Credit: © ISTOCKPHOTO/ CATSCANDOTCOM

The Channeled Scablands of the northwestern United States stand in sharp contrast to the surrounding wheat fields. Here, tortuous channels cut deep into the dark volcanic bedrock, giant gravel ripples reach nine metres in height, and piles of rock debris dot the landscape. The hunt for the origins of the alien landscape began nearly 90 years ago and finally led to the discovery of the former glacial Lake Missoula. From at least 19,000 to 13,000 years ago, the lake sat along a finger of the Cordilleran ice sheet. But the lake was far from stable: at least 25 times during that period, the ice that dammed the lake disintegrated, and water gushed towards the Pacific Ocean.

The floods may have left behind more than just a scoured surface. Jennifer McIntosh and colleagues at the University of Arizona suggest that flood waters also filled the region's aquifers on their way to the ocean (Geophys. Res. Lett. doi:10.1029/2010GL044992; in the press ). The aquifer system, known collectively as the Columbia River Basalt Aquifers, stretches across the eastern half of Washington state and northeast Oregon and is an important source of both domestic and agricultural water. Very little water has been added to the system recently, and the bulk of the water is thought to be a remnant from the last glacial period.

Yet, an ice-age source of groundwater is hard to reconcile with data that suggest the climate then was even more arid than today. Instead, based on radiocarbon- and oxygen-isotope measurements of the groundwater, McIntosh and her colleagues suggest that it came from Lake Missoula. The radiocarbon dates from the groundwater match the dates of the floods and coincide with periods when vast amounts of fresh water poured into the Pacific Ocean. Furthermore, the oxygen-isotope composition of the groundwater more closely resembles the proposed composition of the Cordilleran ice sheet than that of rain or snow during the glacial period.

The rushing flood waters must have been able to infiltrate the aquifers rapidly, draining through fractures and faults, and perhaps eroding inlets into the aquifers.