We’re not in a drought, so why the dry ponds?

Trail enthusiasts and cyclists have been alarmed in recent months by scenes of bone-dry ponds near Penitencia Creek in northeast San Jose. But they’ll soon be relieved to know the water has come back to some parts. Repairs to a culvert pipe and valve connecting two ponds in the area, known as the Dr. Robert W. Gross Ponds (or Bob Gross Ponds), have allowed water to fill some of the pond system. Earlier in the season the pipe failed and required Valley Water to turn off all water to the surrounding pond system. The work was completed last month, which means residents should be happy to enjoy evening tranquil strolls along the pond trails with water in sight.

However, not all ponds in the area will return to a full condition. Actually, many other ponds in the county are currently dry and could remain that way. And that’s not necessarily a bad thing. The reason why is actually good news for our water supply. Our groundwater aquifers, which provide nearly half the water used in our county, are actually full. Santa Clara County residents’ diligent conservation efforts, as well as two robust winters since 2016, have allowed our groundwater levels to recover to pre-drought conditions. In fact, groundwater levels in many wells are at or near historic highs. But if there’s so much water, why are the ponds dry?

Percolation ponds are water supply facilities, built strategically in areas where gravel and permeable material allow water to seep into our aquifers. Their primary and most important purpose is to allow water to filter underground. This replenishes water pumped out of the ground from deep drinking water aquifers that is used in homes, businesses and agriculture. The current and fortunate situation is that we don’t need to fill these ponds because our groundwater supply is in great shape.

During 2014 and 2015 many percolation ponds went dry due to little water available for groundwater replenishment. Valley Water took the opportunity to clean and maintain many ponds. By removing sediment that had accumulated over the years, the rate at which water seeped into the ground increased greatly. Plenty of precipitation from the winter of 2016-2017 filled both local and state reservoirs and restored a healthy Sierra snowpack, bringing an end to the recent historic drought. With all of that rain, for the first time since 2013, Valley Water was able to resume normal replenishment efforts in our region’s groundwater aquifers, compared to a scaled-back approach in drought years. The recovery of local groundwater basins to pre-drought conditions was swift and successful.

Groundwater pumping continues to be relatively low due to excellent community conservation efforts. In 2018 we scaled back groundwater replenishment based on full groundwater conditions. This year we had another wet winter resulting in extraordinary groundwater conditions with new historic highs in many deep aquifer wells. When groundwater basins are full, artesian pressures in deep aquifers can be very high. If wells drilled into these aquifers are not properly cared for or capped, groundwater can flow at the ground surface causing problems. Due to the healthy condition of deep aquifers in northern Santa Clara County, Valley Water is scaling back replenishment operations. Many percolation ponds may go dry longer than normal. Rest assured that your water supplies are in great shape and that Valley Water continues to closely monitor groundwater conditions.

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In the Penitencia Creek area, the various pond systems are connected in a series so that there is a “cascading” effect (there’s a total of 18 ponds connected in the series). Once one of the ponds that make up the Bob Gross Ponds system is full, water flows to the next pond, and then the next, and so on downstream throughout the various systems ending in the Lower Capitol Ponds. Now that the pipe and valve have been repaired, water releases to the Bob Gross Ponds will be occasionally increased to keep the first three of the Bob Gross Ponds full and the City Park Pond partly filled. The remaining ponds downstream will remain empty.

Valley Water is committed to ensuring a reliable water supply for the residents of Santa Clara County. Healthy groundwater aquifers are a key part of that mission. While we continue monitoring and adjusting operations to keep a healthy water supply, we encourage the community to keep up excellent water-saving habits. Residents can make changes for the long-term with the help of our rebates and programs, available at www.watersavings.org.

To learn more about the role percolation ponds play in our water picture, check out the following fact sheet.