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retaining a secluded site with quiet deep water and, over the years, it developed a research unit there that regularly performs acoustic experiments with small ships and submarines.
    Breckenridge learned that naval technicians had constructed a sonic profile of the lake bathymetry by towing acoustic sources through the water, broadcast at different levels and captured by hydrophone receivers. Although the military project focused on the lake bottom’s shallowest sediments, Breckenridge guessed that their comprehensive data might also have something to say about the bedrock below them.
    The Navy’s seismic data for the lake’s southern arm showed gradual slopes dropping off the east and west shores, but the east side alone was marked by a stark subsurface bench running directly down the lake from Green Monarch Ridge. This underwater bench, as well as a similar one still visible above lake level today, can be interpreted as the result of the tunneling jets of water that disintegrated the ice dam.
    The naval data also revealed distinct stratigraphic units of debris on the lake bottom. Breckenridge believes that the deepest and thickest of these layers correspond to sediments left behind after the most recent ice-dam failure. Beneath that debris, the sonar outlined a classic U-shaped bedrock basin with a nearly flat center—the shape that defines glacier-carved valleys all over the world. Furthermore, the actual bottom of the lake was much deeper than he had anticipated. Along its south arm, the lake level of 2,050 feet, combined with a water depth of 1,150 feet, means that the top sediment layer lies 900 feet above sea level. Naval bathymetry showed that the depth of the bedrock basin approaches an astonishing 700 feet
below
sea level. This means that 1,600 feet of sediments rest below the water. In other words, the sediments themselves are much deeper than the deepest water in the lake.
    While the hydrologic forces of high-pressure tunneling beneath an ice mass can reach impressive speeds, no model or study has shown that they can generate enough power to carve bedrock to any great extent below sea level. Breckenridge determined that the bottom of Lake Pend Oreille is actually an overdeepened glaciated basin that has been refilled with the debris of numerous Ice Age floods. It was created in much the same way as well-studied elongate lake valleys in British Columbia, glacier-carved valleys in the Alps, and coastal fjords in Norway. “So,” concluded Breckinridge, “those ice caves, which Largé thought were created by the glaciers, were in fact formed by the big floods. And the bottom of Lake Pend Oreille, which many geologists believed must have been dredged by the floods, was actually carved by ice.”
Flowing through the Country
    After the last of the Lake Missoula floods passed down the Columbia to the sea, vegetation sprouted on the massive gravel dumps and sand bars left in its wake. Mammoths and camels returned to browse their former haunts, and anadromous fish swam back upstream to spawn. People were there with them, spreading back across the landscape as conditions allowed.
    The first written descriptions and accurate maps of this reclaimed world were forged by fur agent David Thompson. When Thompson surveyed his way from the Rockies to the Pacific between 1807 and 1812, he carefully plotted each new drainage that he entered, taking countless sextant shots and compass bearings. One of his maps depicts the interlaced rivulets formed by the Clark Fork River’s delta at the northeast corner of Lake Pend Oreille—the site of the ice dam that impounded Glacial Lake Missoula. Mountain ridges crawl across the landscape, marking the drainage divides for all the surrounding rivers and outlining the limits of Lake Missoula’s expanse upstream along the Clark Fork to Missoula and south through the Bitterroot Valley, as well as east and north up the Flathead River through the Mission Valley to Flathead Lake.
    Thompson