Using Paleo Valleys to Recharge Aquifers, with Dr. Rosemary Knight

Rows of almond trees line each side of the photograph. Every few feet, a little black stick pokes out of the ground, each one connected by a cord.

While irrigation techniques have become more efficient, like this micro irrigation system being used to water an almond orchard in California, the state has used up too much of its groundwater. Directing rainwater to paleo valleys can help recharge groundwater sources. Image courtesy of USDA Natural Resources Conservation Service.

Script by: Jericho Rajninger   Audio by: Jericho Rajninger  Blurb by: Keya Pardasani

What are Paleo Valleys?

Paleo Valleys were created at the end of the last ice age by melting glaciers that carved into the sediments deposited in the Central Valley. When the glaciers receded, high-velocity meltwater carried sediments and grain material into the valley. These sediments are extremely porous, and the permeability means they can absorb 60 times more water than surrounding clay. Because of this, they provide a pathway that can draw surface water hundreds of meters down to aquifers, which also hold water in soil. 

Using Paleo Valleys

Paleo valleys can greatly benefit aquifer recharge. Because of the high permeability, paleo valleys can hold large quantities of water and provide a direct route for water to the aquifers. As the result of drought and pumping from aquifers, California groundwater is significantly depleted. Paleo valleys can help replenish the groundwater. In addition, paleo valleys can store excess water during storms, which can then help replenish groundwater. Thus, identifying and enhancing paleo valleys could increase our capacity to hold and store water below ground in California. 

Challenges in Locating Valleys

In the Sierra Nevada Mountains, each of the major rivers draining watersheds are likely to have associated Paleo Valleys. However, locating these paleo valleys has proved challenging for scientists and researchers. Finding the valleys requires substantial soil testing, and it has taken scientists longer than anticipated to uncover just  a few of the paleo valleys suspected to exist. 

That is where airborne electromagnetic imaging (AEM) comes in. This technology uses electromagnetic pulses and responses (usually from airplane overflights) to distinguish the soil types in these regions. This method has been used to visualize and more easily identify paleo valleys in the Sierras and the Central Valley.  Dr. Rosemary Knight and her team have been using this technique to map out optimal locations where the subsurface provides a good storage capacity. These results have helped Knight create a three-dimensional map showing the makeup of the land, essential to identifying paleo valleys. 

About Our Guest

Dr. Rosemary Knight is a geophysics professor at the Stanford Doerr School of Sustainability. Her research focuses on developing geophysical methods for imaging. She specifically uses electromagnetic methods to explore how the movement of water in the subsurface is distributed, and what processes cause this. She has conducted many studies and published multiple papers on modeling these land characteristics, and her developments have revolutionized the ability to identify subsurfaces. 

Further Reading


Ethan:  I’m Ethan Elkind and you’re listening to Climate Break: climate solutions in a hurry. Today’s proposal? Recharging aquifers through ancient, buried riverbeds. Rosemary Knight, a geophysics professor at the Stanford Doerr School of Sustainability, explains.

Dr. Knight: We can describe these as “paleo valleys,” valleys from the past, that were created at the end of the last ice age when the melting of glaciers carved valleys out into the sediments that had deposited in the Central Valley and then, because of the velocity of the melt water, brought down very coarse grain materials: sands, gravels, cobbles.

Ethan: This sediment is extremely porous, able to draw surface water in parts of California’s Central Valley down to aquifers hundreds of meters underground.

Dr. Knight: We have all this above-the-ground, engineered infrastructure in California that moves water around. What we need to figure out is where the natural infrastructure is, below the ground surface that can take the water from above the surface deep within our groundwater system.

Ethan: To locate paleo valleys, Knight’s team gathers electromagnetic data, which functions like X-ray goggles.

Dr. Knight: It’s very hard to understand a system that you can’t see or sufficiently sample. What we’re trying to do right now is map out optimal locations where the subsurface provides the kind of storage capacity that we need. 

Ethan:  As climate change makes floods and droughts more severe, paleo valleys, which likely exist across the world, could help us recharge our groundwater systems, making them more reliable and sustainable. To learn more, visit

Using Paleo Valleys to Recharge Aquifers, with Dr. Rosemary Knight