More than 15,000 years ago the largest and most powerful scientifically documented freshwater flood to occur on earth happened in the Pacific Northwest. During the last ice age, ice sheets, at times reaching over 10,000 feet in thickness, covered much of Canada. A small lobe of the Cordilleran Ice Sheet called the Purcell Trench lobe advanced southward blocking the river in the Clark Fork Valley in northern Idaho and Montana with a 2,000 foot high and 30 miles wide ice dam. A glacial lake was created that covered much of present-day western Montana under approximately 2,000 feet of water in a 200-mile-long lake roughly the size of Lake Erie and Lake Ontario combined.
Eventually, 400-520 cubic miles of water burrowed into the ice dam, shattering it. The water roared out of the lake, swept across northern Idaho into eastern Washington, then rushed southwest across the Columbia Plateau, and split at a cliff--part of the flood traveled east up the Snake River but the main thrust traveled down the Snake and Columbia Rivers. The floodwaters, moving up to 60 miles per hour, stripped away the soil, scoured out hundred of miles of deep canyons called coulees (Grand Coulee being the largest), created the 3 ½ mile wide, 400-feet-high Dry Falls, the largest waterfall to ever exist, and the 198 feet Palouse Falls, and left 300 foot high gravel bars and huge granite boulders called glacial erratics. The muddy torrent carried water, chunks of ice, trees, gravel, soil, and boulders--some the size of small houses. The first floods carried more debris than the succeeding floods.
The floodwaters traveled westward through the Columbia River Gorge at velocities as high as 90 mph. Geologists estimate the flow which raced through and scoured the gorge was likely 4,000 times the flow of the river today. The waters’ height exceeded 1,000 feet in the gorge. If it happened today, tourists at Crown Point (Vista House) would be running for their lives.
The present site of Vancouver, Washington was beneath 400 feet of water before it dissipated. The cataclysmic flood laid the gravel and sediment that today forms the 10-mile-long gravel bar that encompasses the ridge north of Portland International Airport called the Heights.
The rampaging flood swamped Portland, Oregon beneath more than 400 feet of swirling debris and ice-choked water. Ripple marks can be found in Portland on the east side of the Reed College Campus. Willamette Falls existed before the flood but was shoved far upriver by the waters’ scouring effects. An estimated 50 cubic miles of rich topsoil stripped from the gently rolling hills of eastern Washington’s Palouse region was re-deposited in the Tualatin, Yamhill and Willamette valleys.
The flood spilled into the upper Willamette Valley as far south as Eugene still 200 feet deep. Rich silt left by the flood reached 100 feet deep in places. Numerous boulders (erratics), carried by the flood (some embedded in icebergs), were strewn about as though they were pebbles. Several erratics are scattered here and there throughout the Tualatin Commons northwest of the Tualatin/Sherwood exit off I-5 (see default photo). The roaring flood reached the Kalama Narrows, about 30 miles north of Vancouver and 40 miles north of Portland where some of the water backed up creating the 11,000 square miles Lake Allison. Eventually the floodwaters flowed on to the mouth of the Columbia River and far out into the abyssal plains of the Pacific Ocean.
During a period of several thousand years a single large flood, a few, or possibly as many as 100 of these floods scoured the 600-mile path when the glacial ice dam repeatedly reformed, the lake filled up again, and the ice dam broke again. Each flood was separated by decades or centuries. The National Park Service has proposed this 600-mile path as an Ice Age Floods National Geological Trail—a five-state system of marked travel routes and new interpretive facilities featuring significant landforms created by the colossal floodwaters.
Instructions for logging waymark: A photograph is required of you (or your GPS receiver, if you are waymarking solo) and the glacial erratic.