SwimZone

Lake · Mercer County · OH

Is Grand Lake safe to swim?

Advisory

Blue-green algae is above the safe-contact threshold.

Updated July 22, 2026 · EPA CyAN satellite

Recent rain
0.1 in
Water type
Lake
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
Advisory
Tested
2,895,896
cells/mL
Advisory over
100,000
cells/mL

Cyanobacteria 2,895,896 cells/mL (satellite, 7-day max), imaged 2026-07-21.

Sampled Jul 21 · EPA CyAN satellite

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There's an advisory on Grand Lake right now.

Get an alert the day it clears, and every time a new one goes up.

What's in the water at Grand Lake right now

As of July 22, 2026, Grand Lake in Mercer County, Ohio is not recommended for swimming. Satellite imagery measured 2,895,896 cells/mL of blue-green algae on July 21, 2026, above the 100,000 cells/mL advisory threshold. Rainfall over the past week totaled 0.1 in, which can wash bacteria and nutrients into the water.

How this read is built

Blue-green algae at Grand Lake

Grand Lake is a lake in Mercer County, Ohio. Satellite imagery measured 2,895,896 cells/mL of blue-green algae on July 21, 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.

Bloom history at Grand Lake

From EPA CyAN's satellite archive

2020: peak 3,956,426 cells/mL — above the 100,000 advisory level202021: peak 3,337,206 cells/mL — above the 100,000 advisory level212022: peak 3,532,019 cells/mL — above the 100,000 advisory level222023: peak 3,433,231 cells/mL — above the 100,000 advisory level232024: peak 4,070,268 cells/mL — above the 100,000 advisory level242025: peak 4,307,874 cells/mL — above the 100,000 advisory level252026: peak 3,337,206 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, Grand Lake 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 4,307,874 cells/mL in September 2025.

When to swim at Grand Lake

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 September. 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 Grand Lake

Is it safe to swim in Grand Lake right now?
Grand Lake is not recommended for swimming today. Satellite imagery measured 2,895,896 cells/mL of blue-green algae on July 21, 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 Grand Lake have blue-green algae (cyanobacteria)?
Yes. Satellite monitoring measured 2,895,896 cells/mL of blue-green algae (cyanobacteria) at Grand Lake on July 21, 2026, above the 100,000 cells/mL advisory level. Avoid swimming and keep children and pets out of the water.
Can dogs swim in Grand Lake?
No. Blue-green algae at Grand Lake is above the safe-contact threshold (2,895,896 cells/mL on July 21, 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 Grand Lake have blue-green algae blooms?
Over 7 summers of satellite monitoring since 2020, Grand Lake 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 4,307,874 cells/mL in September 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 Grand 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 Grand Lake, and a recent-rainfall overlay, all refreshed daily. It was last updated July 22, 2026.

Before you swim: the basics

Swim spots near Grand Lake

All Ohio →

Of the 6 monitored waters nearest Grand Lake, 4 are flagged today.

WaterDistanceTodayAlgae readSampled
Lake LoramieLake · Shelby County14 miAdvisory1,737,893 cells/mLJul 21
Bresler Upground ReservoirReservoir · Allen County20 miClear to swimNone detectedJul 19
Ferguson ReservoirReservoir · Allen County28 miAdvisory558,757 cells/mLJul 19
Dunns PondPond · Logan County33 miAdvisory985,424 cells/mLJul 21
Kiser LakeLake · Champaign County37 miAdvisory1,839,344 cells/mLJul 21
Clarence J Brown ReservoirReservoir · Clark County56 miClear to swimNone detectedJul 21