Hetch Hetchy Reservoir, located in California's Yosemite National Park, is a crucial source of water for the San Francisco Bay Area. The reservoir was created in 1923 when the O'Shaughnessy Dam was built across the Tuolumne River. The hydrology of the area is primarily influenced by the Sierra Nevada Mountains and the Pacific Ocean, which provide a reliable and consistent snowpack that feeds the water supply. The Hetch Hetchy Reservoir is fed by the Tuolumne River and also receives water from the Cherry Creek and Falls Creek watersheds. The water from the reservoir provides drinking water to over 2.6 million people and supports agricultural and recreational activities. Despite controversy over the dam's construction, the Hetch Hetchy Reservoir has become an essential resource for California's Bay Area.