Missoula Floods

Long a popular destination for outdoor enthusiasts, the Columbia River Gorge is one of North America’s most scenic and impressive canyons.  The canyon stretches in excess of 80 miles as it passes directly through the Cascade Range, forming a nearly sea-level passage through outcroppings that loom as much as 4,000 feet above the river.

This stretch of the Columbia River is also renowned for its impressive concentration of waterfalls.  Extensive snowmelt and year-round rains course through the landscape into creeks and streams that rush over the steep slopes and canyon walls to join the Columbia.  The rugged rock faces and sculpted amphitheaters create an impressive stage for these cascades.  Most of this rock is basalt, along with the bold edifices and monoliths such as Beacon Rock, and its exposure is the result of several cataclysmic events known as the Missoula Floods.  These floods ripped through eastern Washington and sculpted much of the Columbia River Gorge as we see it today.

Basalt is an igneous rock formed from lava that has cooled and solidified.  It is also described as mafic rock due to the high concentrations of magnesium (‘ma’) and iron (‘feric’) that give the rock its dark appearance.  Unlike its igneous cousin, black obsidian, basalt contains little silica and thus retains a matte-like appearance.  Over 10 million years ago this Columbia River Basalt slowly oozed from volcanic dikes over what is now the Columbia River Plateau, through the Columbia River Gorge, and out into the Pacific Ocean.

Much later, as recently as the last ice age, the global temperature dropped and much of the planet’s water froze into ice sheets up to 2 miles thick.  This lowered sea levels by as much as 400 feet.  Just south of the border shared by The United States and Canada, the Cordilleran Ice Sheet expanded, and the Purcell Trench lobe of the sheet created a natural dam on the Columbia River’s fourth largest tributary, the Pend Oreille River.  The backed-up flows eventually became the massive Lake Missoula, which grew to depths of over 2,000 feet.  Concurrently, the Okanogan and Columbia River lobes of the sheet created Lake Columbia, submerging the present day Spokane area.

Between 13,000 and 15,000 years ago, as the ice age came to an end and the Cordilleran Ice Sheet receded, the dams that contained Lake Missoula and Lake Columbia weakened and released, unleashing a series of torrential floods that scoured the land and exposed the region’s ancient basalt flows.  Evidence indicates there were at least 25 massive floods, the largest of which is estimated to have discharged 2.7 million cubic meters of water per second.  This staggering current is roughly equivalent to 13 times the average flow of the Amazon River.

The evidence of these floods is in the Gorge itself, through the path of the lower Columbia, and across the entire Columbia River Plateau.  Aerial views of the Scablands provide amazingly clear illustrations of the flood path and the erosive impact of these incredible events.