San Francisco Peninsula Reservoir Wet Weather Operations
Winter 2024-2025
Update November 19, 2024
Rain is expected in the forecast for this week. SFPUC staff do not expect water levels in Crystal Springs, San Andreas, or Pilarcitos reservoirs to approach spillway height. Staff will continue to monitor the dams and reservoir levels per normal wet weather operations.
Please read below how you can monitor reservoir levels and stream flows, as well as how to sign up for alerts from your local jurisdiction.
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.
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FAQ: Crystal Springs Reservoir During Winter/Wet Weather
How do we manage the reservoir level?
Crystal Springs Reservoir is a water supply reservoir. It is not a flood control reservoir and the SFPUC is not a flood control Agency. 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 and forecasted rain and high tides to minimize flows to San Mateo Creek. During heavy rain, we do not release more than the small amount required for environmental flows. We do not model San Mateo Creek flows, however we do visually inspect the creek as we release water.
We have been releasing as much water as we believe is feasible, taking in consideration of locations in the creek where the channel is restricted, runoff from rainfall in the urban watershed below the dam, and tides.
What is a spillway?
A spillway is a structure that protects the dam by safely passing water over the dam and downstream once the reservoir has reached capacity. It is designed to safely pass an extreme event called the probable maximum flood. However, how much water it would pass in a specific event depends upon the amount of runoff resulting from rainfall coming into the reservoir when it is full.What happens when the reservoir level reaches the spillway?
In cases of extreme weather when the reservoir level reaches 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 amount of that flow over the spillway is in direct relation to the amount of runoff from rainfall coming in.
How do we use forecasts to inform our operations?
Rainfall forecasts are not perfect. Some rain events – such as New Year’s Eve 2022/2023 – far exceeded forecasts, whereas others can turn out to be weaker than forecasted. The intensity of the rain also greatly affects how quickly the reservoir and creek flows will rise.As a result, we cannot predict with certainty the timing and volume of potential flow over the spillway. Likewise, we cannot precisely predict flows to San Mateo Creek. Development downstream, which is managed by San Mateo County and other cities, and rainfall coming in from other sources all have an impact on the creek.
What can we do to help?
The SFPUC continuously monitors reservoir levels and current forecasts. We strive to provide notice to downstream agencies and municipalities 24 hours prior to a likely spill. The spill may take place a few hours earlier or a day or two later because of the inability to precisely predict the weather.In the case of Crystal Springs, advanced notice will allow San Mateo County, Town of Hillsborough and City of San Mateo to determine what communication to provide to their communities.
We have also provided links to the USGS gauges on our website below so those who are interested can track this on their own. Lower Crystal Springs Reservoir will pass all inflowing rain over the spillway once the gauge elevation reaches 294.6 feet.
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:
11162753 - SAN MATEO C BL LO CRYSTAL SP RES NR SAN MATEO CA
Data on San Andreas Reservoir:
San Andreas Lake a Dam NR Millbrae CA - USGS Water Data for the Nation
IF YOU LIVE ALONG THE SAN MATEO CREEK
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 https://www.smcalert.info/ and be sure to register with your address
LOCAL JURISDICTION PREPARATIONS
San Mateo Residents: visit www.cityofsanmateo.org/floodprep for information on sandbags and storm safety.
Hillsborough Residents: visit https://www.hillsborough.net/ for up to date information.
Mid Coastal Areas of San Mateo County
Stay up to date at the City of Half Moon Bay Website.
Pilarcitos
The gauge for Pilarcitos Reservoir is found here:
The rate of discharge from the reservoir, in cubic feet per second, is found here:
Half Moon Bay
The flow of Pilarcitos Creek near Half Moon Bay can be found here (Please note that this gauge is measuring all the runoff collected in the watershed downstream of our reservoir and may vary significantly from the flow at the dam)
