San Francisco Peninsula Reservoir Wet Weather Operations

How we operate the system. 
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. 

When the water reaches the spillway:
In such cases of extreme weather and 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 we use forecasts to inform our operations.
Rainfall forecasts are not perfect. Some rain events – such as New Year’s Eve – far exceeded forecasts, whereas others, such as this week – were weaker than forecasted. The intensity of the rain also greatly affects how quickly the reservoir and creek flows will rise. 

Therefore, 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 we can do
The SFPUC continues to monitor reservoir levels and current forecasts. We will 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. 

The 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.