Lopez Lake: toxic algae advisories
Lopez 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.
5 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 Lopez Lake
The state has an advisory on file for Lopez 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.
Satellite estimate (not an advisory)
No satellite reading on file for Lopez Lake in recent weeks. Clouds, ice, or a lake shape the satellite's estimate near its center misses can cause this. It is a gap in the data, not a reading.
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 |
|---|---|
| Lopez Lake at Ranger dock (310LPZRDK) | Caution advisory, last posted Sep 4, 2026. No all-clear recorded. |
| at Cottonwood Cove (310LPZCOT) | Caution advisory, last posted Sep 4, 2026. No all-clear recorded. |
| various locations across lake | Caution advisory, last posted Jun 30, 2025. No all-clear recorded. |
| Lopez Lake | Caution advisory, last posted Jun 25, 2024. No all-clear recorded. |
| Lopez Lake at the marina | The state's latest record, from Nov 28, 2022, recommends no advisory. |
| Spot not described | The state's latest record, from May 27, 2021, recommends no advisory. |
| Lopez Lake | The state's latest record, from Oct 30, 2020, recommends no advisory. |
| Lopez Lake, near Lopez Lake Marina | Caution advisory, last posted Aug 31, 2018. No all-clear recorded. |
Advisory history
Every advisory record the state has published for Lopez 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 | Lopez Lake at Ranger dock (310LPZRDK) | Caution | Updates from routine monitoring program-organization |
| Sep 4, 2026 | at Cottonwood Cove (310LPZCOT) | Caution | Updates from routine monitoring program-organization |
| Sep 4, 2026 | at Cottonwood Cove (310LPZCOT) | None | Updates from routine monitoring program-organization |
| Sep 4, 2026 | Lopez Lake at Ranger dock (310LPZRDK) | Visual observation | Updates from routine monitoring program-organization |
| Jun 25, 2026 | at Cottonwood Cove (310LPZCOT) | General awareness | Updates from routine monitoring program-organization |
| Jun 30, 2025 | various locations across lake | Caution | Lake-wide advisory |
| Jun 26, 2025 | various locations across lake | Caution | Lake-wide advisory |
| Jun 26, 2025 | various locations across lake | Caution | Lake-wide advisory |
| Jun 19, 2025 | various locations across lake | Caution | Lake-wide advisory |
| Jun 11, 2025 | various locations across lake | Caution | Lake-wide advisory |
| May 29, 2025 | various locations across lake | Caution | Lake-wide advisory |
| May 20, 2025 | various locations across lake | Caution | Lake-wide advisory |
| May 15, 2025 | various locations across lake | Caution | Lake-wide advisory |
| May 2, 2025 | various locations across lake | Caution | Lake-wide advisory |
| Aug 26, 2024 | various locations across lake | Caution | Lake-wide advisory |
| Jul 30, 2024 | various locations across lake | Caution | Suspected bloom, investigation ongoing |
| Jul 5, 2024 | various locations across lake | Caution | Suspected bloom, investigation ongoing |
| Jul 5, 2024 | various locations across lake | General awareness | Suspected bloom, investigation ongoing |
| Jun 28, 2024 | at Cottonwood Cove (310LPZCOT) | Caution | Lake-wide advisory; Updates from routine monitoring program-organization |
| Jun 28, 2024 | Lopez Lake at Ranger dock (310LPZRDK) | Caution | Updates from routine monitoring program-organization |
| Jun 25, 2024 | Lopez Lake | Caution | Updates from routine monitoring program-organization |
| Jun 12, 2024 | Lopez Lake at Ranger dock (310LPZRDK) | Visual observation | Updates from routine monitoring program-organization |
| Jun 12, 2024 | at Cottonwood Cove (310LPZCOT) | Visual observation | Lake-wide advisory; Updates from routine monitoring program-organization |
| May 21, 2024 | at Cottonwood Cove (310LPZCOT) | General awareness | Updates from routine monitoring program-organization |
| May 21, 2024 | at Cottonwood Cove (310LPZCOT) | None | Updates from routine monitoring program-organization |
| May 21, 2024 | Lopez Lake at Ranger dock (310LPZRDK) | None | Updates from routine monitoring program-organization |
| Nov 8, 2023 | Lopez Lake | None | Updates from routine monitoring program-organization |
| Aug 31, 2023 | Lopez Lake at Ranger dock (310LPZRDK) | Caution | Updates from routine monitoring program-organization |
| Aug 21, 2023 | Lopez Lake | Caution | Floating/planktonic bloom observed; Lake-wide advisory; Updates from routine monitoring program-organization |
| Aug 17, 2023 | Lopez Lake | Caution | Bottom/benthic bloom observed; Lake-wide advisory; Updates from routine monitoring program-organization |
| Aug 9, 2023 | Lopez Lake | Caution | Bottom/benthic bloom observed; Lake-wide advisory; Updates from routine monitoring program-organization |
| Aug 4, 2023 | Lopez Lake | Caution | Lake-wide advisory; Updates from routine monitoring program-organization |
| Jul 25, 2023 | Lopez Lake | Caution | Lake-wide advisory; Updates from routine monitoring program-organization |
| Jul 21, 2023 | Lopez Lake | Caution | Lake-wide advisory; Updates from routine monitoring program-organization |
| Jul 14, 2023 | Lopez Lake | Caution | Lake-wide advisory; Updates from routine monitoring program-organization |
| Jun 29, 2023 | Lopez Lake at Ranger dock (310LPZRDK) | NA - refer to Report Details | Rec general awareness |
| Jun 29, 2023 | at Cottonwood Cove (310LPZCOT) | NA - refer to Report Details | Rec general awareness |
| May 25, 2023 | Lopez Lake at Ranger dock (310LPZRDK) | NA - refer to Report Details | Rec general awareness |
| May 25, 2023 | Lopez Lake at Ranger dock (310LPZRDK) | NA - refer to Report Details | Rec general awareness |
| May 25, 2023 | at Cottonwood Cove (310LPZCOT) | NA - refer to Report Details | Rec general awareness |
| Dec 9, 2022 | Lopez Lake | NA - refer to Report Details | Posted general awareness |
| Dec 9, 2022 | at Cottonwood Cove (310LPZCOT) | None | Advisory based on visual and toxins |
| Dec 9, 2022 | Lopez Lake at Ranger dock (310LPZRDK) | None | Updates from routine monitoring program-organization |
| Nov 28, 2022 | Lopez Lake at the marina | None | Updates from routine monitoring program-organization |
| Nov 28, 2022 | at Cottonwood Cove (310LPZCOT) | None | Advisory based on visual and toxins |
| Nov 10, 2022 | at Cottonwood Cove (310LPZCOT) | None | Advisory based on visual and toxins |
| Nov 10, 2022 | Lopez Lake at the marina | None | Updates from routine monitoring program-organization |
| Oct 19, 2022 | Lopez Lake | NA - refer to Report Details | Posted general awareness |
| Sep 1, 2022 | Lopez Lake | NA - refer to Report Details | Posted general awareness |
| Aug 18, 2022 | Lopez Lake | None | Lake-wide advisory |
| Aug 16, 2022 | Lopez Lake | Caution | Lake-wide advisory |
| Aug 12, 2022 | Lopez Lake | Caution | Lake-wide advisory |
| Aug 12, 2022 | Lopez Lake | Caution | Lake-wide advisory |
| Jul 29, 2022 | Lopez Lake at the marina | Caution | Updates from routine monitoring program-organization |
| Jun 30, 2022 | at Cottonwood Cove (310LPZCOT) | None | Advisory based on visual and toxins |
| Jun 30, 2022 | Lopez Lake at the marina | None | Updates from routine monitoring program-organization |
| May 26, 2022 | at Cottonwood Cove (310LPZCOT) | None | Advisory based on visual and toxins |
| May 23, 2022 | Lopez Lake at the marina | None | Updates from routine monitoring program-organization |
| May 27, 2021 | Spot not described | None | Testing results below posting triggers |
| Oct 30, 2020 | Lopez Lake | None | |
| Aug 31, 2018 | Lopez Lake, near Lopez Lake Marina | Caution |
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.