Is Tongue River Reservoir safe to swim?

As of July 20, 2026, Tongue River Reservoir in Big Horn 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 0.1 in, which can wash bacteria and nutrients into the water.

How this read is built

Tongue River Reservoir is a reservoir in Big Horn County, Montana covering about 3.4 sq mi, the 29th-largest of the 124 lakes SwimZone monitors in Montana. 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, Tongue River Reservoir has reached blue-green algae advisory levels in 4 of them, most recently in August 2025. Its highest recorded reading was 458,124 cells/mL in August 2025.

Is it safe to swim in Tongue River Reservoir right now?

Tongue River 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 Tongue River Reservoir have blue-green algae (cyanobacteria)?

No blue-green algae bloom is detectable at Tongue River 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 Tongue River Reservoir?

No bloom is detected at Tongue River 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 Tongue River Reservoir have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Tongue River Reservoir has reached blue-green algae advisory levels in 4 of them, most recently in August 2025. Its highest recorded reading was 458,124 cells/mL in August 2025.

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 Tongue River 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 Tongue River Reservoir, and a recent-rainfall overlay, all refreshed daily. It was last updated July 20, 2026.