In the 20th century, many levees and dams were built on the San Joaquin and all of its major tributaries. These engineering works changed the fluctuating nature of the river forever, and cut off the Tulare Basin from the rest of the San Joaquin watershed. Once habitat for hundreds of thousands of spawning salmon and millions of migratory birds, today the river is subject to tremendous water-supply, navigation and regulation works by various federal agencies, which have dramatically reduced the flow of the river since the 20th century.