The beach area at Bernice State Park on Grand Lake will remained closed through Tuesday after E. Coli was found in the water earlier this week.
Recent samples show the bacteria level in the water has decreased, but not enough to reopen the beach.
"What is important for the public to know is that water testing results confirm that these are isolated areas of concern," said Spokesman Justin Alberty with the Grand River Dam Authority.
"Grand Lake is large, with plenty of other locations for swimming and access. As always, we encourage the public to practice common sense and follow safe practices when swimming."
Elevated levels also were found about a half mile east of the Cowskin Bay Bridge on the Elk River arm of Grand Lake. Those levels were found during the Seneca-Cayuga Tribe Environmental Department's testing in the area.
The site is near the tribe's ceremonial grounds, where the Strawberry Ceremonies will be hosted this weekend.
Officials are asking the public to take precautions and use common sense when swimming in the area.