 According to the U.S. Census Bureau, the county has a total area of , of which is land and (7.0%) is water.It is the smallest county in Illinois by area.In recent years, average temperatures in the county seat of Hennepin have ranged from a low of in January to a high of in July, although a record low of was recorded in January 1999 and a record high of was recorded in June 1988.