Five beaches are off-limits in the Lake Carmel Park district in the town of Kent. They have been closed for nearly ten days, affecting about 800 families.
The Health Department says the lake has blue-green algae, which releases toxins and can cause skin irritations and gastro-intenstinal problems in both people and pets.
The town plans to treat the lake with an algaecide.
The town's septic systems, dating back to the 1940's, are beginning to fail, resulting in an unbalanced lake and the algae bloom that has closed the beaches.
Residents are frustrated, and both the town and country are trying to find a solution.
"When the conditions are right, the algae that are already in the lake will bloom, and grow significantly," said Anne Bittmar of the Putnam County Department of Health. "And when the bloom is that large it can put toxins in the water."
"I don't think that we can let this go. I don't understand how anybody could actually sit back and let this go," said Kent resident Laurette Zion. "I mean look around, it's gorgeous, this is our home."