 Washington Township was organized in 1858, taking its name from Lake Washington.According to the United States Census Bureau, the township has a total area of 15.2 square miles (39.5 km²), of which 12.0 square miles (31.2 km²) of it is land and 3.2 square miles (8.3 km²) of it (21.05%) is water.As of the census of 2000, there were 797 people, 324 households, and 238 families residing in the township.