Thousands of years ago during the last Ice Age, rivers flowed from giant glaciers in the Sierra Nevada down to the Central Valley, carving into rock and gouging channels at a time when the sea level was about 400 feet lower. When the glaciers retreated, meltwater coursed down and buried the river channels in sediment.
These channels left by ancient rivers lie hidden beneath California's Central Valley. Scientists call them paleovalleys, or incised valley fill deposits. As much as 100 feet deep and more than a mile wide in places, they are filled with coarse-grained sand, gravel and cobbles.
Because these paleovalleys are highly permeable, scientists have pointed to them as ideal pathways for water to quickly percolate down and recharge groundwater. That makes the underground channels especially valuable now in the Central Valley, where agricultural wells and pumps continue to draw down groundwater levels, and where managers of water agencies are looking for areas to capture floodwater to replenish depleted aquifers when California's next big floods come.
The paleovalleys are rare geologic features in the Central Valley, where much of the ground has layers of impermeable clay and silt that impede the downward movement of water. Through years of painstaking research, scientists have pieced together data to identify a few of these areas, but they believe there may be a dozen or more of them along the eastern edge of the Central Valley.
In a new study, scientists have for the first time produced a detailed map of one of these paleovalleys, located on the Kings River alluvial fan between Fresno and Selma. By flying a helicopter equipped with an electromagnetic imaging system, they collected data to effectively peer beneath the ground and map what they say is one of the "fast paths" for recharging the valley's aquifers.
"The central question is, is there an efficient way to find these massive features that could provide the natural infrastructure we need to recharge California's groundwater?" said Rosemary Knight, a geophysics professor at Stanford University and the study's lead author. "We need a fast, reliable, cost-effective way to find these pathways."
Knight and other Stanford scientists focused on a paleovalley that starts at the Kings River, upstream of Centerville, and extends at least 29 miles toward the town of Easton, stretching more than a mile across. Working with study coauthor Graham Fogg, a hydrogeology emeritus professor at UC Davis, the scientists produced a three-dimensional map of the paleovalley and confirmed its location exactly where Fogg and his colleagues had discovered it in previous research.
Knight said the study, which was published Thursday in the journal Environmental Research Letters, showed that an airborne electromagnetic system can effectively map other large-scale channels that could serve as water pathways for managed aquifer recharge.
When the helicopter flew over the area in 2020, it dangled a large, hexagonagonal transmitter loop beneath it. Developed in Denmark, the system uses electricity to create a magnetic field. As it hovers over the earth, the system measures the degree to which different underground materials resist the flow of electrical current. This data provides a cross-section of the subsurface down to about 1,000 feet.
Knight likened it to doing an MRI of the earth, or using a metal detector to find buried metal based on its electrical properties.