Q. At Mary Beth Doyle Park in February, we spotted a big pump that was pumping water into an enormous fabric bag. I think the pump must be moving the water out of the way so that they can re-dig the ponds at the park. But what on earth does the water bag do?

A. The pond is occasionally pumped down to remove sediment from Malletts Creek. Some sediment is normal, but too much can be a problem for aquatic creatures. Sediment also carries phosphorus, and when it moves downstream it encourages algae blooms in Ford Lake and Lake Erie. The wetland system at Mary Beth Doyle Park is designed to remove pollutants, and also to reduce downstream flooding from large storms.

Washtenaw County Water Resources takes care to limit the impact downstream from this periodic sediment removal. The “dewatering filter bag” is a cheap and effective way to filter the water that is pumped from the pond into Malletts Creek.

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