Is Upper Peoria Lake safe to swim?

As of August 20, 2026, Upper Peoria Lake in Woodford County, Illinois carries a swim caution. EPA's CyAN satellite showed a possible blue-green algae signal here (July 21, 2026), but the read is low-confidence and is not confirmed by a water sample. Modeled rainfall over the past week was about 3.7 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

Upper Peoria Lake is a lake in Woodford County, Illinois covering about 24.2 sq mi, the 3rd-largest of the 68 lakes SwimZone monitors in Illinois. EPA's CyAN satellite showed a possible blue-green algae signal here (July 21, 2026), but the read is low-confidence and is not confirmed by a water sample. 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 EPA CyAN satellite imagery since 2020, Upper Peoria Lake has shown a satellite-estimated bloom-level signal in 4 of them, most recently in July 2026. Its highest satellite estimate was about 680,000 cells/mL in July 2026. 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 4 of 7 monitored summers, historically peaking around July. 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 Upper Peoria Lake right now?

Upper Peoria Lake carries a swim caution today. EPA's CyAN satellite showed a possible blue-green algae signal here (July 21, 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 Upper Peoria Lake have blue-green algae (cyanobacteria)?

Unclear. EPA's CyAN satellite showed a possible blue-green algae signal at Upper Peoria Lake on July 21, 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 Upper Peoria Lake?

Be careful. The satellite signal at Upper Peoria 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 Upper Peoria Lake have blue-green algae blooms?

Over 7 summers of EPA CyAN satellite imagery since 2020, Upper Peoria Lake has shown a satellite-estimated bloom-level signal in 4 of them, most recently in July 2026. Its highest satellite estimate was about 680,000 cells/mL in July 2026. 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 Upper Peoria 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 Peoria 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?→