Is Belle Fourche Reservoir safe to swim?

As of July 20, 2026, Belle Fourche Reservoir in Butte County, South Dakota is clear to swim. Satellite imagery detected no blue-green algae bloom as of July 19, 2026. Rainfall over the past week totaled 0.0 in, which can wash bacteria and nutrients into the water.

How this read is built

Belle Fourche Reservoir is a reservoir in Butte County, South Dakota covering about 9.5 sq mi, the 5th-largest of the 127 lakes SwimZone monitors in South Dakota. 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, Belle Fourche Reservoir has not reached a blue-green algae advisory level. Conditions can still change quickly, so check the current read before you swim.

Is it safe to swim in Belle Fourche Reservoir right now?

Belle Fourche 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 Belle Fourche Reservoir have blue-green algae (cyanobacteria)?

No blue-green algae bloom is detectable at Belle Fourche 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 Belle Fourche Reservoir?

No bloom is detected at Belle Fourche 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 Belle Fourche Reservoir have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Belle Fourche Reservoir has not reached a blue-green algae advisory level. Conditions can still change quickly, so check the current read before you swim.

How many South Dakota lakes and beaches are unsafe for swimming today?

Today, 89 South Dakota waters are above a safe-contact threshold and 3 more warrant caution, out of 142 monitored waters SwimZone tracks in the state.

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