Is Big Lake safe to swim?

As of August 20, 2026, Big Lake in Brown County, Illinois carries a swim caution. EPA's CyAN satellite showed a possible blue-green algae signal here (August 19, 2026), but the read is low-confidence and is not confirmed by a water sample. Modeled rainfall over the past week was about 1.5 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

Big Lake is a lake in Brown County, Illinois covering about 0.5 sq mi, the 63rd-largest of the 68 lakes SwimZone monitors in Illinois. EPA's CyAN satellite showed a possible blue-green algae signal here (August 19, 2026), but the read is low-confidence and is not confirmed by a water sample.

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, Big Lake has shown a satellite-estimated bloom-level signal in 6 of them, most recently in September 2025. It shows a bloom-level signal most summers. Its highest satellite estimate was about 4,000,000 cells/mL in September 2023. 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 most monitored summers (6 of 7), 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 Big Lake right now?

Big Lake carries a swim caution today. EPA's CyAN satellite showed a possible blue-green algae signal here (August 19, 2026), but the read is low-confidence and is not confirmed by a water sample. Conditions can change quickly with wind, heat, and rain, so check the posted signs and look at the water before you get in.

Does Big Lake have blue-green algae (cyanobacteria)?

Unclear. EPA's CyAN satellite showed a possible blue-green algae signal at Big Lake on August 19, 2026, but the read is low-confidence and is not confirmed by a water sample. Avoid any scummy or discolored water and check your state or county health department.

Can dogs swim in Big Lake?

Be careful. The satellite signal at Big Lake is low-confidence and unconfirmed, but dogs are much more vulnerable to blue-green algae than people. Keep them away from any green, scummy, or discolored water and check local advisories.

How often does Big Lake have blue-green algae blooms?

Over 7 summers of EPA CyAN satellite imagery since 2020, Big Lake has shown a satellite-estimated bloom-level signal in 6 of them, most recently in September 2025. It shows a bloom-level signal most summers. Its highest satellite estimate was about 4,000,000 cells/mL in September 2023. These are satellite estimates, not water samples or posted advisories; check your state or county health department for official status.

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

Today, 0 Illinois waters are above a safe-contact threshold and 54 more warrant caution, out of 130 monitored waters SwimZone tracks in the state.

How is Big 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 Big 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?→