Lake · Beaverhead County · MT
Is Upper Red Rock Lake safe to swim?
No advisory or algae bloom reported for this water today.
Updated July 20, 2026 · EPA CyAN satellite
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What's in the water at Upper Red Rock Lake right now
As of July 20, 2026, Upper Red Rock Lake 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 0.3 in, which can wash bacteria and nutrients into the water.
Blue-green algae at Upper Red Rock Lake
Upper Red Rock Lake is a lake in Beaverhead County, Montana covering about 4.6 sq mi, the 22nd-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.
Bloom history at Upper Red Rock Lake
From EPA CyAN's satellite archive
Over 7 summers of satellite monitoring since 2020, Upper Red Rock Lake has reached blue-green algae advisory levels in 3 of them, most recently in July 2025. Its highest recorded reading was 558,757 cells/mL in July 2025.
Frequently asked: swimming in Upper Red Rock Lake
- Is it safe to swim in Upper Red Rock Lake right now?
- Upper Red Rock Lake 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 Upper Red Rock Lake have blue-green algae (cyanobacteria)?
- No blue-green algae bloom is detectable at Upper Red Rock Lake 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 Upper Red Rock Lake?
- No bloom is detected at Upper Red Rock Lake 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 Upper Red Rock Lake have blue-green algae blooms?
- Over 7 summers of satellite monitoring since 2020, Upper Red Rock Lake has reached blue-green algae advisory levels in 3 of them, most recently in July 2025. Its highest recorded reading was 558,757 cells/mL in July 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 Upper Red Rock Lake'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 Upper Red Rock Lake, and a recent-rainfall overlay, all refreshed daily. It was last updated July 20, 2026.