Climate patterns were different during Ice Ages. The Rocky Mountain region of North America is mostly arid today, but more precipitation and lower rates of evapotranspiration led to the formation of vast lakes during cooler climate phases. Most of these lakes gradually disappeared in non-dramatic fashion after the climate became warmer and drier. Evaporation changed the former sites of these freshwater lakes into empty basins, salt plains, and much smaller salt lakes. But the demise of Glacial Lake Missoula caused a spectacular flood, perhaps the largest deluge in earth’s history.
A southern lobe of the Cordilleran Ice Sheet blocked the flow of the Clark Fork River near the border of present day Idaho and Montana, creating a glacial lake as big as Lake Erie and Lake Ontario combined. At times it was almost 2000 feet deep, though it periodically lowered and partially drained. The ice dam itself was an astonishing 2000 feet high. The warm climate phase that marked the end of the Ice Age beginning about 15,000 years ago melted the ice dam, and the tremendous volume of water in Lake Missoula burst across Idaho and eastern and central Washington, finally emptying through the Columbia River valley into the Pacific Ocean near the present day town of Astoria, Oregon. This massive flood created a landscape known as the “channeled scablands.” The geological formations that serve as evidence of this cataclysm are impressive and picturesque.
The largest floods in the history of North America occurred in the Pacific northwest following the end of Ice Ages.
These geological landforms were caused by post Ice Age floods.
Below is a link to many more photos of these formations.
The flood carried large boulders encased in icebergs. These “erratics” can be found throughout the channeled scablands. There are dry falls–350 foot tall hills under where 300 feet of Lake Missoula water formerly flowed in what were temporary waterfalls. Huge ripple marks can be seen on Camas Prairie. Other amazing formations are the kolk potholes where swirling eddies gouged out deep troughs. Strandlines and lake deposits visible on the sides of mountains are evidence the dissolution of glacial lakes occurred repeatedly in this region–perhaps more than 34 times during the Pleistocene.
The scouring of these intermittent Ice Age floods eroded most of the topsoil in this region and much of the scabland is unsuitable for crops. But there are some exceptions. The tops of some hills were above the flood and still have enough soil for growing crops, and some soil eroded from mountains into some valleys where crops can also be grown. But for the most part agricultural activity here is limited to livestock grazing.
Humans began colonizing North America about the same time this cataclysmic flood occurred. Any people in the path of the deluge perished. Members of the sparse population living on the edge of the flood witnessed an unusual, awe-inspiring event, a story they likely told their children and grandchildren. It may be the origin of ancient flood myths found in Native American lore. Flood myths are known in cultures worldwide and probably are based on inherited memories of local floods that occurred at the end of the Ice Age when glaciers melted and sea level rose rapidly.
“Repeated Sedimentation and Expanse of Glacial Lake Missoula Sediments: A Lake Level History of Garden Gulch, Mountain, USA”
Quaternary Science Review January 2017