Is South Branch Lake safe to swim?

As of July 22, 2026, South Branch Lake in Penobscot County, Maine is clear to swim. Satellite imagery detected no blue-green algae bloom as of July 21, 2026. Rainfall over the past week totaled 0.4 in, which can wash bacteria and nutrients into the water.

How this read is built

South Branch Lake is a lake in Penobscot County, Maine. 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.

Over 7 summers of satellite monitoring since 2020, South Branch Lake has reached blue-green algae advisory levels in 1 of them, most recently in June 2020. Blooms here have been occasional rather than annual. Its highest recorded reading was 110,913 cells/mL in June 2020.

It is currently peak season for blue-green algae, when warm, calm water lets blooms form fastest. This lake has bloomed in 1 of 7 monitored summers, 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.

Is it safe to swim in South Branch Lake right now?

South Branch Lake 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 South Branch Lake have blue-green algae (cyanobacteria)?

No blue-green algae bloom is detectable at South Branch Lake 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 South Branch Lake?

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

Over 7 summers of satellite monitoring since 2020, South Branch Lake has reached blue-green algae advisory levels in 1 of them, most recently in June 2020. Blooms here have been occasional rather than annual. Its highest recorded reading was 110,913 cells/mL in June 2020.

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

Today, 15 Maine waters are above a safe-contact threshold and 33 more warrant caution, out of 457 monitored waters SwimZone tracks in the state.

How is South Branch 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 South Branch Lake, and a recent-rainfall overlay, all refreshed daily. It was last updated July 22, 2026.

• Blue-green algae and dogs→
• What does blue-green algae look like?→
• When is algae season?→