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.
