‘Fast paths’ for the water below

From the air, scientists map underground channels that could help replenish aquifers

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

Kings River Conservation Distric A HELICOPTER tows an electromagnetic system near Pine Flat Dam during a survey of the Kings River alluvial fan in December 2020. Data from the system, which was developed in Denmark, provide a cross-section of the subsurface down to about 1,000 feet.