top of page

Inland Northwest Geology

Geologists tell us that millions of years ago our area underwent a scourging by fire. All along the border of North Idaho and Eastern Washington time after time the earth’s surface opened in giant crevices and out of its core flowed molten lava that traveled in a southwesterly pattern across the state of Washington. 

Evidence of this lava flow exists all around us with huge outcroppings of basalt, the channeled scab land to the west of us, distinct layers along the Spokane River, columnar basalt and uses of basalt in landscaping and building foundations.

The next big geologic event was the Cataclysmic Ice Age Floods some 12,000 to 17,000 years ago. J. Harlen Bretz was the first to put forth the theory of this flood; for more than four decades he defended his theories on the flood before they were generally accepted by the scientific community. In 1922 he began his field research in the Channeled Scablands of central Washington and through his research Bretz was confident that a flood had occurred. In 1930 Joseph Pardee viewed giant ripple marks left on the lake bottom sediments of Glacial Lake Missoula and deduced that they could only have formed by powerful currents that flowed over the bottom of the lake.

Looking back over the decades following the Bretz and Pardee theories scientists have put together the events of this monster flooding. Glacial Lake Missoula lay in a bowl where Missoula, Montana sits today. The ice was over 2000 feet thick and extended west and north along what we know as the Clark Fork River to the spot where that river empties into Lake Pend Oreille.

As the earth warmed the ice plug burst open and massive amounts of water came shooting out at the rate of 10 times the combined flow of all the rivers in the world. It traveled south and turned west at Coeur d’Alene and came raging down the Spokane River Valley, then spread over the land to the south and west of Spokane repeating this pattern over and over again. 

Giant icebergs traveling on the flood waters carried rocks from western Montana to central Washington and even as far as the Willamette Valley in Oregon. Along this course glacial moraine formed deposits of sand, gravel, river rock and giant boulders. Twenty-five lakes in Spokane County were gouged out.

Two important results of this flood impact our area every day. One is the Spokane Valley-Rathrdrum Prairie Aquifer traveling though sand and gravel laid down during the flood. The aquifer lies 50-300 feet beneath the soil, covering an area of more than 325 square miles, extending from the southern part of Lake Pend Oreille to the Little Spokane River. It is the designated a sole source aquifer, with a never ending supply of clean water to Spokane, Post Falls and Coeur d’Alene. 

The second benefit is the rich volcanic-laced soil that was carried along with the flood waters, deposited south and west of Spokane and blown about by the winds into gentle rolling hills. This soil called loess provides the base for one of the richest soft white wheat producing areas in the world..our Palouse Country. This is additional information provided by: Reanette Boese Spokane County Utilities Water Resources.

Most of the flood water from these glacier generated floods, or j’kulhlaups, flowed down the Rathdrum Prairie, through the Spokane Valley east of Spokane, and north through the Hillyard trough to the southern portion of the Little Spokane River valley. Some of the water from the catastrophic floods was deflected through the Blanchard channel to the north part of the Deer Park basin. Terraces near the Spokane-Little Spokane River confluence suggest that the last major late Wisconsin floods may have used the Little Spokane as their main course (Kiver and Stradling, 1985). 

Sedimentation in the Little Spokane River basin was also affected by the floods, but it further depended on whether the valley was covered by a glacial lake or not and by the amount of water in the flood. If no lake occupied the valley, the great velocity of the water picked up and carried large amounts of sediment along with it, depositing poorly sorted gravels and boulders as the flood waned.

If the catastrophic flood waters encountered a deep glacial lake, boulders and other coarse debris would immediately be deposited as the flood wave encountered the low energy environment. A large flood wave passing through the glacial lake would rework coarse sediment on the bottom and form rip-up clasts. The sediments left behind by the flood would be mostly sand sized (Kiver and others, 1991).

This little peek at our amazing geologic history is intended to spark your interest in learning more about an aspect of our Inland Northwest that you possibly haven’t thought about. There are publications at local book stores and the libraries. 

Two DVDs give excellent accounts:

NOVA, call 800-949-8670 and ask for the Mystery of the Megaflood.

Another: The Great Ice Age Floods.. available from MWIA c/o National Parks Section, 1008 Crest Dr., Coulee Dam WA 99116, phone: 509- 633-9441 ext. 110.

A group, The Ice Age Floods Institute, is a 501(c) (3) tax-exempt, non-profit organization and is centered around EWU and its geology department. They offer tours to various areas affected by the flood. They are diligently working on a trail system that features historic markers where folks can learn more about this dramatic event. Check out their web site –

10 views0 comments

Recent Posts

See All


bottom of page