Lake Pend Oreille is a remnant from the last Ice Age when outburst floods associated with periods of glaciation crept south across the northern border of the United States. The glacial ice blocked river drainages, including the Clark Fork near the Idaho-Montana border. These blockages created huge, ice-dammed lakes. The largest of these was Glacial Lake Missoula, which extended 200 miles east.
Geologists theorize that about every several dozen years glacial meltwater that deepened behind the ice dam began to tunnel through the 20- to-30-mile-long ice dam. The impoundment disintegrated and the entire dam suddenly let loose, releasing an earth-shaking glacial-outburst flood. The largest floods consisted of up to 530 cubic miles of bashing, grinding, roaring water that raced through Idaho, across eastern Washington and down the Columbia River valley all the way to the Pacific Ocean.
After the sudden breakup of the ice dam holding back humongous Lake Missoula, most of the floodwaters burst out of the intermontane valley that today holds Lake Pend Oreille. The valley was filled with up to thousands of feet of glacial ice that at times extended all the way to Bayview and beyond. At other times the ice dam may have only advanced as far as Green Monarch Ridge.
The generally rounded, lush, green, forested mountainsides and clear blue waters make it difficult to fathom that this idyllic setting for the ice dam and breakout area has anything in common with the barren, scarred landscape wrought by the same Ice Age floods downstream in the Channeled Scabland of Washington. The striking contrast is for two reasons – one climatic and the other geologic.