Fed primarily by rainwater and snowmelt (in winter and spring), the Donald C. Tillman Water Reclamation Plant in Van Nuys (in summer and fall), and urban discharge, it is one of the few low-elevation perennial rivers in Southern California. Some water usually reaches the ocean, even in the driest summers; although there are historical accounts of the river running dry, there has been constant flow of the river every month since recording of stream flow began in 1929. This is helped by the concrete channel, which limits absorption of water into the earth. Flow, while generally low in volume, can be extremely brisk even in summer.