After California became part of the United States in 1846 , the ranching economy gradually shifted towards agriculture ( a transition quickened by the Great Flood of 1862 and subsequent drought of 1863-64 which killed almost three-quarters of the livestock in Los Angeles County ) and the San Gabriel River became a crucial water source for farms . The California Gold Rush brought a huge influx of people to the state , and the high demand for food transformed the San Gabriel River Basin into one of the nation 's most productive agricultural regions . The Southern Pacific Railroad arrived in 1876 and the Union Pacific Railroad in the early 1900s , the latter line passing through the Whittier Narrows ; this enabled the San Gabriel River region to become a major exporter of agricultural products
Great Flood of 1862
Great Flood of 1862
San Gabriel River (California)