Is Jack Lake safe to swim?

As of August 20, 2026, Jack Lake in Mason County, Illinois 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 1.9 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

Jack Lake is a lake in Mason County, Illinois covering about 2.3 sq mi, the 20th-largest of the 68 lakes SwimZone monitors in Illinois. 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, Jack Lake has shown a satellite-estimated bloom-level signal in 7 of them, most recently in July 2026. It shows a bloom-level signal most summers. Its highest satellite estimate was about 2,800,000 cells/mL in August 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 (7 of 7), historically peaking around August. 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 Jack Lake right now?

Jack 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 Jack Lake have blue-green algae (cyanobacteria)?

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

Can dogs swim in Jack Lake?

Use caution. EPA's CyAN satellite estimated elevated blue-green algae at Jack 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 Jack Lake have blue-green algae blooms?

Over 7 summers of EPA CyAN satellite imagery since 2020, Jack Lake has shown a satellite-estimated bloom-level signal in 7 of them, most recently in July 2026. It shows a bloom-level signal most summers. Its highest satellite estimate was about 2,800,000 cells/mL in August 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 Jack 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 Jack 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?→