USACE CWMS - Chehalis Watershed

The Corps Water Management System (CWMS) includes four interrelated models to assist with water management for the basin:
- GeoHMS (Geospatial Hydrologic Modeling Extension)
- ResSIM (Reservoir System Simulation)
- RAS (River Analysis System)
- FIA (Flood Impact Analysis)
The Wynoochee River basin is 195 square miles and located on the Olympic Peninsula in the State of Washington. It is one of several tributaries feeding the Chehalis River before it drains at the mouth of Grays Harbor. The basin extends about 40 miles in the north-south direction and about 5 miles in the east-west direction.
The Wynoochee River flows 68 miles from its headwaters on the southern slopes of the Olympic Mountains to the Chehalis River. For the first 8 miles, Wynoochee River drops in elevation from nearly 5,000 feet to 1,000 feet then flows 16 miles through a forested valley known as the Weatherwax basin. Wynoochee Dam is located midway through the Weatherwax basin and regulates flows from about one quarter of the drainage area above the control point, Wynoochee River above Black Creek. USACE is authorized to use flood control storage in the reservoir at Wynoochee Dam.
Below the Weatherwax basin the river travels 4 miles through a twisting canyon, then winds 13 miles among gravel bars confined by steep valley slopes. The lower 27 miles of the river meanders through pastures and hayfields. Upstream of the dam tributaries are small. Downstream, the larger tributaries are Big Creek (9.7 square miles), Schafer Creek (12.9 square miles), and Black Creek (25.1 square miles).
The upper basin has rugged mountains and is not populated. The lower basin is primarily agricultural and is sparsely populated with houses and barns. The City of Montesano (population 4,000), is the only city within the basin and has the largest potential for economic damages or threat to life safety in a large flood event.
The agricultural area of the Wynoochee valley consists of 3,980 acres in the flood plain along the lower 27 miles of the river. These bottom lands are among the most productive of Grays Harbor County. Cultivated land is used for feed crops, primarily hay and silage, with clover and timothy hay the most common types. There are few major developments along the river. The city of Aberdeen has an industrial water intake at RM 8.1; there is a commercial gravel pit operation at RM 3.2; US Highway 410 crosses at RM 2.7; and the North Pacific Railroad crosses at RM 1.6.