Pend Oreille River - Geology

Geology

Much of the Pend Oreille valley consists of relatively ancient metamorphic rock, uplifted over 500 million years ago from the former floor of the Precambrian sea that covered the region during that period. Granite batholiths overlain by layers of sedimentary rock compose most of the higher terrain, such as the Cabinet Mountains. The age of the rocks along the Pend Oreille generally decreases as one travels downstream (north), and the terrain is also more rugged towards the north than in the south. About 200 million years ago, increased tectonic activity caused the uplift of the Idaho Batholith, a portion of which cooled and eroded to become the present main body of the Bitterroot Range, a major physiographic feature of the watershed which sweeps from northwest to southeast along the entire Clark Fork valley (and the border of Idaho and Montana), by about 70 million years ago. This time period is generally accepted as when the entire Rocky Mountains system was formed, although age of the rocks varies with location.

In the previous Ice Age, a massive glacier of the Cordilleran Ice Sheet advanced southwards through the Idaho Panhandle, burying the Lake Pend Oreille and upper Pend Oreille River valley with ice hundreds to thousands of feet thick. This formed a pair of ice dams, one gigantic, and one significantly smaller. The one that caused the water of the Clark Fork and other smaller streams to back up into Glacial Lake Missoula, which stretched over two hundred miles southwest across western Montana, containing some 500 cubic miles (2,100 km3) of water, was over 2,000 feet (610 m) high. The second one blocked the upper Pend Oreille River near Sandpoint, creating an enlarged Lake Pend Oreille that could have connected with a similarly enlarged Kootenay Lake in the north. Water pressure and glacial melt destroyed the larger of the two ice dams several times, causing massive amounts of water to rush out across eastern Washington through the upper Pend Oreille River. A mistaken belief is that these cataclysmic floods, known as the Missoula Floods, traveled down the Pend Oreille Valley into the Columbia. Rather, it is now thought that the water completely breached the western divide of the Pend Oreille River valley and rushed out towards the direction of Spokane.