Evidence of these slides appear on the west sides of Mts. Jumbo and Sentinel. They look like stream channels but no water flows here now, and the absence of rounded rocks suggests that no stream ever did. The Missoula Aquifer is a stratum of porous sand, gravel, and cobbles 10-70 feet beneath the valley floor that contains extractable groundwater. A layer of clay forms its lower boundary.
Porous soil covers the top.
The floodwater from the rapid discharge of Glacial Lake Missoula 10,000 years ago, deposited the millions of tons of cobbles over the bedrock and clay that form today's aquifer. The aquifer begins at the mouth of Hellgate Canyon and continues southwest beneath the city. It discharges into the Bitterroot River, which joins the Clark Fork again shortly thereafter. The water moves through the aquifer at 8-30 feet per day. Look for signs around town indicating the depth of the aquifer.