Is Horseshoe Lake safe to swim?

As of July 20, 2026, Horseshoe Lake in Penobscot County, Maine is clear to swim. Satellite imagery detected no blue-green algae bloom as of July 19, 2026. Rainfall over the past week totaled 1.4 in, which can wash bacteria and nutrients into the water.

How this read is built

Horseshoe Lake is a lake in Penobscot County, Maine covering about 0.4 sq mi, the 331st-largest of the 331 lakes SwimZone monitors in Maine. Satellite imagery detected no blue-green algae bloom as of July 19, 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.

Over 7 summers of satellite monitoring since 2020, Horseshoe Lake 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 3,064,947 cells/mL in August 2022.

Is it safe to swim in Horseshoe Lake right now?

Horseshoe Lake is clear to swim today. Satellite imagery detected no blue-green algae bloom as of July 19, 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 Horseshoe Lake have blue-green algae (cyanobacteria)?

No blue-green algae bloom is detectable at Horseshoe Lake as of July 19, 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 Horseshoe Lake?

No bloom is detected at Horseshoe Lake 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 Horseshoe Lake have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Horseshoe Lake 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 3,064,947 cells/mL in August 2022.

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

Today, 21 Maine waters are above a safe-contact threshold and 27 more warrant caution, out of 469 monitored waters SwimZone tracks in the state.

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