Is West Valley Reservoir safe to swim?

As of August 20, 2026, West Valley Reservoir in Modoc County, California carries a swim caution. EPA's CyAN satellite estimated a possible blue-green algae bloom here (August 19, 2026); this is a satellite estimate, not a water sample or an official advisory. Modeled rainfall over the past week was about 0.0 in (Open-Meteo, a gridded estimate that can differ from a local gauge), which can wash bacteria and nutrients into the water.

How this read is built

West Valley Reservoir is a reservoir in Modoc County, California covering about 1.3 sq mi, the 107th-largest of the 186 lakes SwimZone monitors in California. EPA's CyAN satellite estimated a possible blue-green algae bloom here (August 19, 2026); this is a satellite estimate, not a water sample or an official advisory.

Blue-green algae (cyanobacteria) blooms are most common from midsummer into early fall, when the water is warm, calm, and rich in nutrients from runoff. They can produce toxins that make people sick and can be rapidly fatal to dogs, so skip water that looks green, scummy, or like spilled paint, and keep children and pets out of it.

Over 7 summers of EPA CyAN satellite imagery since 2020, West Valley Reservoir has shown a satellite-estimated bloom-level signal in 4 of them, most recently in July 2026. Its highest satellite estimate was about 1,700,000 cells/mL in September 2025. These are satellite estimates, not water samples or posted advisories; check your state or county health department for official status.

It is currently peak season for blue-green algae, when warm, calm water lets blooms form fastest. This lake has bloomed in 4 of 7 monitored summers, historically peaking around September. Early summer, before the water fully warms, has generally been the safer window here; check the current read before you go either way.

Is it safe to swim in West Valley Reservoir right now?

West Valley Reservoir carries a swim caution today. EPA's CyAN satellite estimated a possible blue-green algae bloom here (August 19, 2026); this is a satellite estimate, not a water sample or an official advisory. Conditions can change quickly with wind, heat, and rain, so check the posted signs and look at the water before you get in.

Does West Valley Reservoir have blue-green algae (cyanobacteria)?

Possibly. EPA's CyAN satellite estimated a possible blue-green algae bloom at West Valley Reservoir on August 19, 2026. This is a satellite estimate, not a water sample or an official advisory, so treat it as a reason to be careful: avoid discolored or scummy water and check your state or county health department for the official status.

Can dogs swim in West Valley Reservoir?

Use caution. EPA's CyAN satellite estimated elevated blue-green algae at West Valley Reservoir on August 19, 2026 (a satellite estimate, not confirmed by sampling). Dogs are far more sensitive to cyanobacteria than people, so keep them out of the water and away from surface scum, and check local advisories.

How often does West Valley Reservoir have blue-green algae blooms?

Over 7 summers of EPA CyAN satellite imagery since 2020, West Valley Reservoir has shown a satellite-estimated bloom-level signal in 4 of them, most recently in July 2026. Its highest satellite estimate was about 1,700,000 cells/mL in September 2025. These are satellite estimates, not water samples or posted advisories; check your state or county health department for official status.

How many California lakes and beaches are unsafe for swimming today?

Today, 0 California waters are above a safe-contact threshold and 21 more warrant caution, out of 445 monitored waters SwimZone tracks in the state.

How is West Valley Reservoir's water quality measured?

SwimZone builds this read from EPA CyAN satellite imagery for blue-green algae, official beach and fecal-bacteria advisories where a program monitors West Valley Reservoir, and a recent-rainfall overlay, all refreshed daily. It was last updated August 20, 2026.

• Blue-green algae and dogs→
• What does blue-green algae look like?→
• When is algae season?→