Is Lake Butte des Morts safe to swim?

As of July 20, 2026, Lake Butte des Morts in Winnebago County, Wisconsin is not recommended for swimming. Satellite imagery measured 742,033 cells/mL of blue-green algae on July 19, 2026, above the 100,000 cells/mL advisory threshold. Rainfall over the past week totaled 0.0 in, which can wash bacteria and nutrients into the water.

How this read is built

Lake Butte des Morts is a lake in Winnebago County, Wisconsin covering about 13.4 sq mi, the 11th-largest of the 337 lakes SwimZone monitors in Wisconsin. Satellite imagery measured 742,033 cells/mL of blue-green algae on July 19, 2026, above the 100,000 cells/mL advisory threshold. 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, Lake Butte des Morts has reached blue-green algae advisory levels in 7 of them, most recently in July 2026. This is a lake that blooms most summers. Its highest recorded reading was 1,689,285 cells/mL in September 2024.

Is it safe to swim in Lake Butte des Morts right now?

Lake Butte des Morts is not recommended for swimming today. Satellite imagery measured 742,033 cells/mL of blue-green algae on July 19, 2026, above the 100,000 cells/mL advisory threshold. Conditions can change quickly with wind, heat, and rain, so check the posted signs and look at the water before you get in.

Does Lake Butte des Morts have blue-green algae (cyanobacteria)?

Yes. Satellite monitoring measured 742,033 cells/mL of blue-green algae (cyanobacteria) at Lake Butte des Morts on July 19, 2026, above the 100,000 cells/mL advisory level. Avoid swimming and keep children and pets out of the water.

Can dogs swim in Lake Butte des Morts?

No. Blue-green algae at Lake Butte des Morts is above the safe-contact threshold (742,033 cells/mL on July 19, 2026). Cyanobacteria toxins can be rapidly fatal to dogs, which swallow it while swimming and lick it from their fur. Keep dogs out of the water and away from any scum along the shore.

How often does Lake Butte des Morts have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Lake Butte des Morts has reached blue-green algae advisory levels in 7 of them, most recently in July 2026. This is a lake that blooms most summers. Its highest recorded reading was 1,689,285 cells/mL in September 2024.

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

Today, 91 Wisconsin waters are above a safe-contact threshold and 11 more warrant caution, out of 564 monitored waters SwimZone tracks in the state.

How is Lake Butte des Morts'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 Lake Butte des Morts, and a recent-rainfall overlay, all refreshed daily. It was last updated July 20, 2026.