Originating in the Sierra Nevada, the North and Middle Forks of the American River join just upstream of the city of Auburn (Figure 1). From the North and Middle Forks confluence, the river flows past Auburn before entering Folsom Lake. Flows are released from Folsom Dam into Lake Natoma, which serves primarily as a reregulating reservoir. From Nimbus Dam, water is released downstream into the lower American River. It then flows through the American River Parkway (Figure 2), located in the densely populated Sacramento metropolitan area, for a distance of 23 miles before reaching its confluence with the Sacramento River. The average annual runoff is about 2.7 million acre-feet. The annual flow regime of the river, however, has changed significantly since construction of Folsom and Nimbus Dams in 1955. Nimbus Salmon and Steelhead Hatchery is located just below Nimbus Dam (Figure 3). Built in the early 1950's to compensate impacts of the Folsom project, it is operated by the California Department of Fish and Game under contract with the Federal Government. The lower American River floodplain is about 4,800 acres in size with an additional 4,000 acres of undeveloped adjacent lands. Of the 8,800 acres, about 5,000 acres are administered by Sacramento County as the American River Parkway (Figure 2). The parkway supports over 5 million visitor-use days annually with an estimated nonmarket value of $96 million (California Department of Fish and Game 1986), making it a very valuable regional asset.