Is Wachusett Reservoir safe to swim?

As of July 20, 2026, Wachusett Reservoir in Worcester County, Massachusetts carries a swim caution. Satellite imagery measured 29,238 cells/mL of blue-green algae on July 19, 2026, elevated but below the 100,000 cells/mL advisory threshold. Rainfall over the past week totaled 0.0 in, which can wash bacteria and nutrients into the water.

How this read is built

Wachusett Reservoir is a reservoir in Worcester County, Massachusetts covering about 6.2 sq mi, the 2nd-largest of the 39 lakes SwimZone monitors in Massachusetts. Satellite imagery measured 29,238 cells/mL of blue-green algae on July 19, 2026, elevated but below the 100,000 cells/mL advisory threshold. Larger, warmer waters like this can hold a bloom for weeks once nutrients and calm weather line up.

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, Wachusett Reservoir 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 Wachusett Reservoir right now?

Wachusett Reservoir carries a swim caution today. Satellite imagery measured 29,238 cells/mL of blue-green algae on July 19, 2026, elevated but below 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 Wachusett Reservoir have blue-green algae (cyanobacteria)?

Low levels. Satellite monitoring measured 29,238 cells/mL at Wachusett Reservoir on July 19, 2026, below the 100,000 cells/mL advisory threshold. Keep an eye out for surface scum, which can concentrate quickly.

Can dogs swim in Wachusett Reservoir?

Use caution. Blue-green algae at Wachusett Reservoir are elevated (29,238 cells/mL on July 19, 2026). Dogs are far more sensitive to cyanobacteria than people, so keep them out of the water and away from surface scum.

How often does Wachusett Reservoir have blue-green algae blooms?

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

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

Today, 5 Massachusetts waters are above a safe-contact threshold and 246 more warrant caution, out of 608 monitored waters SwimZone tracks in the state.

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