Hidden ‘Paleo Valleys’ Could Help California Survive Droughts

Hidden ‘Paleo Valleys’ Could Help California Survive Droughts

“Paleo valleys,” carved by ice age rivers and now underground, could provide spaces to recharge California’s depleted groundwater

An irrigation canal separates an almond orchard and a field that lies fallow in May 2021 in the Central Valley region of California. California’s Central Valley region has seen the worst drought in a millennium. This has led to water shortages from rivers and reservoirs, which has forced farmers and others into using groundwater. Credit: Justin Sullivan/Getty Images

When glaciers covered the Sierra Nevada mountains several times over the last million years, rivers flowing down the mountains grew more powerful, cutting channels a mile wide and 100 feet deep into what is now California’s Central Valley. High-velocity meltwater rushed down the slopes carrying scoured gravels and rocks. The cobble fell out of the water column as the flow slowed down, filling the canyons. These “paleovalles” were long unknown below the ground. But, they are now being mapped with airborne imaging systems. Their detection could help with a modern problem: California needs to balance flood and drought extremes worsened by climate changeby storing water underground.

Californians have long used groundwater as a backup supply when rivers run low. Groundwater and surface water are linked, so pumping groundwater from wells can lower streams and rivers even more. Groundwater levels have been declining for decades because of overpumping and engineered infrastructure that prevents natural refilling, such as levees that block water from seeping down through floodplain soil. Depleted aquifers in the Central Valley now have three times the capacity of California’s approximately 1,400 reservoirs.

People can help restore hydrologic health and percolate underground water through managed aquifer replenishment. But clay soils make up 65 to 80 percent of the Central Valley, and water moves through them slowly. Paleo valleys, also known as incised-valleyfills by scientists, could be a useful tool. Because they are filled with gravel, they are more porous than clay soils and can absorb about 60 times more water.

When large storms hit, Californians could capture floodwaters and spread them atop the youngest paleo valleys, which lie just below the surface. The water would be quickly absorbed and stored for long periods of drought. This would increase water tables over a large area, making surface flows stronger, healthier, and protecting fish in streams. It also stops subsidence, which is a phenomenon where land crumbles due to the loss of water below it. “These geologic ‘fast paths’ are the natural infrastructure that we need for managing our groundwater resources,” says Stanford University geophysicist Rosemary Knight, lead author of a new study published on Tuesday in Environmental Research Letters.

Graham Fogg, a professor emeritus of hydrogeology at the University of California, Davis, first suggested using paleo valleys for managed aquifer recharge nearly 40 years ago. He believes that each of the approximately a dozen rivers flowing off the Sierra Nevada contains corresponding paleovalles. After a quarter century of searching, he and his graduate student have only managed to find three. They used a tedious method that analyzed tens of thousands soil samples taken from wells. Fogg, Knight, and their co-authors found a quicker way to find these valleys. They flew a helicopter equipped with an AEM instrument to “see” underground. This technology, similar to magnetic resonance imaging, distinguishes soil types by their electromagnetic responses. Fogg and one student had previously identified a paleo valley using soil data. The researchers used AEM for AEM mapping. Fogg states, “I thought it important to figure out if AEM would detect these things.” “These results indicate that it will .”

Knight used AEM data for a three-dimensional map that showed clays in blue and sands and gravels in red. She says these maps will be useful for researchers to identify paleo-valleys elsewhere. Burke Minsley, a U.S. Geological Survey geophysicist, said that Knight’s data provides important details about the valley’s geometry and depth, as well as connectivity to underground flow paths. This information is far beyond what log data can provide.

Since Knight and Fogg began this research to look for a paleo valley using AEM a few years ago, California’s Department of Water Resources (DWR) has followed the scientists’ recommendation to conduct a reconnaissance project using AEM to map 100 depleted groundwater basins. The data from the DWR project, which is more detailed but still has some data, are still being processed. Katherine Dlubac is the project manager for AEM surveys in the department. She says it “looks promising so far” for paleovalles. She says that recharge is a very important issue in California. “It’s an important priority for us at DWR… to help locals obtain more information about their water resources.” Dlubac said that the state will go back to collect more detailed data. This could begin in the next six-months.

Because of the unique size and accessibility of paleo valleys, Fogg and Knight believe that areas of land higher than them can be used as recharge zones. The subsurface has been terra invisible, which is one reason groundwater has become scarce. Fogg’s decades-long vision is finally coming to fruition with these 3-D maps, which help people see what’s below the ground. It’s “gratifying,” he says, “for people to take notice and make use of the information.”



    Erica Gies is author of Water Always Wins: Thriving in an Age of Drought and Deluge (University of Chicago Press, 2022). She wrote our December 2018 article “Sponge Cities” about restoring natural water resources in urban areas. Credit: Nick Higgins

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