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San Francisco Peninsula Reservoir Wet Weather Operations

Wet Weather Operations

The San Francisco Public Utilities Commission owns and operates the Crystal Springs Reservoir system on the Peninsula, which includes Lower Crystal Springs Dam on San Mateo Creek. This reservoir is a key source of drinking water for the San Francisco Peninsula and not a flood control facility. 

Location of water flow gauge at Lower Crystal Springs

January 27, 2023 Update

Lower Crystal Springs Dam

Dam Integrity:

Lower Crystal Springs Dam is a substantial structure which survived both the 1906 and 1989 earthquakes with little to no damage. The dam’s integrity remains solid and is performing as designed. 

Dam Releases:

As part of our prudent reservoir operations, before upcoming large rain events, operators will release water out of Lower Crystal Springs Dam into San Mateo Creek to create space for expected rain inflow. These releases are planned with consideration of current rain and high tides to minimize impacts to San Mateo Creek. During heavy rain, we do not release more than the small amount required for environmental flows. 

Reservoir Levels and Spillway:

Through February 7, 2023, water levels in Crystal Springs Reservoir are not expected to reach the dam's spillway.  However, there is a small chance that water may reach the dam’s spillway during subsequent storms. In such cases of extreme weather that is outside of our control, and the reservoir levels reach the spillway, any water flowing into the reservoir would flow out of the reservoir via the dam’s spillway, which would increase flows in San Mateo Creek. The chances of this are small, and SFPUC closely monitors reservoir levels.

Creek and Reservoir Level Information:

You may access real-time data on creek and reservoir levels at the USGS Current Conditions for California: Streamflow website

Data on Lower Crystal Springs Dam:

11162750  -  LOWER CRYSTAL SPRINGS RES NR SAN MATEO CA  The reservoir will pass all inflowing rain over the spillway once the gauge elevation reaches 294.6 feet.

Data on flows below Lower Crystal Springs Dam: 



Residents who live along the creek or who have previously experienced flooding from the creek should take additional precautions to prepare for wet weather.

Encourage your neighbors to sign up for SMC Alerts and be sure to register with your address


San Mateo Residents: visit for information on sandbags and storm safety.

Hillsborough Residents: visit for information on sandbags and storm safety.


Mid Coastal Areas of San Mateo County 

January 13, 2023

SFPUC Personnel will reduce releases from Pilarcitos Reservoir the afternoon of January 13th back to the minimum required flow in advance of the heavy rains forecasted for the weekend.

We expect water levels to reach the spillway this weekend. In that case, rainfall inflow into the reservoir will flow downstream into Pilarcitos Creek, as it has in recent rain events.


 For more information and to receive alerts from the city of Half Moon Bay, sign up here.