San Francisco significantly restored its local water supply storage this week with the completion of the Calaveras Dam Replacement Project.

The 31-billion-gallon Calaveras reservoir has been kept well below capacity since 2001, due to concerns of seismic reliability of its original dam, which was built in 1925. The upgrade, which included building a brand new dam at the site, gives the SFPUC the ability to fill the reservoir with water levels not seen at the reservoir in almost 20 years. The new dam is located directly adjacent to the old dam, and has been built to withstand a 7.25 magnitude earthquake on the nearby Calaveras Fault.

Approximately 85 percent of the SFPUC’s drinking water comes from Hetch Hetchy Reservoir in the Sierra Nevada Mountains. The remaining 15 percent originates from five Bay Area reservoirs. The Calaveras Reservoir, when full, is the largest Bay Area Reservoir, accounting for 40 percent of the local supply. Having the Calaveras Reservoir at full capacity is a vital component of maintaining water reliability for the 2.7 million customers who rely on the SFPUC for drinking water.

The Calaveras Dam Replacement Project is the largest project of the $4.8 billion Water System Improvement Program to repair, replace, and seismically upgrade key components of the Hetch Hetchy Regional Water System.