Missoula Floods

For a Google Earth .kmz file to explore the Missoula Floods of Oregon, scroll to the very bottom of the page.

One of the coolest geologic happenings in the that I know of in the United States happened right here in the Pacific Northwest. It all happened during the Pleistocene, starting about 2 million years, when last ice age was taking place. During this time there was an ice sheet called the Cordilleran Ice Sheet that stretched from the peninsula of Washington, northward to southern Alaska, east to the western boundary of the Northwest Territories of Canada, and stretching south to northern Montana. The southeastern portion of the ice sheet moved south and blocked a portion of the Clark Fork River on the Montana/Idaho border (Figure 1). This ice dam caused the river to back up creating a huge lake that contained over 500 cubic miles of water and was 2000 ft deep at the ice dam (http://www.iafi.org/floods.html). Eventually the lake became large enough to break through the ice dam and this could have happened in several ways or a combination of the following: 1) By the water in the lake floating the ice dam and releasing the water, 2) pressure at the base of the dam being so great that water seeped into cracks widening it till the dam burst and 3) lastly there could have been so much hydrostatic pressure from the lake water pushing on the ice dam that the pressure became too great and the dam burst.

The bursting dam created a rush of water that was referred to as the Missoula Floods, and these floods created some of the coolest landforms and deposits across the Pacific Northwest. The water spilled out across the northern panhandle of Idaho and across eastern Washington carving out what is now referred to as the Channeled Scablands, and then turned south and flowed along the path of the Columbia River, backing up several major river valleys, and finally washed out into the Pacific Ocean (Figure 1). Missoula Lake has been estimated to have drained over a period of 40 hours and the speed of the flood waters has been calculated 50 million cfs (cubic feet per second) (http://www.wou.edu/las/physci/taylor/g473/haggerty.pdf). These floods occurred many times, in fact estimates set the number of floods at more than 80 (Reichert, 2012). But since this website focuses on Oregon geology, let’s talk about the various effects that the floods had in our neck of the woods.

The first portion of the flood waters that interacted with Oregon was when the floodwaters reached Wallula Gap, which is a narrow area among the Columbia River Basalts on the Oregon/Washington border through which the Columbia River flows. This blockage caused water to back up creating Lake Lewis. The water flowed through Wallula Gap at high pressures and later ponded again when the gorge narrows at the Dalles, Oregon creating Lake Condon, and lastly another narrowing at Rainier, Oregon at an area called the Kalama Narrows caused the water to back up the Willamette River creating Lake Allison (Orr and Orr, 1999; Figure 2). The flood waters ripping through the Columbia Gorge were moving so fast that the water behaved very turbulently with extreme eddying called kolks that behaved like underwater tornadoes. These kolks would act as drills into the surrounding basalts

and leave behind kolk lakes and some of the most famous can be found in the Tom McCall Nature Preserve just west of the Dalles, Oregon. These floods stripped sediments and gravels from the areas the water rushed over and steepened the Columbia Gorge walls in the process.

One thing to keep in mind as the flood waters raced over the Pacific Northwest they stripped various areas of sediments, as I stated above, but as the water slowed when the Columbia Gorge narrowed the waters would flow backwards up other river systems. This can be seen in several river valleys, but the one we will focus on is the Willamette Valley. As the flood waters slowed at the Kalama Narrows water backed up the Willamette Valley and going as far south as modern day Eugene. The amount of water was tremendous, in Portland the flood waters reached depths of up to 400 ft, in the town of Lake Oswego the flood water depths were 200 ft, and in Forest Grove and Hillsboro the flood water depth was 100 ft (Orr and Orr, 1999). These depths are based on the height of silt deposits found around the Willamette Valley. The flow of the Missoula Flood waters were contrary to the normal flow of the Willamette River and as a result of slowing waters much of the coarser sands and gravels in the Missoula Flood waters were dropped near Portland. However, silts and clays are much finer and were carried southward and deposited over much of the Willamette Valley. Silt deposits in the valley are up to 100 ft deep and account for much of the great farming soil and is the main reason grapes in the valley make tremendous wines (Orr and Orr, 1999).

One of the the most interesting deposits that can be found all along the Missoula Flood path, but particularly in the Willamette Valley, are deposits of glacial erratics. A glacial erratic is a rock that is different in both size and composition than the rocks or sediment that surrounds it. Basically an erratic is a very strange rock found in a location is doesn’t belong. The way that it is presumed erratics get to these other locations is by using calved glaciers, or icebergs, to float to their new location. Essentially, as the Missoula Floods broke through their ice dam, pieces of the dam and any other glacial ice that was floating in the lake could be swept up by the flood waters and washed downstream. Once the flood water receded these icebergs would come to rest and melt, depositing the glacial erratic. The largest example of an erratic in the Willamette Valley would be the one found at Erratic State Park that lies between Sheridan and McMinnville, Oregon (Figure 3). Here there is a glacial erratic deposited by the Missoula Floods that originally weighed 160 tons, but over the years 70 tons have been removed by visitors leaving a still very large erratic at 90 tons (Orr and Orr, 1999). The composition of the rock is similar to the composition of the Northern Rocky Mountains, over 500 miles away (http://www.oregonstateparks.org/index.cfm?do=parkPage.dsp_parkPage&parkId=96). That's pretty amazing to me. Very few places enjoy the results of massive floods as much as we do in the Willamette Valley.