Los Angeles County has a number of watercourses which discharge into the Pacific Ocean. Three of these are of major importance in that they traverse the Coastal Plain area of the County. These three are the Los Angeles River, the San Gabriel River, and Ballona Creek. They have a combined drainage area of approximately 1,645 square miles, most of which is within Los Angeles County. Such area not only comprises over 40 percent of the land area of the County but, more important, includes within its boundaries, the great majority of the County's population. The Coastal Plain area of Los Angeles County, prior to installation of flood control works, was probably subject to a greater potential flood hazard than any area of similar size and density of population in the United States. It has been subjected periodically to floods that, descending from the San Gabriel and Santa Monica Mountains, have rushed across the valley floor towards the Pacific Ocean altering topographic features and causing loss of life and property.