Is Lost Valley Reservoir safe to swim?

As of July 20, 2026, Lost Valley Reservoir in Adams County, Idaho is clear to swim. Satellite imagery detected no blue-green algae bloom as of July 19, 2026. Rainfall over the past week totaled 0.1 in, which can wash bacteria and nutrients into the water.

How this read is built

Lost Valley Reservoir is a reservoir in Adams County, Idaho covering about 1.1 sq mi, the 41st-largest of the 63 lakes SwimZone monitors in Idaho. Satellite imagery detected no blue-green algae bloom as of July 19, 2026.

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 satellite monitoring since 2020, Lost Valley Reservoir has reached blue-green algae advisory levels in 1 of them, most recently in August 2024. Blooms here have been occasional rather than annual. Its highest recorded reading was 365,109 cells/mL in August 2024.

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

Lost Valley Reservoir is clear to swim today. Satellite imagery detected no blue-green algae bloom as of July 19, 2026. Conditions can change quickly with wind, heat, and rain, so check the posted signs and look at the water before you get in.

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

No blue-green algae bloom is detectable at Lost Valley Reservoir as of July 19, 2026 (below the satellite's roughly 10,000 cells/mL detection limit). Cyanobacteria can still appear within hours in warm, calm, nutrient-rich water, so look before you swim.

Can dogs swim in Lost Valley Reservoir?

No bloom is detected at Lost Valley Reservoir today, but dogs are much more vulnerable to blue-green algae than people, and blooms can form within hours. Always keep dogs away from water that looks green, scummy, or discolored, and rinse them off after swimming.

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

Over 7 summers of satellite monitoring since 2020, Lost Valley Reservoir has reached blue-green algae advisory levels in 1 of them, most recently in August 2024. Blooms here have been occasional rather than annual. Its highest recorded reading was 365,109 cells/mL in August 2024.

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

Today, 9 Idaho waters are above a safe-contact threshold and 1 more warrant caution, out of 66 monitored waters SwimZone tracks in the state.

How is Lost 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 Lost Valley Reservoir, and a recent-rainfall overlay, all refreshed daily. It was last updated July 20, 2026.