The Chehalis Basin is a large watershed located in western Washington, primarily within Lewis County and Grays Harbor County. It encompasses the Chehalis River and its tributaries, covering an area of approximately 2,700 square miles. The basin is known for its diverse ecosystems, including forests, wetlands, and agricultural lands. The Chehalis River flows through the basin, eventually emptying into Grays Harbor and the Pacific Ocean. The region is prone to flooding, which has led to various flood management efforts. The basin supports a variety of wildlife and is important for local communities, agriculture, and recreation.