 Rock Lake Township was organized in 1876, and named for Rock Lake.According to the United States Census Bureau, the township has a total area of 34.6 square miles (89.6 km²), of which 33.1 square miles (85.8 km²) of it is land and 1.5 square miles (3.8 km²) of it (4.28%) is water.As of the census of 2000, there were 282 people, 108 households, and 83 families residing in the township.