Ancient shorelines on Glacial Lake Missoula on Mount Jumbo.
(Photo: National Park Service) The confluence of the Clark Fork and Blackfoot Rivers was once covered under as much as 1,000 feet of cold, iceberg-filled meltwater created by Glacial Lake Missoula. Lakes had been occasionally present in western Montana much earlier, during the long history of the mountain building and uplift of the area. But these were warm-water lakes, and many of the prehistoric animal groups (including camels, elephants, and giraffes) formerly abundant in western North America lived nearby.

Lake Missoula was a result of the huge continental ice sheet that began forming some 1 to 2 million years ago in the Hudson’s Bay region.  Over thousands of centuries it moved southward, eventually covering most of the continent north of the Missouri River and generally north of the Ovando-Flathead Lake-Spokane area.  During this period, the massive ice sheet moved farther southward several times, as a series of ice “lobes” alternately extended and retreated south of the main ice mass.

During the last of these southward movements, one ice lobe carved the valley of Flathead Lake, while another moved southward across the Clark Fork in the general vicinity of Pend O’rielle Lake in Idaho (see map). A few miles downstream from present-day Noxon, Montana, ice filled the river canyon and formed a dam hundreds of feet thick.  When this happened, the waters of the Clark Fork, Blackfoot, Bitterroot, Flathead and other streams—already swollen with meltwater from the advancing glacier, which had by now reached the Ovando and Kalispell areas—had no place to go and, therefore, slowly backed up behind the ice dam to form Lake Missoula.

Map of Glacial Lake Missoula The center of the lake was approximately in the Flathead Valley area, but a network of long bays also extended up the river valleys, as far as Drummond, Darby, and Clearwater Junction (see map).  Over many years, the lake eventually reached a great enough depth (up to 2000 feet in places) that the enormous mass of backed-up water began to work its way through the ice dam, probably by first floating the adjacent portion of the ice mass out of its way.

Removal of the ice dam initially might have occurred slowly, but once most of the ice was out of the way, the full weight of all the backed-up water (over 500 cubic miles, about one-half the amount of water in Lake Michigan) surged through the narrow canyon. The gigantic flood that ensued spread across much of east central Washington, through the Spokane area, and headed for the Columbia River and into the Pacific Ocean.  Although the initial outflow of water through the ice dam was enormous and sudden, it probably took several weeks to drain the entire Lake Missoula.  Nevertheless, within a short time the Bonner area was again a river valley, covered with a layer of soft lake-bottom mud.

This process repeated itself many times during the history of Lake Missoula. The ice dam would build and the lake would fill until the water would once again burst the dam, and the lake would drain. Each time the lake built up, shoreline beaches formed at the high-water level, on the sides of the valleys and hills around and within the lake. Evidence of these beaches can be observed today as a series of parallel lines high up on Mount Sentinel and Mount Jumbo on the east side of the Missoula valley.