San Andrés Creek, now called San Andreas Creek, is a perennial stream that flows 5.9 miles (9.5 km) southeasterly along the San Andreas Fault from Sweeney Ridge in San Mateo County, California, providing the inflow to and outflow from San Andreas Reservoir, and then entering Lower Crystal Springs Reservoir, where it was a historic tributary to San Mateo Creek. San Mateo Creek then carries its waters over Crystal Springs Dam northeast to San Francisco Bay.