Is Alton Slough safe to swim?

As of July 20, 2026, Alton Slough in St. Charles County, Missouri is not recommended for swimming. Satellite imagery measured 1,737,893 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

Alton Slough is a lake in St. Charles County, Missouri covering about 0.6 sq mi, the 23rd-largest of the 32 lakes SwimZone monitors in Missouri. Satellite imagery measured 1,737,893 cells/mL of blue-green algae on July 19, 2026, above the 100,000 cells/mL advisory threshold.

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, Alton Slough 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 2,180,635 cells/mL in August 2025.

Is it safe to swim in Alton Slough right now?

Alton Slough is not recommended for swimming today. Satellite imagery measured 1,737,893 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 Alton Slough have blue-green algae (cyanobacteria)?

Yes. Satellite monitoring measured 1,737,893 cells/mL of blue-green algae (cyanobacteria) at Alton Slough 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 Alton Slough?

No. Blue-green algae at Alton Slough is above the safe-contact threshold (1,737,893 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 Alton Slough have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Alton Slough 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 2,180,635 cells/mL in August 2025.

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

Today, 14 Missouri waters are above a safe-contact threshold and 1 more warrant caution, out of 34 monitored waters SwimZone tracks in the state.

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