization |
| May 23, 2022 | Oso Flaco Lake boardwark at first alcove | Caution | Updates from routine monitoring program-organization |
| May 23, 2022 | Oso Flaco Lake boardwark at first alcove | None | Updates from routine monitoring program-organization |
| Aug 26, 2021 | boardwalk at first alcove | 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.