 The northeast arm of Lower Crystal Springs Reservoir is formed by San Mateo Creek's tributary, San Andreas Creek which descends to the Reservoir southeast along the San Andreas Rift.Another tributary, Laguna Creek, flows northwards from Woodside with its source on Edgewood County Park and Natural Preserve, and historically fed Laguna Grande and then joined San Mateo Creek just upstream from Crystal Springs Canyon, where San Mateo Creek turned east to flow through the canyon.Laguna Grande was submerged when an earthen dam (this was the first Crystal Springs Dam) was constructed in 1877, forming Upper Crystal Springs Reservoir.