The Au Sable River is one of the defining natural resources of Big Creek Township. One of Michigan's most celebrated rivers, the Au Sable's headwaters are in Otsego, Crawford and Roscommon counties. It then flows eastward through Oscoda County and Big Creek Township, eventually draining into Lake Huron in the city of Oscoda. The main stream is 129 miles long, and all of the combined tributaries total 476 miles in length.

The Au Sable is known for trout fishing, especially browns. The western portion of the river in Big Creek Township is fast and narrow. At Camp 10 Road, it begins to widen out for the Mio Pond impoundment. Canoe rental services are available in Mio for trips starting below the dam or upstream, above the pond. For more information on the Au Sable, see the Michigan DNR webpage.