Actually fasteddiecopeman is correct. During non drought years, Ninety-five percent of SoCal water comes from the snowpack of the Sierra Nevada (Northern California).
The State Water Project and the federal Central Valley Project deliver melted snowpack mostly through the Sacramento-San Joaquin River Delta.
Currently, as the region enters the fourth year of a drought, supplies from Northern California have been curtailed to 5 percent. More water is being pumped (basically imported from out of state) from the Colorado River.
We're touchy about the water situation in NorCal since we were quicker to react to conservation efforts during the drought than SoCal was. On tap is a plan to build two huge underground tunnels from the Sacramento Delta to greatly increase the delivery capacity to SoCal.
If you're interested: Where Southern California gets its water - CAdrought.com