Is Little Boy Lake safe to swim?

As of August 20, 2026, Little Boy Lake in Cass County, Minnesota carries a swim caution. EPA's CyAN satellite showed an elevated blue-green algae signal here (August 19, 2026), below bloom levels and unconfirmed by sampling. Modeled rainfall over the past week was about 0.1 in (Open-Meteo, a gridded estimate that can differ from a local gauge), which can wash bacteria and nutrients into the water.

How this read is built

Little Boy Lake is a lake in Cass County, Minnesota covering about 2.2 sq mi, the 183rd-largest of the 975 lakes SwimZone monitors in Minnesota. EPA's CyAN satellite showed an elevated blue-green algae signal here (August 19, 2026), below bloom levels and unconfirmed by sampling.

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 EPA CyAN satellite imagery since 2020, Little Boy Lake has shown a satellite-estimated bloom-level signal in 2 of them, most recently in September 2024. Those signals have been occasional rather than annual. Its highest satellite estimate was about 220,000 cells/mL in September 2024. These are satellite estimates, not water samples or posted advisories; check your state or county health department for official status.

It is currently peak season for blue-green algae, when warm, calm water lets blooms form fastest. This lake has bloomed in 2 of 7 monitored summers, historically peaking around September. Early summer, before the water fully warms, has generally been the safer window here; check the current read before you go either way.

Is it safe to swim in Little Boy Lake right now?

Little Boy Lake carries a swim caution today. EPA's CyAN satellite showed an elevated blue-green algae signal here (August 19, 2026), below bloom levels and unconfirmed by sampling. Conditions can change quickly with wind, heat, and rain, so check the posted signs and look at the water before you get in.

Does Little Boy Lake have blue-green algae (cyanobacteria)?

Elevated but below bloom levels. EPA's CyAN satellite showed an elevated blue-green algae signal at Little Boy Lake on August 19, 2026, unconfirmed by sampling. Keep an eye out for surface scum, which can concentrate quickly.

Can dogs swim in Little Boy Lake?

Use caution. EPA's CyAN satellite estimated elevated blue-green algae at Little Boy Lake on August 19, 2026 (a satellite estimate, not confirmed by sampling). Dogs are far more sensitive to cyanobacteria than people, so keep them out of the water and away from surface scum, and check local advisories.

How often does Little Boy Lake have blue-green algae blooms?

Over 7 summers of EPA CyAN satellite imagery since 2020, Little Boy Lake has shown a satellite-estimated bloom-level signal in 2 of them, most recently in September 2024. Those signals have been occasional rather than annual. Its highest satellite estimate was about 220,000 cells/mL in September 2024. These are satellite estimates, not water samples or posted advisories; check your state or county health department for official status.

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

Today, 0 Minnesota waters are above a safe-contact threshold and 451 more warrant caution, out of 1178 monitored waters SwimZone tracks in the state.

How is Little Boy 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 Little Boy Lake, and a recent-rainfall overlay, all refreshed daily. It was last updated August 20, 2026.

• Blue-green algae and dogs→
• What does blue-green algae look like?→
• When is algae season?→