That would be the Colorado River, although I think that the largest river entirely in the Southern California region would be somewhere in LA county due to the large watershed from the tallest mountains in Southern California which are the San Bernardino Mountains in Riverside county.
Sacramento River.
American river
Mississippi river
Mississippi
Brahmaputra, Ganga river is the largest river in India.
california
The Mojave Desert is the largest desert in California.
Mississippi River is the name of the state and the largest river in the USA.
The largest river in the Carribean Island is the Artiboninte River which is 280km long.
Yangtze River
The Volta river.
The nile river