The river rises in four main forks in the Sierra Nevada which unite as arms of the Lake Oroville reservoir in the foothills northeast of Oroville in eastern Butte County. In terms of drainage areas the largest is the North Fork, which drains about 60% of the entire upper Feather River watershed. The Middle Fork is the second largest, draining about 32% of the upper basin. The South Fork and the West Branch are much smaller, each drains less than 5% of the upper basin. The main stem Feather River begins at Oroville Dam, the outlet of Lake Oroville. From there the river flows generally south across the Sacramento Valley, east of the Sutter Buttes, past Oroville and Yuba City-Marysville. The Feather receives the Yuba River from the east at Yuba City and the Bear River from the east south of Yuba City. It empties into the Sacramento River from the north, about northwest of Sacramento.