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 Willamette River, a major tributary of the Columbia River, is 187 miles long. Flowing northward between the Oregon Coast Range and Cascade Range, the river and its tributaries form a basin called the Willamette Valley. The valley, fed by rainfall on the western side of the Cascades, is one of the most fertile agriculture regions of the United States.
The main stem of the Willamette River is formed by the confluence of the Middle and Coast Forks of the Willamette River near Springfield, Oregon. The main stem flows north for 187 miles to the Columbia River. Significant tributaries of the Willamette River, from source to mouth, include the Middle and Coast Fork Willamette, the McKenzie, Long Tom, Marys, Calapooia, Santiam, Luckiamute, Yamhill, Molalla, Tualatin, and Clackamas rivers. The main stem of the Willamette River has an elevation of 438 feet at its headwaters and loses 428 feet in elevation between source and mouth or about 2.3 feet per mile. The gradient is slightly steeper from the source to Albany than from Albany to Oregon City. The main stem of the Willamette varies in width from about 330 to 660 feet. The average flow at the mouth is approximately 32,400 cubic feet per second and contributes 12 to 15 percent of the total flow of the Columbia River. The Willamette’s flow varies seasonally, averaging about 8,200 cfs in August to more than 79,000 cfs in December.
The Willamette River basin contains thirteen USACE dams. The primary purpose of these projects is to prevent flood damages to the downstream metropolitan areas of the Willamette Valley but other purposes include hydropower generation, fish and wildlife, water quality, recreational use and water supply. The regulation of flood and conservation storage in each reservoir is coordinated with the regulation of storage in all of the other reservoirs in the basin.
Communities along the main stem at risk of flooding include Springfield and Eugene in Lane County; Harrisburg in Linn County; Corvallis in Benton County; Albany in Linn and Benton Counties; Salem in Marion County; Newberg in Yamhill County; Oregon City, West Linn, Milwaukie, and Lake Oswego in Clackamas County, and Portland in Multnomah and Washington counties. The Willamette River is known for flooding because of the high amounts and variations of precipitation in the valley. The largest flood on the Willamette River, in recorded history, occurred in 1861 when rainstorms and warm temperatures combined with a well-above-average snowpack in the Cascades. From Eugene to Portland, thousands of acres of riverside farmland were washed away and many towns in the valley were damaged or destroyed. Peaking at 635,000 cubic feet per second, the 1861 flood inundated approximately 353,000 acres of land. Although the Willamette River is regulated and controlled by a complex system of dams, severe flooding is still a concern. In 1996, a low elevation snowpack combined with massive rainfall and warm temperatures, caused some of the costliest floods to ever affect the Willamette Valley.