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The Chehalis River flows approximately 125 miles in southwestern Washington north-northwesterly to Grays Harbor and the Pacific Ocean, draining an area of approximately 2,700 square miles. The Chehalis River Basin is the second largest basin in Washington State. It is bounded on the west by the Pacific Ocean, on the east by the Deschutes River Basin, on the north by the Olympic Mountains, and on the south by Cowlitz River Basin. The basin includes portions of Grays Harbor, Lewis, Mason, and Thurston Counties and the Cities of Aberdeen, Centralia, Chehalis, and Hoquiam, and the Confederated Tribes of the Chehalis Reservation. Population in the Chehalis River Basin was about 141,000 in 2000.
Increasing demands for water for municipal, fisheries, agricultural, industrial, and recreational uses will affect the water resources of the basin. A better understanding of the ground-water flow system and its relation to rivers and streams is needed to effectively manage the basin's water resources.
In cooperation with the Chehalis Basin Partnership and the Washington Department of Ecology, the USGS is studying the ground-water system in the Chehalis River Basin and how it interacts with rivers and streams in the basin.