Lake Missoula Floods

From the moment they dipped their paddles into the Snake River, the Lewis and Clark expedition would not only be following the path of multiple flood basalt flows, but also the route of the colossal Glacial Lake Missoula floods that so markedly scoured the basaltic landscape.^[2]See on this site Columbia River Basalts. Three of the most popular narratives of the Missoula floods, often dubbed the Ice Age Floods, are John Eliot Allen, Marjorie Burns, and Scott Burns, … Continue reading During the last glacial period in the Pleistocene,^[3]During the Pleistocene Epoch, which began about 2.588 million years ago, much of southwestern Canada was repeatedly glaciated by a series of Cordilleran ice sheets that extended into northern … Continue reading a glacial lobe from the massive Cordilleran ice sheet advanced southeastward of present-day Lake Pend Oreille near the Idaho-Montana border. It dammed the natural outlet of Clark Fork River, thus creating a nearly two hundred-mile long impoundment in western Montana now referred to as Glacial Lake Missoula. Following the catastrophic failure of the approximately 2,000-foot high ice dam, along with multiple other large-scale releases of lake waters between about 15,000 and 18,500 calendar years ago,^[4]The cyclic ice damming/filling and subsequent release of Glacial Lake Missoula floodwaters occurred numerous times in the late Wisconsin, most probably between 12,700 and 15,300 radiocarbon years … Continue reading floodwaters repeatedly burst out of Lake Missoula and rushed westward seeking a route to the sea.^[5]It is believed these flooding events were separated by a few years to six decades, per Richard B. Waitt, Jr., “Periodic Jökulhlaups from Pleistocene Glacial Lake Missoula—New Evidence from … Continue reading The largest and earliest floods may have flowed freely through the northern Columbia River Valley,^[6]Gerardo Benito and James (Jim) E. O’Connor, “Number and Size of Last-Glacial Missoula Floods in the Columbia River Valley between the Pasco Basin, Washington, and Portland, Oregon,” … Continue reading but subsequent releases from Lake Missoula encountered Glacial Lake Columbia, itself created by another Cordilleran glacial lobe damming the Columbia River. With no outlet in that direction, Missoula floodwaters spilled over the low drainage divide to the south in a myriad of stupendous cascades and torrents moving upwards of thirty to sixty miles per hour or more.^[7]One can find different Missoula flood speed estimates in virtually every reference work of this phenomenon, depending on whether a researcher is referring to flow in a hydraulically impounded reach … Continue reading The sheer force, volume, and speed of this ice and rock-laded water instantaneously denuded the landscape, stripping away soil and vegetation and carving deep canyons along its flood path throughout an area now known as the Channeled Scablands, while also depositing oversized sand and gravel bars.^[8]Two exceptional geologists, J [no period after the J] Harlen Bretz and Joseph T. Pardee, are credited with deducing the erosional mechanism and water source that created the Channeled Scablands. … Continue reading

The entire expedition route on the Snake and Columbia Rivers is marked by evidence of these gargantuan floods, including the 184-foot high Palouse Falls, the only active waterfall left along glacial flood path in the Channeled Scablands. Located about five miles up what the captains named “Drewyers River,” the expedition members did not see the falls because they did not reconnoiter that far upstream. Had they seen it, certainly the beauty of the falls and its basalt flow architecture^[9]The lip of Palouse Falls is notched into oldest flow of the Frenchman Springs Member of the Wanapum Basalt, and the underlying cliffs are comprised of the Grande Ronde Basalt. would have been worthy of mention, but how curiously undersized the river and narrow falls were compared to its broad, eroded valley and outsized plunge pool might not have been noted by the explorers. Such an insight would require imaging the entire landscape being abraded and torn asunder under hundreds of feet of tumultuous, ice- and rock-laded Missoula floodwaters.

Close examination of the expedition journals reveals that Lewis, Clark, and the other journal keepers took note of key physical evidence of epic flooding events. For example, Clark noted on 11 and 12 October 1805, primarily in his courses and distances, the “stoney” composition of the islands in the Snake River, while Gass remarked on 11 October 1805 that the “bed and shores of the river are very stony” and the stones were of “a round smooth kind.” These were pre-Missoula flood deposits, which are the predominantly stony sediments found from the confluence of Snake and Clearwater Rivers to about 7.5 miles eastward of the mouth of the Palouse River. An excellent exposure of these “stoney” deposits can be found on the north bank of the Snake River at Central Ferry, fortunately still visible above the impounded water of Lake Bryan behind Little Goose Dam. These pebble, cobble, and boulder-sized stones are comprised of many different lithologies^[10]According to the geological map of this region, and verified through my own field observations, these light to dark gray, greenish gray, olive gray, grayish red, grayish red purple, grayish orange … Continue reading but their markedly well-rounded shapes disclose a final turbulent, shared journey from perhaps as far distant as the ancestral Salmon River.

Clark was a bit more descriptive of flood depositional features on 15 October 1805:

The Islands of different Sizes and all of round Stone and Sand.

Because the easternmost discharge of the Missoula floods was the Palouse River (which the floods re-routed from its original course down Washtucna Coulee), the “round Stone” Clark described on 15 October (downstream of the Snake-Palouse River confluence) was a Missoula flood outburst gravelly deposit. Geologists can differentiate between pre-Missoula flood deposits such as the aforementioned Central Ferry exposure and Missoula flood gravels because the latter deposits are almost completely comprised of basalt gravels (>95%), direct evidence that the Missoula floods swept over, and plucked, wrenched and scoured rocks from a nearly uniform basaltic landscape. Ever diligent in his observations, on 16 October 1805, Clark would correctly describe another one of these Missoula flood outburst deposits as a “gravelley bare.”