Iowa is characterized by a humid continental climate and an extensive network of rivers, streams, and tributaries that supply water to major reservoirs and dams. The Des Moines River Basin, which includes the Raccoon River, provides the largest water source for the state. The Missouri River forms the western boundary of the state and is the second-largest river in Iowa. Other major rivers include the Cedar, Iowa, and Wapsipinicon. Snowmelt and rainfall are the primary sources of water in Iowa, with the state receiving an average of 32 inches of precipitation per year. The state experiences both droughts and floods, with the most severe flood in recent history occurring in 2008. The Iowa Watershed Approach is a comprehensive approach to flood management that involves watershed management, floodplain restoration, and green infrastructure.