Is Lake Ontario safe to swim?

As of July 22, 2026, Lake Ontario in New York is clear to swim. Satellite imagery detected no blue-green algae bloom as of July 21, 2026. Rainfall over the past week totaled 0.6 in, which can wash bacteria and nutrients into the water.

How this read is built

Lake Ontario is a lake in New York covering about 7,276.5 sq mi, the 1st-largest of the 133 lakes SwimZone monitors in New York. Satellite imagery detected no blue-green algae bloom as of July 21, 2026. 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, Lake Ontario 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 Lake Ontario right now?

Lake Ontario 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 Lake Ontario have blue-green algae (cyanobacteria)?

No blue-green algae bloom is detectable at Lake Ontario 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 Lake Ontario?

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

Over 7 summers of satellite monitoring since 2020, Lake Ontario 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 York lakes and beaches are unsafe for swimming today?

Today, 12 New York waters are above a safe-contact threshold and 188 more warrant caution, out of 495 monitored waters SwimZone tracks in the state.

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