Mad River: toxic algae advisories
Mad River, Humboldt County. See all Humboldt County waters.
What the state's records say
Danger advisory, last posted Sep 15, 2026. No all-clear recorded.
9 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: Toxins may be at levels that can seriously harm people and animals. Stay out of the water and keep children and pets away from the water and the shore.
Before you go, check the state's official algae page and any signs posted at the water.
Dogs at Mad River
The state has an advisory on file for Mad River (Danger advisory), last posted Sep 15, 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 |
|---|---|
| At Blue Lake Rancheria | The state's latest record, from Sep 30, 2026, recommends no advisory. |
| Mad River at Pump Station 4 | Algal mat alert sign, last posted Sep 15, 2026. No all-clear recorded. |
| Town of Blue Lake | Algal mat general awareness sign, last posted Sep 23, 2025. No all-clear recorded. |
| CDFW Mad River Hatchery | Algal mat general awareness sign, last posted Jul 18, 2024. No all-clear recorded. |
| near Hatchery Road Bridge | Caution advisory, last posted Aug 31, 2021. No all-clear recorded. |
| at County Boat Ramp | Caution advisory, last posted Aug 31, 2021. No all-clear recorded. |
| near Blue Lake | Caution advisory, last posted Jun 29, 2021. No all-clear recorded. |
| Arcata | Caution advisory, last posted Jun 29, 2021. No all-clear recorded. |
| Mad River at Blue Lake | Caution advisory, last posted Oct 19, 2020. No all-clear recorded. |
| Mad River at Blue Lake Bridge | Danger advisory, last posted Jul 17, 2018. No all-clear recorded. |
Advisory history
Every advisory record the state has published for Mad River, 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 30, 2026 | At Blue Lake Rancheria | None | Testing results below posting triggers; Updates from routine monitoring program-organization |
| Sep 16, 2026 | At Blue Lake Rancheria | None | Testing results below posting triggers; Updates from routine monitoring program-organization |
| Sep 15, 2026 | Mad River at Pump Station 4 | Algal mat alert sign | Advisory based on visual and toxins; Bottom/benthic bloom observed; Suspected illness reported |
| Sep 4, 2026 | Mad River at Pump Station 4 | Algal mat alert sign | Bottom/benthic bloom observed; Suspected bloom, investigation ongoing; Suspected illness reported; Visual monitoring only-present bloom |
| Sep 4, 2026 | Mad River at Pump Station 4 | Algal mat alert sign | Bottom/benthic bloom observed; Suspected bloom, investigation ongoing; Suspected illness reported; Visual monitoring only-present bloom |
| Sep 2, 2026 | At Blue Lake Rancheria | None | Testing results below posting triggers; Updates from routine monitoring program-organization |
| Aug 13, 2026 | At Blue Lake Rancheria | None | Testing results below posting triggers; Updates from routine monitoring program-organization |
| Aug 3, 2026 | At Blue Lake Rancheria | None | Testing results below posting triggers |
| Sep 23, 2025 | Town of Blue Lake | Algal mat general awareness sign | Visual monitoring only-absent bloom |
| Sep 10, 2025 | Town of Blue Lake | Algal mat alert sign | Suspected illness reported; Under investigation |
| Jul 18, 2024 | CDFW Mad River Hatchery | Algal mat general awareness sign | No cyano bloom w/photos; Posted algal mat general awareness |
| Jul 11, 2024 | CDFW Mad River Hatchery | Algal mat alert sign | Suspected bloom, investigation ongoing; Suspected illness reported; Under investigation |
| Aug 31, 2021 | near Hatchery Road Bridge | Caution | Advisory based on visual and toxins |
| Aug 31, 2021 | at County Boat Ramp | Caution | Advisory based on visual and toxins |
| Jun 29, 2021 | near Blue Lake | Caution | Advisory based on visual and toxins |
| Jun 29, 2021 | Arcata | Caution | Advisory based on visual and toxins |
| Oct 19, 2020 | Mad River at Blue Lake | Caution | Bottom/benthic bloom observed |
| Jul 17, 2018 | Mad River at Blue Lake Bridge | Danger |
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.