Why aren’t our reservoirs full?

There’s no doubt about it, the winter of 2018-2019 has been a wet one for Santa Clara County. A handful of atmospheric rivers have delivered a healthy amount of rain to our region. So much so, that this season six out of our 10 reservoirs have spilled at one point during January and February.

As of mid-March, water levels in our reservoirs are at around 101 percent of their historical average, which means normal storage for this time of year. Collectively our 10 reservoirs are at around 63 percent of total capacity. With so much rain, why aren’t our reservoirs all filled?

Valley Water manages 10 water supply reservoirs to provide Silicon Valley with clean, safe and reliable drinking water. Rarely will we ever see all of our reservoirs 100 percent full because the water captured is used to keep our taps running every day. In addition, five of our dams have storage restrictions in place due to seismic concerns, limiting our ability to have those reservoirs full.

Water released from our reservoirs is sent to water treatment plants or used to replenish our groundwater basins where it is later pumped from wells. In addition to playing a critical role in our water supply, reservoir releases also help sustain healthy creek ecosystems and can help reduce flood risks downstream.

Throughout the year, Valley Water works to replenish our groundwater supply. With the water captured in our reservoirs, we are able to replenish our groundwater basins as needed through a system of percolation ponds. Groundwater replenishment also happens naturally when water is released to creeks. In the past year, due to the community’s outstanding conservation efforts and Valley Water’s sustainable groundwater management, our groundwater levels are in good shape.

Reservoir releases also occur year-round to help the environment. These releases help keep our creeks flowing and support habitat for plants and wildlife. Water management requires a careful balance of these critical factors for our county’s prosperity and public safety.

Currently, there are five reservoirs that have restrictions set by the state Division of Safety of Dams. Evaluations by the state agency in the recent past deemed the dams at these reservoirs as seismically unstable in the event of a significantly large earthquake. To protect the public, storage restrictions have been placed on the Almaden, Anderson, Calero, Coyote and Guadalupe reservoirs. To prevent the reservoirs from exceeding these limits during winter, we may make controlled releases before and after storms. This helps return the reservoirs to safe operating levels.

During the winter season, Valley Water operates its reservoirs to capture stormwater for later use throughout the year. The goal is to fill the reservoir by the end of the rainy season to maximize the amount of water available for water supply.  If rain early in the season is going to fill the reservoir, Valley Water will often release some water to make room for a moderate amount of future runoff with the expectation that there is significant rainy season ahead to still fill the reservoir.

This winter we increased reservoir releases from Lexington Reservoir as an interim safety measure for concerns downstream. This was a precaution after recent analyses revealed that a four-mile stretch of the Lower Guadalupe River Flood Protection Project between Tasman Drive and Airport Parkway will no longer provide protection against a flood that has one in 100 chance of occurring in any given year. The Guadalupe River is a large waterway that is fed by several creeks in our county. During heavy storms, higher flows in these contributing streams can increase flood risks in the river. Lowering Lexington Reservoir provides more room to store storm runoff, reducing the amount of flow going downstream. However, this was a special circumstance and not a normal operation.

As of mid-March, we cut back on releases from Lexington and plan to allow storage to increase with the hope that late season rains can continue to fill the reservoir. Fortunately, with our groundwater basins in good condition and the significant rainfall in our region and statewide, we have ample water supply for next year.

Planning for future wet and dry years is no easy task. With no crystal ball to reveal what the rainfall outlook will be each year, we have to be diligent about capturing an ample water supply. We acknowledge and thank Santa Clara County residents’ conservation efforts and encourage everyone to keep it up.

To continue receiving updates on our county’s water supply, environmental stewardship, flood protection efforts and more, sign up for our monthly newsletter. You can also learn more about our reservoirs and current projects at www.valleywater.org.