The unreliability of the San Diego River compelled the Franciscan fathers to construct the area�s first dam in 1813 to conserve drinking and irrigation water for the Mission San Diego de Alcal�. This water-driven circumstance continued and expanded in the ensuing American era. Lacking a reliable water source at the turn of the 20th century, San Diego County was destined to experience modest growth. The region�s semiarid conditions, cyclical droughts, and existing river systems determined that the only effective way to maintain a ready water supply was to conserve runoff and river floodwaters behind dam-created reservoirs. Between 1888 and 1934, private and municipal interests constructed a series of massive structures that made San Diego County the dam-building center of the world. The county featured some of America�s first multiple arch dams and earliest hydraulic fill dams. Into the mid-1940s, dammed reservoirs remained the principle water source for county consumers and made the municipal expansion of the city of San Diego possible.