Mr Lin asked a very interesting question: "Where does our water come from?"
Two sources: If you live south of Calaveras Blvd, you get your water from the San Luis Reservoir via the California and South Bay Aqueduct. See Map of CA State Water Project.
If you live north of Calaveras Blvd, you get your water from Hetch Hetchy.
During the 1906 San Francisco earthquake, the water system failed, and fire destroyed half of the city. The City and County of San Francisco decided that it must have it's own fresh water supply. SF asked the US government to dam Hetch Hetchy Valley (near Yosemite) to collect the water from the melting snow in the Sierra Nevada.
The water comes down the Hetch Hetchy Aqueduct to the reservoirs in the mountains around the SF Bay Area. The water for Milpitas and Fremont is stored in the Calaveras Reservoir, which used to be the largest earth fill dam in the world. It is 210 feet (64 m) tall and was completed in 1925. The reservoir has a capacity of 100,000 acre-feet (123,000,000 m³). The reservoir is also fed by two creeks, Arroyo Hondo and Calaveras Creek, and is next to the Calaveras Fault.
The drinking water for Milpitas and East San Jose is processed at the Penitencia Water Treatment Plant.
Early California History pg 6
Map of Santa Clara County Flood Zones pg 59
San Jose Drinking Water
Milpitas Drinking Water
Map of Milpitas Water Service Area (pg 21)