Thermalito Afterbay: toxic algae advisories
Thermalito Afterbay, Butte County. See all Butte County waters.
What the state's records say
The state's latest record, from Sep 4, 2026, recommends no advisory.
That reflects what the state knew on that date. It is not a finding that the water is safe now: blooms can return within days, and many are never reported.
Before you go, check the state's official algae page and any signs posted at the water.
Dogs at Thermalito Afterbay
The state's latest record, from Sep 4, 2026, recommends no advisory. For Thermalito Afterbay, that is not the same as an all-clear for today. Look at the water before your dog goes in: keep it out of green, blue-green or scummy water and away from mats on the shore.
Satellite estimate (not an advisory)
No satellite reading on file for Thermalito Afterbay 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 |
|---|---|
| swim beach | The state's latest record, from Sep 4, 2026, recommends no advisory. |
| Swim Beach | The state's latest record, from Jul 14, 2021, recommends no advisory. |
| Thermalito Afterbay Swim Beach | The state's latest record, from Sep 3, 2020, recommends no advisory. |
| Lake Oroville Thermalito Afterbay Swim Beach | The state's latest record, from Dec 10, 2019, recommends no advisory. |
Advisory history
Every advisory record the state has published for Thermalito Afterbay, 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 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| Aug 21, 2026 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| Aug 10, 2026 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| Jul 31, 2026 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| Jul 28, 2026 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| Jul 17, 2026 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| Jul 15, 2026 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| Jul 3, 2026 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| Jun 30, 2026 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| Jun 12, 2026 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| Jun 8, 2026 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| May 29, 2026 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| May 22, 2026 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| Sep 3, 2025 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| Aug 22, 2025 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| Aug 18, 2025 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| Aug 13, 2025 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| Aug 1, 2025 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| Jul 25, 2025 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| Jul 18, 2025 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| Jul 14, 2025 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| Jul 3, 2025 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| Jul 1, 2025 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| Jun 20, 2025 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| Jun 11, 2025 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| Jun 9, 2025 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| May 29, 2025 | swim beach | None | Updates from routine monitoring program-organization |
| May 23, 2025 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| Aug 30, 2024 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| Aug 23, 2024 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| Aug 15, 2024 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| Aug 12, 2024 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| Jul 29, 2024 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| Jul 19, 2024 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| Jul 12, 2024 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| Jul 3, 2024 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| Jun 27, 2024 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| Jun 14, 2024 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| Jun 6, 2024 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| May 30, 2024 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| May 23, 2024 | swim beach | None | Updates from routine monitoring program-organization; Visual monitoring only-absent bloom |
| Aug 31, 2023 | swim beach | None | Updates from routine monitoring program-organization |
| Aug 28, 2023 | swim beach | None | Updates from routine monitoring program-organization |
| Aug 10, 2023 | swim beach | None | Updates from routine monitoring program-organization |
| Aug 8, 2023 | swim beach | None | Updates from routine monitoring program-organization |
| Jul 28, 2023 | swim beach | None | Updates from routine monitoring program-organization |
| Jul 19, 2023 | swim beach | None | Updates from routine monitoring program-organization |
| Jun 30, 2023 | swim beach | None | Updates from routine monitoring program-organization |
| Jun 26, 2023 | swim beach | None | Updates from routine monitoring program-organization |
| Jun 15, 2023 | swim beach | None | Updates from routine monitoring program-organization |
| Jun 7, 2023 | swim beach | None | Updates from routine monitoring program-organization |
| Sep 7, 2022 | swim beach | None | Updates from routine monitoring program-organization |
| Sep 6, 2022 | swim beach | None | Updates from routine monitoring program-organization |
| Aug 18, 2022 | swim beach | None | Updates from routine monitoring program-organization |
| Aug 9, 2022 | swim beach | None | Updates from routine monitoring program-organization |
| Aug 1, 2022 | swim beach | None | Updates from routine monitoring program-organization |
| Jul 25, 2022 | swim beach | None | Updates from routine monitoring program-organization |
| Jul 13, 2022 | swim beach | None | Updates from routine monitoring program-organization |
| Jul 8, 2022 | swim beach | None | Updates from routine monitoring program-organization |
| Jul 1, 2022 | swim beach | None | Updates from routine monitoring program-organization |
| Jun 27, 2022 | swim beach | None | Updates from routine monitoring program-organization |
| Jun 17, 2022 | swim beach | None | Updates from routine monitoring program-organization |
| Jun 10, 2022 | swim beach | None | Updates from routine monitoring program-organization |
| Jun 3, 2022 | swim beach | None | Updates from routine monitoring program-organization |
| Jun 3, 2022 | swim beach | None | Updates from routine monitoring program-organization |
| Jun 1, 2022 | swim beach | None | Updates from routine monitoring program-organization |
| Jul 14, 2021 | Swim Beach | None | Testing results below posting triggers |
| Sep 3, 2020 | Thermalito Afterbay Swim Beach | None | |
| Dec 10, 2019 | Lake Oroville Thermalito Afterbay Swim Beach | None |
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.