The Black River flows through both Thurston and Grays Harbor Counties as an important tributary of the Chehalis River, representing one of the largest remaining wetland systems in western Washington. This unique waterway originates at Black Lake in Thurston County, drains waters from the Black Hills, and merges with the Chehalis River southwest of Olympia before eventually emptying into Grays Harbor Estuary. Remarkably, due to the Black Lake Ditch dug in 1922, which connects the Black River to Percival Creek, this is the only waterway in Washington that drains to both the Pacific Coast and Puget Sound. The river supports extensive areas of undisturbed swamp habitat and features sedge meadows, riparian zones with red alder, Oregon ash, Scouler's willow, Sitka willow, and other deciduous vegetation along its banks. The Black River Unit of the Billy Frank Jr. Nisqually National Wildlife Refuge protects the northern portion of the river south of Black Lake, managing crucial habitat for numerous species while offering anglers access to productive waters throughout both counties.