Clark Fork River near Piltzville The Clark Fork and Blackfoot Rivers converge just outside Milltown, Montana, which was once known as Riverside. The two rivers are among the Rocky Mountain headwaters of the Columbia River.

The Clark Fork River originates roughly 120 miles upstream of the confluence near the Warm Springs Ponds. Here, Silver Bow Creek and Warm Springs Creek and other feeder streams merge near Opportunity, Montana. Both the upper Clark Fork and Silver Bow Creek, contaminated with mine wastes, are Superfund sites.

From its origins, the Clark Fork River flows north and west for the next 350 miles to the Lake Pend Oreille, just over the Montana-Idaho border. The Clark Fork along the way gathers tributaries large and small such as the Little Blackfoot, Flint Creek, Rock Creek, the Blackfoot, the Rattlesnake, the Bitterroot and the Flathead Rivers. By the time the Clark Fork enters Lake Pend Oreille, it is Montana's largest river, averaging more than 22,000 cubic feet of water per second.

Lower Blackfoot River near Bonner The Blackfoot starts on the Continental Divide northeast of Lincoln. From there it flows more than 130 miles to its meeting point with the Clark Fork River. Along the way it picks up numerous tributaries, including the Landers Fork, Nevada Creek, the North Fork of the Blackfoot, the Clearwater and Gold Creek.

Native Missoulian author Norman Maclean described the Blackfoot in his renowned book A River Runs Through It, writing,

“The straight line on the map also suggests [the Blackfoot's] glacial origins; it has no meandering valley, and its few farms are mostly on its southern tributaries which were not ripped up glaciers; instead of opening into a wide flood plain near its mouth, the valley, which cut overnight by a disappearing lake when the great ice dam melted, gets narrower and narrower until the only way a river, an old logging railroad, and an automobile can fit into it is for two of them to take to the mountainsides.”

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.