Roy Breckenridge, who passed away this fall at the age of 71, grew up in the Spokane Valley in a family of four that stressed common-sense resourcefulness and creative ingenuity. When Mrs. Breckenridge ran out of ice in her big cooler during a 1961 family camping trip to the Columbia Ice Fields, it naturally fell to young Roy to recharge it.
Back in those days, great glaciers now retreating into the high country ran right down to the road between Banff and Jasper, so Roy thought nothing of grabbing his camp hatchet and setting out. He returned with a perfectly-sculpted wedge of transparent blue glacial ice that looked like it had been removed from the center of the Pleistocene Epoch. After finding a home in Mrs. Breckenridge’s cooler, Roy’s giant ice cube made it all the way back to Spokane, moved over to the family’s big chest freezer, and lasted there for several years before its final demise.
Young Roy grew up to become a geologist, and during the course of his entire career, which concluded with a stint as Idaho’s official State Geologist, he maintained that affinity for ice. Along the way, Breckenridge became an integral member of the tight lineage of geologists following Harlan Bretz and J. T. Pardee who not only described the details of the Ice Age Floods that burst from Glacial Lake Missoula at the end of Pleistocene, but also communicated to the general public their excitement over such an awesome story.
It took decades for both scientists and amateurs to grasp the scale of these floods, but by the early twenty-first century, the saga of a great wall of water exploding out of Lake Pend Oreille to crash over the scablands and back up behind Wallula Gap before tearing through the Columbia Gorge and on to the Pacific stood as a defining chapter in our region’s history.
Even though the basics of this story have been available for almost a century, the devil, as Roy continually used to point out, is in the details. He and his colleagues studied the events that took place around Lake Pend Oreille and the lower Clark Fork River at the end of the Pleistocene, trying to figure out how the Cordilleran Ice Sheet formed a wall that was tall enough and strong enough to back up an enormous Glacial Lake Missoula. They wanted to know how such a massive dam could have failed so catastrophically, and exactly what happened when three thousand square miles of deep water roared across the landscape. They were curious as to how the ice dam then re-formed to replay the same scene, with subtle variations, many times over.
This is how they tell the story now: Around 20,000 years ago, the Purcell Lobe of the Cordilleran Ice Sheet moved down the pathway of the Purcell Trench, filling the ancestral valley occupied today by Lake Pend Oreille. When this frozen river pushed through the basin, it collided with the ancient rocks of Green Monarch Ridge on the eastern rim. Green Monarch provided a solid terminal buttress for the ice. As the glacier continued to advance, more and more ice piled up behind the ridge, gradually thickening into the dam that created the first of a series of Glacial Lake Missoulas.
Whenever the depth of Lake Missoula approached 2,000 feet, the water’s sheer weight began to compromise the ice cleaving to the base of Green Monarch Ridge. As small cracks appeared, streams of water flowed into the weak points, boring tunnels beneath the ice plug. The combined forces from the weight of the dam and the volume of Lake Missoula pressurized the water flowing into these tunnels so that the pathways enlarged very quickly. Jets of water churning along Green Monarch’s solid wall undermined the dam and caused its sudden failure, resulting in a catastrophic flood to the Pacific.
After the last glacier retreated, Lake Pend Oreille remained as the largest and deepest body of water in the Idaho Panhandle. Its natural surface level lies about 2,050 feet above sea level; the ice-carved mountains that surround it reach to 6,000 feet and more. Its waters plunge as much as 1,150 feet deep along its southern reach.
During his investigations around the lake, one question that intrigued Breckenridge and others was whether such great depth was the result of the grinding ice sheet or the repeated slashing floods. Although the carving power of glaciers is well documented, some geologists contended that the pressurized water shooting from the ice dams would have eaten away enough bedrock to significantly deepen Lake Pend Oreille. Various YouTube video clips take during modern dam removals certainly testify to the power of such pressurized jets.
Breckenridge believed that a definitive answer must lie on the bottom of the lake. He knew that the most dynamic part of a glacier is its forward toe, where the ice’s bulldozing power performs its most drastic razing of the landscape. Since the ice lobe that filled the Pend Oreille basin would have repeatedly gouged its southern edge, and since the main discharges of Glacial Lake Missoula would have been ripped through that same area, Breckenridge and his team were drawn to the lake’s southern arm. The sediments there, they reasoned, might well show the difference between what had been sculpted by ice and what had been eroded by floods.
By chance, during World War II the US Navy had established a training base along the southwestern edge of the lake, exactly where the floodwaters had once poured out. Although the Farragut Naval Station was decommissioned in 1946, the Navy understood the advantages of retaining a secluded site with quiet deep water and, over the years, 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 instruments 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.
It took a while, but Breckenridge obtained the raw data from those acoustic surveys. The Navy’s seismic profile of 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 glacial-carved valleys all over the world. Furthermore, the actual bottom of the lake was much deeper than anyone 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 seismic charts show 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, glacial-carved valleys in the Alps, and coastal fjords in Norway.
The work of Roy and his colleagues proved that sometimes it takes a little resourcefulness and ingenuity to separate ice features from flood features. Mrs. Breckenridge would have been proud, but not surprised.
Thanks to Dana Komen for the cooler story and photo. This essay by Jack Nisbet is adapted from a chapter of his most recent book, Ancient Places. For more information, visit www.jacknisbet.com