Missoula Floods

The Missoula Flood sequence begins at the southern margin of the Cordilleran Ice Sheet, the continental glacier that covered most of British Columbia and northern Washington during the last glacial maximum. As the ice sheet advanced southward into northern Idaho around 17,000 years ago, a lobe of ice pushed across the Clark Fork River near present-day Lake Pend Oreille, creating an ice dam approximately 600 metres tall. The dam backed up the Clark Fork waters, producing Glacial Lake Missoula: a body of water that at maximum extent contained approximately 3,000 cubic kilometres and stretched from the dam site eastward across most of western Montana, flooding what is now Missoula and surrounding valleys. Maximum lake depth at the dam was approximately 600 metres; the lake reached high enough to flood the valleys that today contain Polson, Hot Springs, and Plains in Montana. The geological record (varved sediments preserved in lake-floor deposits) shows that the lake filled and drained repeatedly, with each filling cycle taking decades and each draining cycle taking days. Current best estimates suggest 40 to 100 separate fill-and-drain cycles over approximately 2,000 years.

• Cordilleran Ice Sheet pushed south into northern Idaho ~17,000 years ago; ice lobe dammed the Clark Fork River near present-day Lake Pend Oreille; ice dam ~600 metres tall
• Glacial Lake Missoula at maximum extent ~3,000 cubic kilometres; stretched eastward across western Montana; flooded valleys at Polson, Hot Springs, Plains; maximum depth at dam ~600 metres
• Repeated fill-and-drain cycles: varved sediments in lake-floor deposits record 40 to 100 separate cycles over ~2,000 years; each fill cycle decades, each drain cycle days
• Lake dammed primarily by the Purcell Trench lobe of the Cordilleran Ice Sheet; secondary dams and lake configurations also documented at smaller scales

When the ice dam failed, the entire 3,000 cubic kilometres of Glacial Lake Missoula released through the Clark Fork drainage and into eastern Washington. Floodwater velocity reached 60 to 80 miles per hour; the discharge exceeded the combined flow of all the world's modern rivers by a factor of 10 to 20. The floodwaters scoured the basalt bedrock of eastern Washington, producing the Channeled Scablands: a network of dry, scoured channels (coulees), potholes (kolks), and exposed basalt that today is one of the most distinctive landscapes in North American geology. Major Channeled Scabland features include Dry Falls (a former waterfall site approximately 5 kilometres wide and 120 metres tall, larger than modern Niagara Falls by a factor of approximately 10), the Grand Coulee, the Palouse Canyon, and the Drumheller Channels. The floodwaters carried staggering volumes of debris: gravel, sand, basalt boulders, and finer sediments suspended in the rushing water. As floodwaters slowed at downstream points (the Ephrata area, the lower Columbia River canyon, and the eddies where the Columbia River cut through narrow gaps), they dropped their sediment load and built massive depositional features: the Ephrata Fan (a giant gravel bar over 100 kilometres long), gravel bars at Crescent Bar and other Columbia River eddies, and the Pasco Basin gravel fields.

• Floodwater velocity 60-80 mph; discharge exceeded combined flow of all modern rivers by factor of 10-20x; each individual flood event in days
• Channeled Scablands: scoured network of dry channels (coulees), potholes (kolks), and exposed basalt across eastern Washington; one of North America's most distinctive geological landscapes
• Dry Falls: former waterfall ~5 km wide and 120 m tall, larger than modern Niagara Falls by ~10x; located in present-day Sun Lakes-Dry Falls State Park
• Major depositional features: Ephrata Fan (gravel bar >100 km long), Crescent Bar (Columbia River eddy gravel), Pasco Basin gravel fields

The most consequential Missoula Flood deposit for Pacific Northwest wine is the Touchet Beds: layered slack-water silts that accumulated in southern valleys where floodwaters backed up against narrow drainage points and slowed enough to drop their suspended fine-sediment load. The Touchet Beds are most spectacularly preserved in the Walla Walla Valley (named for the Touchet River, a tributary of the Walla Walla River where the layered deposits were first identified and named by geologists). Touchet Bed exposures in the Walla Walla Valley show 30 to 40 rhythmically layered beds of fine sand and silt, each layer representing a single flood event; the number and consistency of the layers is the primary geological evidence supporting the multiple-flood hypothesis. The Touchet Beds form the principal vineyard soil at many Walla Walla Valley sites: well-drained but fertile, holding water through the dry summer, and producing the structural mid-palate that distinguishes Walla Walla Bordeaux blends and Syrah from neighbouring Columbia Valley sites where Touchet Bed deposits are thinner or absent. The Rocks District of Milton-Freewater AVA on the Oregon side of Walla Walla sits not on Touchet Beds but on cobblestone alluvial deposits from the Walla Walla River carrying basalt cobbles eroded from upstream Columbia River Basalt outcrops; the Rocks District is distinct from the Touchet Beds it neighbours.

• Touchet Beds: rhythmically layered slack-water silts in southern valleys; 30-40 layers in Walla Walla Valley exposures, each representing a single flood event
• Named for the Touchet River, a Walla Walla River tributary; provides primary geological evidence for multiple-flood hypothesis (number and consistency of beds)
• Principal vineyard soil at many Walla Walla Valley sites: well-drained but fertile, holds water through dry summer, produces structural mid-palate in Walla Walla Bordeaux blends and Syrah
• The Rocks District of Milton-Freewater AVA: cobblestone alluvial basalt cobbles from Walla Walla River, NOT Touchet Beds; distinct depositional history neighbouring the Touchet Bed sites

Above the flood path proper (on hilltops and ridges that the floodwaters did not directly inundate), windblown loess accumulated during and after the flood sequence. Loess is windblown silt: as floodwaters dried and as prevailing southwest winds blew across the freshly deposited fine-sediment surfaces, the wind picked up fine particles and redistributed them to sheltered upland sites. The most consequential loess deposits for wine country are the Laurelwood soil series in Oregon's Tualatin Mountains (a windblown silt cap over Columbia River Basalt bedrock) that anchors the Laurelwood District AVA and Tualatin Hills AVA, and the loess deposits across the Palouse region of eastern Washington (which extend into the Walla Walla highlands above the Touchet Beds). The Willamette Valley sits at the downstream terminus of the Missoula Flood path: floodwaters reached the valley through the Columbia River gorge, then backed up against the Coast Range and slowly drained. The most famous physical evidence of the flood reach into the Willamette Valley is the Willamette erratic boulders: granite and metamorphic blocks (sourced from western Montana) that were carried on icebergs floating in the floodwaters and dropped on Willamette Valley hillsides when the ice melted. Some boulders weigh over 90 tonnes; the most famous specimens sit on or near vineyard property at sites in Yamhill-Carlton and Eola-Amity Hills.

• Windblown loess accumulated on hilltops above flood path: fine particles picked up by prevailing southwest winds from freshly deposited flood surfaces; redistributed to sheltered upland sites
• Laurelwood soil series: windblown silt cap over Columbia River Basalt bedrock in Tualatin Mountains; anchors Laurelwood District AVA + Tualatin Hills AVA in Oregon
• Palouse loess: deep windblown silt across eastern Washington (extends to Walla Walla highlands above Touchet Beds); produces the rolling hills of Whitman County dryland wheat country
• Willamette erratic boulders: granite + metamorphic blocks sourced from western Montana, carried on icebergs in floodwaters, dropped on Willamette Valley hillsides; some specimens >90 tonnes; sit on or near vineyard property in Yamhill-Carlton + Eola-Amity Hills