A lake that’s been closed for two years is often polluted by algae blooms, according to the Environmental Protection Agency.

That’s because water levels in the lake are too high to allow the algae to grow.

A lake’s ability to filter pollution depends on a variety of factors, including whether the lake is located near a stream, estuary or river, and whether or not a lake has a lake floor or a reservoir.

Here’s what you need to know about water pollution in your local lakes.