Two major river systems drain and define the two parts of the Central Valley. The Sacramento River, along with its tributaries the Feather River and American River, flows southwards through the Sacramento Valley for about . In the San Joaquin Valley, the San Joaquin River flows roughly northwest for , picking up tributaries such as the Merced River, Tuolumne River, Stanislaus River and Mokelumne River. The Central Valley watershed encompasses over a third of California at , with 46 percent draining into the Sacramento River, 26 percent into the San Joaquin, and 27 percent into Tulare Lake.