Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Washington County is a county located in the U.S. state of Illinois. As of 2000, the population was 15,148. Its county seat is Nashville, Illinois. According to the U.S. Census Bureau, the county has a total area of 564 square miles (1,461 km²), of which 563 square miles (1,457 km²) is land and 2 square miles (4 km²) (0.27%) is water. There are many lakes, rivers, streams, and creeks in this area.