Is Lake Shelbyville safe to swim?

As of July 20, 2026, Lake Shelbyville in Shelby County, Illinois is not recommended for swimming. Satellite imagery measured 681,496 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 Shelbyville is a lake in Shelby County, Illinois covering about 16 sq mi, the 4th-largest of the 75 lakes SwimZone monitors in Illinois. Satellite imagery measured 681,496 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 Shelbyville 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 1,236,468 cells/mL in August 2022.

Is it safe to swim in Lake Shelbyville right now?

Lake Shelbyville is not recommended for swimming today. Satellite imagery measured 681,496 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 Shelbyville have blue-green algae (cyanobacteria)?

Yes. Satellite monitoring measured 681,496 cells/mL of blue-green algae (cyanobacteria) at Lake Shelbyville 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 Shelbyville?

No. Blue-green algae at Lake Shelbyville is above the safe-contact threshold (681,496 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 Shelbyville have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Lake Shelbyville 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 1,236,468 cells/mL in August 2022.

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

Today, 53 Illinois waters are above a safe-contact threshold and 7 more warrant caution, out of 138 monitored waters SwimZone tracks in the state.

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