Is Clark Canyon Reservoir safe to swim?

As of July 20, 2026, Clark Canyon Reservoir in Beaverhead County, Montana is clear to swim. Satellite imagery detected no blue-green algae bloom as of July 19, 2026. Rainfall over the past week totaled 1.1 in, which can wash bacteria and nutrients into the water.

How this read is built

Clark Canyon Reservoir is a reservoir in Beaverhead County, Montana covering about 7.7 sq mi, the 12th-largest of the 124 lakes SwimZone monitors in Montana. Satellite imagery detected no blue-green algae bloom as of July 19, 2026. Larger, warmer waters like this can hold a bloom for weeks once nutrients and calm weather line up.

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, Clark Canyon Reservoir has reached blue-green algae advisory levels in 4 of them, most recently in September 2023. Its highest recorded reading was 985,424 cells/mL in September 2023.

Is it safe to swim in Clark Canyon Reservoir right now?

Clark Canyon 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 Clark Canyon Reservoir have blue-green algae (cyanobacteria)?

No blue-green algae bloom is detectable at Clark Canyon 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 Clark Canyon Reservoir?

No bloom is detected at Clark Canyon 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 Clark Canyon Reservoir have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Clark Canyon Reservoir has reached blue-green algae advisory levels in 4 of them, most recently in September 2023. Its highest recorded reading was 985,424 cells/mL in September 2023.

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

Today, 19 Montana waters are above a safe-contact threshold and 2 more warrant caution, out of 135 monitored waters SwimZone tracks in the state.

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