Is Sunapee Lake safe to swim?

As of July 20, 2026, Sunapee Lake in Merrimack County, New Hampshire is not recommended for swimming. Satellite imagery measured 114,105 cells/mL of blue-green algae on July 19, 2026, above the 100,000 cells/mL advisory threshold. Rainfall over the past week totaled 3.1 in, which can wash bacteria and nutrients into the water.

How this read is built

Sunapee Lake is a lake in Merrimack County, New Hampshire. Satellite imagery measured 114,105 cells/mL of blue-green algae on July 19, 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.

Over 7 summers of satellite monitoring since 2020, Sunapee Lake has not reached a blue-green algae advisory level. Conditions can still change quickly, so check the current read before you swim.

Is it safe to swim in Sunapee Lake right now?

Sunapee Lake is not recommended for swimming today. Satellite imagery measured 114,105 cells/mL of blue-green algae on July 19, 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 Sunapee Lake have blue-green algae (cyanobacteria)?

Yes. Satellite monitoring measured 114,105 cells/mL of blue-green algae (cyanobacteria) at Sunapee Lake on July 19, 2026, above the 100,000 cells/mL advisory level. Avoid swimming and keep children and pets out of the water.

Can dogs swim in Sunapee Lake?

No. Blue-green algae at Sunapee Lake is above the safe-contact threshold (114,105 cells/mL on July 19, 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 Sunapee Lake have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Sunapee Lake has not reached a blue-green algae advisory level. Conditions can still change quickly, so check the current read before you swim.

How many New Hampshire lakes and beaches are unsafe for swimming today?

Today, 2 New Hampshire waters are above a safe-contact threshold and 1 more warrant caution, out of 61 monitored waters SwimZone tracks in the state.

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