The river was once one of the richest river ecosystems in California. However, now over of the river is dry except during floods, because lots of water has been taken away for irrigation. In fact, below Friant Dam, a dam built in the 1940s on the San Joaquin River, 95 percent of the river's flow is dry. Much of the river is polluted, destroying the historic population of chinook salmon, once the southernmost in the United States. Pesticides, selenium, and many more toxic materials are carried by the San Joaquin into San Francisco Bay.