The Missoula Flood

THE MISSOULA FLOOD Dry Falls in Grand Coulee, Washington, was the largest waterfall in the world during the Missoula Flood. Height of falls is 385 ft [117 m]. Flood waters were actually about 260 ft deep [80 m] above the top of the falls, so a more appropriate name might be Dry Cataract. KEENAN LEE DEPARTMENT OF GEOLOGY AND GEOLOGICAL ENGINEERING COLORADO SCHOOL OF MINES GOLDEN COLORADO 80401 2009 The Missoula Flood 2 CONTENTS Page OVERVIEW 2 THE GLACIAL DAM 3 LAKE MISSOULA 5 THE DAM FAILURE 6 THE MISSOULA FLOOD ABOVE THE ICE DAM 6 Catastrophic Flood Features in Eddy Narrows 6 Catastrophic Flood Features in Perma Narrows 7 Catastrophic Flood Features at Camas Prairie 9 THE MISSOULA FLOOD BELOW THE ICE DAM 13 Rathdrum Prairie and Spokane 13 Cheny – Palouse Scablands 14 Grand Coulee 15 Wallula Gap and Columbia River Gorge 15 Portland to the Pacific Ocean 16 MULTIPLE MISSOULA FLOODS 17 AGE OF MISSOULA FLOODS 18 SOME REFERENCES 19 OVERVIEW About 15 000 years ago in latest Pleistocene time, glaciers from the Cordilleran ice sheet in Canada advanced southward and dammed two rivers, the Columbia River and one of its major tributaries, the Clark Fork River [Fig. 1]. One lobe of the ice sheet dammed the Columbia River, creating Lake Columbia and diverting the Columbia River into the Grand Coulee. Another lobe of the ice sheet advanced southward down the Purcell Trench to the present Lake Pend Oreille in Idaho and dammed the Clark Fork River. This created an enormous Lake Missoula, with a volume of water greater than that of Lake Erie and Lake Ontario combined [530 mi3 or 2200 km3]. Lake Missoula had no outlet [Fig. 2]. When the dammed water got deep enough, it started to float the glacier, and the tremendous surge of water under the ice immediately broke up the ice dam, leading to the cataclysmic dumping of Lake Missoula. A wall of water close to 2000 ft [700 m] high surged through the breached dam and poured across eastern Washington at speeds of up to 100 mph [160 kph]. Discharge was about 600 million cfs [ft3/s; 17 million m3/s], about 20 times the size of the Bonneville Flood and a rate that would drain Lake Erie dry in about 8 hours. This was the largest flood discharge known. Floodwaters poured into Lake Columbia and surged right on over the south bank into three major spillways [Fig. 3]. In each of these flood spillways, water scoured the land down to bare bedrock to create the Channeled Scablands of eastern Washington. As floodwaters rushed to the Pacific Ocean, discharge was so great that existing valleys couldn’t carry the floodwaters. Mile-wide valley constrictions simply could not conduct that much water, causing temporary ponding by hydraulic damming that created three ephemeral lakes [Fig. 4]. The floodwaters scoured scablands and coulees, ripped out huge blocks of bedrock, and dumped enormous loads of gravel in bars, often with giant current ripples. Titanic chunks of glacier rafted 100-ton rocks to the Pacific Ocean. There was at least one such catastrophic flood, probably more than 25, perhaps as many as 89. The Missoula Flood 3 Figure 2—Lake Columbia and Lake Missoula created by ice dams. Figure 1—Columbia River drainage as the Cordilleran ice sheet advanced southward. Glaciers shown in blue. Figure 4—Flood discharge across Idaho, Washington, and Oregon. Figure 3—Lake Missoula floodwaters swept across Idaho and eastern Washington. THE GLACIAL DAM Lake Missoula was created when the Clark Fork River was dammed by an advancing ice sheet from Canada. This occurred near the border of Montana and Idaho at the present Lake Pend Oreille [Figs. 5, 6], where the west-flowing Clark Fork River is bounded by the Cabinet Mountains on the north and the Bitterroot Range on the south. The main front of the Cordilleran ice sheet never quite reached the Clark Fork Valley, but an outlet glacier, or lobe, called the Purcell lobe because it led the ice advance down the Purcell Trench of northern Idaho, crossed the Clark Fork Valley and rammed into the north end of the Bitterroot Range [Fig. 7]. The left abutment of the glacial dam, where the ice ground against very competent bedrock, was formed by the Green Monarch Ridge. Figure 6—View up the Purcell Trench, path of the ice lobe advance, from Green Monarch Ridge; Lake Pend Oreille fills the glacial furrow. Figure 5—Glacial damsite area showing present topography. The Missoula Flood 4 Although the Clark Fork River at first probably flowed around/over/under the thin distal edge of the advancing glacial lobe, relentless advance of the ice eventually sealed it against Green Monarch Ridge to create a formidable dam. Clark Fork River waters began to accumulate to create Lake Missoula [Fig. 8]. Figure 8—At the maximum ice advance Lake Missoula Figure 7—Advancing ice of the Purcell lobe ground against the reached a depth of about 2000 ft at the dam. [Ice margin from Green Monarch Ridge, damming the Clark Fork River. Richmond and others, 1965] When the waters of Lake Missoula rose behind the Green Monarch glacial dam to about 1100 ft deep, lake level reached a low point on the drainage divide at Pole Creek, through which the lake water spilled [Fig. 9]. Well rounded cobbles and pebbles of mixed rock types at the very top of the spillway [Fig. 9 inset] attest to river flow through the spillway. This spillway may have been active for some time, because the valley is considerably underfit; that is, it is larger than would have been created by its present stream [Fig. 10A, B]. Figure 10A—Topography of the Green Monarch spillways. Figure 9— View down the Clark Fork River toward the ice dam. Early Lake Missoula waters spilled out through the Pole Creek spillway. Inset shows river gravels at the top of the spillway. Figure 10B—Early Lake Missoula, at a depth of Continuing advance of the thickening glacier eventually about 1100 ft, spilled out at Pole Creek. blocked the Pole Creek spillway, forcing lake level higher. At a depth of about 1700 ft, a higher spillway opened [spillway 2, Fig. 10C], also indicated by river gravels at the top. Three more spillways opened successively, the highest at an elevation of about 4100 ft [Fig. 11]. There is no evidence that Lake Missoula stabilized at this elevation – that is, there is no prominent maximum elevation strandline around the lakeshore – but this elevation is close to the maximum known elevation of the lake. Figure 10C—At 1700 ft depth, a higher spillway opened. The Missoula Flood 5 Figure 11—Green Monarch Ridge viewed across Lake Pend Oreille, Clark Fork Valley at left. Numbers on spillways correspond to Fig. 10. LAKE MISSOULA Lake Missoula was created when the glacier dammed the Clark Fork River and water backed up as the lake grew for 50 or 60 years. Lake Missoula backed up to about Deer Lodge on the Clark Fork River, past Darby in the Bitterroot Valley, and up against the Flathead glacier lobe at Flathead Lake. The Clark Fork River flows through rugged, mountainous topography, so when this country flooded, Lake Missoula was very long, somewhat serpentine in map view, with deeply indented shorelines [Fig. 12]. Lake Missoula eventually reached an areal extent of 2900 mi2 [7500 km2]. Shorelines around the perimeter of the lake [Fig. 13] show the lake reached an elevation of about 4250 ft [1295 m]. This made the water at Missoula about 950 ft [290 m] deep and an astounding 2000 ft [700 m] deep at the ice dam! Lake volume at highstand was an incredible 530 mi3 [2200 km3], more than the volume of Lake Erie and Lake Ontario combined. Figure 12—Glacial Lake Missoula at an elevation of 4150 ft. EN, Eddy Narrows; PN, Perma Narrows; RLP, Rainbow Lake Pass. Figure 13—Lake Missoula shorelines above University of Montana campus at Missoula. The geology of Lake Missoula is quite simple; almost all of the lake floor consists of Belt Supergroup metasedimentary rocks of Precambrian age [1.47 to 1.40 billion years]. The enormously thick sediments - perhaps as much as 12 miles [20 km] thick - were deposited in a rift basin by rivers heading in nearby [at the time] Siberia to the west and Australia to the southwest. Mostly sands and clays, these sediments were metamorphosed to quartzites and argillites, respectively, which are very tough rocks, resistant to erosion except where highly fractured. The metasedimentary rocks contain numerous intrusions of basalt. Conveniently, Belt rocks extend only a short distance beyond the site of the glacial dam, so that all of the boulders and cobbles of Belt rocks found across Washington and Oregon and, indeed, in the Pacific Ocean, can, with certainty, be attributed to transport by the Missoula Flood. The Missoula Flood 6 Lake sediments accumulated at the bottom of Lake Missoula, many of which show sequences of varved sediments. Varves are annual layers consisting of a lighter summer layer of meltwater-transported sand and silt and a darker winter layer of clay and organic material that settled out when the lake was frozen over [Fig. 14]. A count of the varve couplets thus indicates how long it took to fill the lake up to the point where the glacier dam failed. THE DAM FAILURE Figure 14—Varves in Lake Missoula Lakes are ephemeral features. Dams fail. Big dams fail catastrophically. sediments. One year of deposition is represented by a dark winter layer and We don’t know for sure quite how Lake Missoula’s dam failed, but the most a light summer layer.

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