The main project they control is the State Water Project from which more than two thirds of Californians receive their water. The state project is the largest part of the system in California to deliver water to the needed areas. Overall the water system works well, although the system does have some drawbacks. In terms of flooding, the problems mainly arise with levees. The dams and reservoirs reliably store water, but the levees, especially in 1997 floods, were notorious for breaking to cause flooding.
Levees break in several different ways.
One way is the water rising over the top of the levee and therefore flooding the other side. A second possibility is the erosion of the levees to the point where they begin to leak or break. A third problem occurs from boils which are areas of pressure inside the earthen levees. Boils can start out from animals burrowing into the levees. Then when the pressure becomes too great to be held by the pressure inside the earthen levees.