SwimZone

Lake · Columbiana County · OH

Is Guilford 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
1,465,895
cells/mL
Advisory over
100,000
cells/mL

Cyanobacteria 1,465,895 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 Guilford Lake right now.

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

What's in the water at Guilford Lake right now

As of July 22, 2026, Guilford Lake in Columbiana County, Ohio is not recommended for swimming. Satellite imagery measured 1,465,895 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 Guilford Lake

Guilford Lake is a lake in Columbiana County, Ohio covering about 0.5 sq mi, the 39th-largest of the 41 lakes SwimZone monitors in Ohio. Satellite imagery measured 1,465,895 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 Guilford Lake

From EPA CyAN's satellite archive

2020: peak 855,112 cells/mL — above the 100,000 advisory level202021: peak 905,030 cells/mL — above the 100,000 advisory level212022: peak 763,384 cells/mL — above the 100,000 advisory level222023: peak 985,424 cells/mL — above the 100,000 advisory level232024: peak 1,272,047 cells/mL — above the 100,000 advisory level242025: peak 879,717 cells/mL — above the 100,000 advisory level25Advisory100k cells/mL

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

Over 6 summers of satellite monitoring since 2020, Guilford Lake has reached blue-green algae advisory levels in 6 of them, most recently in September 2025. This is a lake that blooms most summers. Its highest recorded reading was 1,272,047 cells/mL in August 2024.

When to swim at Guilford 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 (6 of 6), 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.

Frequently asked: swimming in Guilford Lake

Is it safe to swim in Guilford Lake right now?
Guilford Lake is not recommended for swimming today. Satellite imagery measured 1,465,895 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 Guilford Lake have blue-green algae (cyanobacteria)?
Yes. Satellite monitoring measured 1,465,895 cells/mL of blue-green algae (cyanobacteria) at Guilford 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 Guilford Lake?
No. Blue-green algae at Guilford Lake is above the safe-contact threshold (1,465,895 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 Guilford Lake have blue-green algae blooms?
Over 6 summers of satellite monitoring since 2020, Guilford Lake has reached blue-green algae advisory levels in 6 of them, most recently in September 2025. This is a lake that blooms most summers. Its highest recorded reading was 1,272,047 cells/mL in August 2024.
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 Guilford 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 Guilford Lake, and a recent-rainfall overlay, all refreshed daily. It was last updated July 22, 2026.

Before you swim: the basics

Swim spots near Guilford Lake

All Ohio →

Of the 6 monitored waters nearest Guilford Lake, 5 are flagged today.

WaterDistanceTodayAlgae readSampled
Pine LakeLake · Mahoning County15 miAdvisory662,435 cells/mLJul 21
Berlin LakeLake · Portage County16 miAdvisory527,938 cells/mLJul 21
Deer Creek ReservoirReservoir · Stark County18 miAdvisory1,103,832 cells/mLJul 21
Evans LakeLake · Mahoning County18 miClear to swimNone detectedJul 21
Lake MiltonLake · Mahoning County21 miAdvisory354,898 cells/mLJul 21
Meander Creek ReservoirReservoir · Mahoning County23 miAdvisory325,944 cells/mLJul 21