SwimZone

Reservoir · Clark County · OH

Is Clarence J Brown Reservoir safe to swim?

Clear to swim

No advisory or algae bloom reported for this water today.

Updated July 22, 2026 · EPA CyAN satellite

Recent rain
6.0 in
Water type
Reservoir
Last updated
Jul 22
Bacteria
E. coli, fecal
Not monitored
Tested
Not monitored

No bacteria advisory or sampling for this water.

Blue-green algae
cyanobacteria
Clear to swim
Tested
None detected
Advisory over
100,000
cells/mL

Cyanobacteria No cyanobacteria bloom detected by satellite (below detection limit), imaged 2026-07-21.

Sampled Jul 21 · EPA CyAN satellite

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Clarence J Brown Reservoir is clear today. Be the first to know when it's not.

SwimZone checks this water every day and pings you the moment a bacteria or algae advisory goes up.

What's in the water at Clarence J Brown Reservoir right now

As of July 22, 2026, Clarence J Brown Reservoir in Clark County, Ohio is clear to swim. Satellite imagery detected no blue-green algae bloom as of July 21, 2026. Rainfall over the past week totaled 6.0 in, which can wash bacteria and nutrients into the water.

How this read is built

Blue-green algae at Clarence J Brown Reservoir

Clarence J Brown Reservoir is a reservoir in Clark County, Ohio covering about 3.2 sq mi, the 11th-largest of the 41 lakes SwimZone monitors in Ohio. Satellite imagery detected no blue-green algae bloom as of July 21, 2026.

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.

Bloom history at Clarence J Brown Reservoir

From EPA CyAN's satellite archive

2020: peak 931,071 cells/mL — above the 100,000 advisory level202021: peak 1,135,594 cells/mL — above the 100,000 advisory level212022: peak 591,375 cells/mL — above the 100,000 advisory level222023: peak 855,112 cells/mL — above the 100,000 advisory level232024: peak 1,135,594 cells/mL — above the 100,000 advisory level242025: peak 1,168,269 cells/mL — above the 100,000 advisory level252026: peak 957,862 cells/mL — above the 100,000 advisory level26Advisory100k cells/mL

Each bar is that summer's peak satellite reading (log scale). The red cap is the part above the advisory level.

Over 7 summers of satellite monitoring since 2020, Clarence J Brown Reservoir has reached blue-green algae advisory levels in 7 of them, most recently in June 2026. This is a lake that blooms most summers. Its highest recorded reading was 1,168,269 cells/mL in June 2025.

When to swim at Clarence J Brown Reservoir

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 June. Early summer, before the water fully warms, has generally been the safer window here; check the current read before you go either way.

Frequently asked: swimming in Clarence J Brown Reservoir

Is it safe to swim in Clarence J Brown Reservoir right now?
Clarence J Brown Reservoir is clear to swim today. Satellite imagery detected no blue-green algae bloom as of July 21, 2026. Conditions can change quickly with wind, heat, and rain, so check the posted signs and look at the water before you get in.
Does Clarence J Brown Reservoir have blue-green algae (cyanobacteria)?
No blue-green algae bloom is detectable at Clarence J Brown Reservoir as of July 21, 2026 (below the satellite's roughly 10,000 cells/mL detection limit). Cyanobacteria can still appear within hours in warm, calm, nutrient-rich water, so look before you swim.
Can dogs swim in Clarence J Brown Reservoir?
No bloom is detected at Clarence J Brown Reservoir today, but dogs are much more vulnerable to blue-green algae than people, and blooms can form within hours. Always keep dogs away from water that looks green, scummy, or discolored, and rinse them off after swimming.
How often does Clarence J Brown Reservoir have blue-green algae blooms?
Over 7 summers of satellite monitoring since 2020, Clarence J Brown Reservoir has reached blue-green algae advisory levels in 7 of them, most recently in June 2026. This is a lake that blooms most summers. Its highest recorded reading was 1,168,269 cells/mL in June 2025.
How many Ohio lakes and beaches are unsafe for swimming today?
Today, 33 Ohio waters are above a safe-contact threshold and 32 more warrant caution, out of 101 monitored waters SwimZone tracks in the state.
How is Clarence J Brown 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 Clarence J Brown Reservoir, and a recent-rainfall overlay, all refreshed daily. It was last updated July 22, 2026.

Before you swim: the basics

Swim spots near Clarence J Brown Reservoir

All Ohio →

Of the 6 monitored waters nearest Clarence J Brown Reservoir, 5 are flagged today.

WaterDistanceTodayAlgae readSampled
Lake ChoctawLake · Madison County13 miAdvisory1,465,895 cells/mLJul 21
Kiser LakeLake · Champaign County20 miAdvisory1,839,344 cells/mLJul 21
Deer Creek LakeLake · Pickaway County36 miAdvisory662,435 cells/mLJul 21
Dunns PondPond · Logan County37 miAdvisory985,424 cells/mLJul 21
Cowan LakeLake · Clinton County41 miClear to swimNone detectedJul 21
Lake LoramieLake · Shelby County43 miAdvisory1,737,893 cells/mLJul 21