The origin of the Spokane aquifer system is one of the most interesting geologic stories worldwide. The Spokane area was repeatedly scoured (as many as 40 times) by large outbursts of water from glacial lakes in Montana during the Pleistocene (Ice Age). With each passing flood wave, sand and gravel was deposited in a large bedrock trough - the valley - that is filled today with more than 600 feet of these materials in places.
The aquifer, then, is contained within these coarse sediments. It is not an underground river at all, but rather
Here's the story:
First, during the Pleistocene (Ice Age) glaciers advanced to the south out of Canada in a configuration envisioned above. One of these advancing lobes in the Sandpoint, Idaho area blocked the Clark Fork River with an enormous dam of ice, thereby creating Glacial Lake Missoula that extended well into western Montana.
Oblique aerial view of the Spokane area about 13,000 years ago showing the ice dam in the Sandpoint area. Note that the Spokane and Hangman valleys were inundated at that time by another small glacial lake, due to another similar ice dam on the Columbia River near Grand Coulee.
Funny thing about ice - it floats! Once the depth of Glacial Lake Missoula rose to a critical point behind the ice dam near the Sandpoint area, the dam lifted and collapsed, releasing about 500 cubic miles of water catastrophically across eastern Washington. The cartoon shown above depicts this incredible event.
Floodwaters from the big dump inundated most of eastern Washington creating what is known as the "channeled scablands." Eventually these floodwaters discharged to the Pacific Ocean through the Columbia River gorge.
Note: Diagrams shown above are from USGS Water Supply Paper 2265
(Molenaar, 1988).
