News & Features

The World’s Greatest Floods

During the last ice age, Glacial Lake Missoula unleashed the most powerful floods ever recorded, shaping the landscape from western Montana all the way to the Pacific Ocean

Driving through the lower Flathead Valley into the Clark Fork River corridor, around Hot Springs and Perma into Plains and Thompson Falls, it would be easy to miss the millennia-old relics of an often overlooked natural marvel: Glacial Lake Missoula, a vast ice-age body of water that unleashed the greatest floods in the world’s recorded history.

But a closer look, if you know what you’re looking at, reveals the remarkable remnants of that Earth-shattering event — series of events, in fact — which shaped the Northwest United States from western Montana to the Pacific Ocean.

Today, there is a burgeoning contingent of scientists, residents, community and business leaders, and a few politicians who want people to take a closer look and to understand what they see: the enduring contours of ancient cataclysms, connecting us to a formative chapter in our modern landscape’s origin story.

An aerial photo overlaid with an illustration depicting Glacial Lake Missoula at the ice dam located near the modern-day Idaho-Montana border. Photo by Dave Bennett; photo illustration by Jared Shear

During the last ice age, between 12,000 and 18,000 years ago, a massive ice sheet, thousands of feet thick, covered much of what is today Canada. A finger of this Cordilleran Ice Sheet extended into the Idaho Panhandle, creating a towering ice dam that blocked the Clark Fork River, which formed Glacial Lake Missoula.

The lake, deeper than 2,000 feet in places, held over 500 cubic miles of water, as much as modern Lake Eerie and Lake Ontario combined. Then the dam broke, unleashing cataclysmic floods, and then broke again, and again, repeatedly, “scores of times” perhaps every 40 to 140 years, according to the Ice Age Floods Institute, lasting until the ice sheet retreated to the north as the ice age ended.

Each time the dam failed, unfathomable amounts of water and ice were released, sweeping through western Montana, Idaho, Oregon and Washington to the Pacific Ocean. Its peak rate of flow was 10 times the combined flow of all rivers in the world today. The enormous Lake Missoula, which encompassed huge swaths of western Montana, drained within days.

The overpowering ice-filled water, as it sprinted to the Pacific Ocean, picked up extraordinary quantities of earth and rock, dozens of cubic miles at a time, and transported and deposited it across the Northwest U.S, building new landforms. As the floods carved through mountains and rolled over or through everything in their way, they also left behind features in the landscape that endure today.

The Ice Age Floods Institute says the floods built gravel bars as tall as 400 feet and moved “boulders weighing many tons as if they were pebbles.” A great deal of debris was carried all the way to the floor of the Pacific Ocean, where extensive deposits of flood sediment have been identified hundreds of miles from the current mouth of the Columbia River.

“These floods are a remarkable part of the North American natural heritage,” the institute asserts. “They have profoundly affected the geography and ways of life in the region, but have until recently remained largely unknown to the general public.”

Lakebed sediments around Niarada. Photo by Dave Bennett

Jim Shelden, among the floods’ foremost experts, is leading the charge in expanding public awareness. Shelden is a geologist who retired in 2009 as U.S. Forest Service Regional Geologist for Region 1. As president of the Ice Age Flood Institute’s Glacial Lake Missoula Chapter, he has dedicated his retirement to furthering our understanding of the floods, both in terms of scientific research and public outreach.

Shelden considers Lake Missoula a fascinating ancient story that is unfolding in modern times as we learn more about the science.

“The hardest part of Lake Missoula is getting your head around how big it was,” he said, noting the floods are the world’s biggest based on flow rates. “Once you start to get your head around how big it was, then all of the stuff you see from here to the ocean gets set in proper context. It amazes you. It’s all your mind can grasp and handle, and sometimes more.”

Shelden’s institute was instrumental in Congress passing the Ice Age Floods National Geologic Trail bill, signed into law by President Obama as part of the Omnibus Public Land Management Act of 2009. The bill authorized funding to develop an educational trail, which Shelden likens to the Lewis & Clark National Historic Trail, with the goal of “enabling the public to view, experience, and learn about the features and story of the Ice Age floods through the collaborative efforts of public and private entities.”

“After much ado and many years, it is authorized as the nation’s first and only national geological trail,” Shelden said.

But no money to date has been appropriated for the trail. In the absence of federal funding, the institute has been partnering locally and doing what it can to further its mission of education, leading guided field trips, erecting scattered informational markers and signage, compiling educational pamphlets, applying for grants, and creating driving tour maps for western Montana, which are available on the institute’s website. Shelden also gives lectures, including a September talk in Kalispell hosted by the Northwest Montana Posse of Westerners.

Gravel deposits piled up around Perma as all the sediments from the north hit the water coming from the east. Photo by Dave Bennett

Beyond public education, Shelden thinks there are tourism opportunities.

“We’d like to use the flood and its stories to enhance and diversify the small rural communities all along the way to give them a bigger niche in the tourist business,” he said, pointing to an example of a tiny town in Oregon that has promoted the flood’s history: “They’ve embraced it, and it’s paying off in tour buses and getting tourists.”

Sanders County in particular is full of lake and flood landmarks, including markings from shorelines, ripples, lakebed sediments, terminal moraines and more, all blown up on a scale that challenges the imagination. The Paradise Center, a museum and community center located in Paradise’s old schoolhouse, has a detailed relief map of Lake Missoula on display.

Shelden notes that in the area around Rainbow Lake near Plains, where the elevation suddenly changes, “300 to 600 feet of water was smoking through at 60 miles per hour, and it cut a coulee right through the mountains.”

“We have a number of really cool features packed right into that area, and it’s really the central part of the spectacular stuff in the lake,” he said.

Dave Bennett, a real estate broker in Thompson Falls, has also been working to promote public awareness. A pilot and photographer on the side, he has been taking photographs for years from his Super Cub airplane to document the area’s many flood features. He has published two calendars of the photos, and his aerial images appear in this story.

“It’s an easy subject to get hooked on,” Bennett said.

Bison graze on a hillside at the National Bison Range in Moiese. Greg Lindstrom | Flathead Living

Shelden notes that the concept of “catastrophic floods changing the planet in a couple days or weeks” was once controversial but is now embraced as deeply supported science. He points to dinosaurs’ mass extinction by a single comet or asteroid strike as another example of a catastrophic event rapidly and thoroughly altering the Earth.

With an abundance of research and physical evidence proving the existence of Lake Missoula and its floods, an expanding body of research into other cataclysmic floods across the world is sweeping across the scientific community.

“Now that we understand the signature of outburst floods,” Shelden said, “we’re finding them all over the world, and it’s really a big deal for the history of the world and shaping the world.”

“Lake Missoula was the first in this huge process,” he added. “It will always have its place in history as the first in this whole line of thinking that changed science.”

Special thanks to the Northwest Montana Posse of Westerners. For information about Montana floodscape features, visit www.iafi.org. To purchase a calendar of Dave Bennett’s aerial photos, visit www.glaciallakemissoulafloods.com.

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