Giant Floods and Raging WatersWhen Ice Age glaciers blocked the Columbia River, the Spokane River and the Clark Fork river in Montana they formed Glacial Lake Columbia, Glacial Lake Spokane and Glacial Lake Missoula. The evidence of these lakes can be seen in the lakebeds visible in various road cuts in eastern Washington and lap marks on hills and gravel deposits in Montana. Glacial Lake Missoula, the largest lake, covered some 3000 square miles and was about 2000 feet deep.
At some time the ice dam blocking the Clark Fork broke, sending nearly 500 cubic miles of water across northern Idaho's Rathdrum Prairie and into eastern Washington. At 45 miles an hour and 10 times the flow of all the rivers in the world today, the raging floodwaters swept into the Spokane Valley and out across the loess-covered basalt plateau. Turbulent, powerful currents eroded the basalt, tearing out whole columns, eroding the loess and scouring the landscape.
Separated into three huge flows up to 600 feet deep the torrents carved the 20 mile-wide Cheney-Palouse Tract, the 14 mile-wide Crab Creek Channel and the 50 mile long Grand Coulee. As you can see on the map at right the waters created numerous cross channels. The flow is from upper right to lower left. The darker blue areas are where the rivers are today.
Blocked by the Horse Heaven Hills on the west and the Blue Mountains to the south the water raced to the Walula Gap where the Columbia River heads west to the Pacific Ocean. Unable to flow out the narrow gap as fast as it came in the flood waters were pushed back and reversed the flow of the Snake River all the way past Lewiston, Idaho. Temporary lakes formed in the scablands and silt, sand, and gravel settled out of the water. Eventually the water drained down to the Pacific and along the way flooded Oregon's Willamette Valley. Portland, Oregon, would have been under 400 feet of water at this time.
Just outside of Spokane there are giant ripple marks in the fields to the north near Airway Heights. In the photo of the ocean beach at right, the sand ripples are smaller versions of these giant ripples. Most ripples are usually less than an inch high and a few inches between the tops (crests) of each ripple. Large ripple marks like these are found elsewhere in the Channeled Scablands and were the most convincing evidence for the Glacial Lake Missoula floods.
In the photos below you can see fields and these giant ripples from the air. Note that the image is made from several photos and that the dark gray circular areas are irrigated crops.
Geological evidence indicates that 89 or more such floods happened over the years as the ice dam blocked the Clark Fork, broke and re-blocked the river's flow. There is some evidence that Moses Coulee to the west of Grand Coulee was formed before Grand Coulee as an earlier ice dam blocked the Columbia River and a flood channel was carved just east of Wenatchee, Washington.
In this LANDSAT satellite view of the area you can plainly see the darker channels and lighter islands of the channeled scablands. The very dark areas are various lakes and rivers.
Here are some basic terms used in the tour. Find more geology terms in the Glossary.
- A mound of gravel and sand deposited by flowing water. Bretz and other geologists identified many large bars in the Channeled Scablands.
- Volcanic rock caused by partial melting of the Earth's crust.
- The deepest part of a river or bay.
- Channeled Scabland
- Area in Washington state where huge floods made channels in a large, deep basalt flow. Named by J Harlan Bretz during the 1920's in various publications.
- Current Ripple
- Mark left on streambed from water current usually less than an inch high and a few inches between the tops (crests) of each ripple. The giant ripples from Lake Missoula floods are as much as 35 feet high and several hundred feet between. See also Ripple Mark.
- Lifting and removal of rock, dirt, sand and the like caused by wind, water, or glacial ice.
- Large rock or boulder carried by water or glaciers and left behind.
- A narrow, winding ridge made of gravel usually formed by streams flowing on a glacier or in a tunnel below the glacier or ice sheet.
Kids' Cosmos… Expanding Minds Beyond the Limits of the Universe
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