Is Thomas Hill Reservoir safe to swim?

As of July 20, 2026, Thomas Hill Reservoir in Randolph County, Missouri carries a swim caution. Satellite imagery measured 96,246 cells/mL of blue-green algae on July 19, 2026, elevated but below 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

Thomas Hill Reservoir is a reservoir in Randolph County, Missouri covering about 6.3 sq mi, the 7th-largest of the 32 lakes SwimZone monitors in Missouri. Satellite imagery measured 96,246 cells/mL of blue-green algae on July 19, 2026, elevated but below 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, Thomas Hill Reservoir has reached blue-green algae advisory levels in 6 of them, most recently in June 2026. This is a lake that blooms most summers. Its highest recorded reading was 721,279 cells/mL in July 2021.

Is it safe to swim in Thomas Hill Reservoir right now?

Thomas Hill Reservoir carries a swim caution today. Satellite imagery measured 96,246 cells/mL of blue-green algae on July 19, 2026, elevated but below 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 Thomas Hill Reservoir have blue-green algae (cyanobacteria)?

Low levels. Satellite monitoring measured 96,246 cells/mL at Thomas Hill Reservoir on July 19, 2026, below the 100,000 cells/mL advisory threshold. Keep an eye out for surface scum, which can concentrate quickly.

Can dogs swim in Thomas Hill Reservoir?

Use caution. Blue-green algae at Thomas Hill Reservoir are elevated (96,246 cells/mL on July 19, 2026). Dogs are far more sensitive to cyanobacteria than people, so keep them out of the water and away from surface scum.

How often does Thomas Hill Reservoir have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Thomas Hill Reservoir has reached blue-green algae advisory levels in 6 of them, most recently in June 2026. This is a lake that blooms most summers. Its highest recorded reading was 721,279 cells/mL in July 2021.

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 Thomas Hill Reservoir'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 Thomas Hill Reservoir, and a recent-rainfall overlay, all refreshed daily. It was last updated July 20, 2026.