Supp III a Basin Description
Supplement Section III — Basin Description Information Base Part A — Basin Description The Chehalis River Basin is the largest river basin in western Washington. With the exception of the Columbia River basin, it is the largest in the state. The basin extends over eight counties. It encompasses large portions of Grays Harbor, Lewis, and Thurston counties, and smaller parts of Mason, Pacific, Cowlitz, Wahkiakum, and Jefferson counties. For purposes of water resources planning under the Washington State Watershed Planning Act of 1998, the Chehalis Basin was divided into two Water Resource Inventory Areas (WRIAs), WRIA 22 and WRIA 23, depicted here with surrounding WRIA numbers and in relation to the whole state of Washington. Chehalis Basin Watershed — County Land Areas County Area (sq.mi.) Area (acres) Percentage Grays Harbor 1,390 889,711 50.3% Thurston 323 206,446 11.7% Lewis 770 493,103 27.9% Mason 206 132,146 7.5% Pacific 66 42,040 2.4% Cowlitz 8 5,427 0.3% Jefferson 2 1,259 0.07% Wahkiakum .1 37 0.002% Total 2,766 1,770,169 Source: Chehalis Watershed GIS Watershed Boundaries The basin 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 the Willapa Hills and Cowlitz River Basin. Elevations vary from sea level at Grays Harbor to the 5,054-foot Capitol Peak in the Olympic National Forest. The basin consists of approximately 2,766 square miles. The Chehalis WRIA 22 River system flows through three distinct eco-regions before emptying into Grays Harbor near Aberdeen (Omernik, 1987): • The Cascade ecoregion (including the Olympic Mountains) is char- acterized by volcanic/sedimentary bedrock formations.
[Show full text]