Virginia officials say the Pamunkey and North Anna branches of Lake Anna in Orange, Louisa and Spotsylvania counties are having a harmful algae bloom.
The Virginia Department of Health is advising the public to avoid contact with the lake in some areas until algae concentrations return to acceptable levels.
Some harmful algae can cause skin rash and gastrointestinal illnesses.
Algae blooms can happen when warm water and nutrients combine to make conditions favorable for algae growth. The department says these conditions have occurred recently with significantly warmer temperatures and sunny weather, which happened after high amounts of rainfall over the last month.
Discolored water or scums that are green or blueish-green should be avoided, because they are more likely to contain toxins.