Letts Creek is a tributary of Mill Creek, the largest tributary in the entire Huron River system, which drains 144 square miles of land. Mill Creek joins the Huron River in Dexter, 13 river miles downstream of Vets Park. Chelsea relies on municipal wells for its drinking water, which are protected in a Wellhead Protection Zone. Any substance, such as oil, that is poured into a storm grate or dropped on the street goes directly into Letts Creek. Volunteers with the Huron River Watershed Council have monitored the conditions of Letts Creek since 1993. The Huron River is 129 miles long, with an additional 370 miles of streams flowing into it, and most of the 1050 lakes and ponds in the Huron River basin were formed by glaciers.