Geology

A large boulder sits in the middle of a green field. Overhead, the sky is blue with puffy, white clouds.
A half mile high glacier that once covered part of the Lake Roosevelt area left behind this seemingly out of place boulder.

NPS / JEFF AXEL

The landscape around Lake Roosevelt National Recreation Area changes dramatically across the almost 130 miles of the lake. From the dry scrubland and flat plateaus of the south to the forested mountains of the north, the geologic story of the area is just as varied. It is a story that spans 250 million years and one that still has secrets to reveal.

 

Ancient Shorelines and Ancient Mountains


The mountains at the north end of Lake Roosevelt tell the story of a violent geologic past. 250 million years ago, the Lake Roosevelt area was underwater, a quiet environment just offshore of a quiet continent. The calm was broken as giant sections of the Earth's crust slowly but surely rammed into each other. Today, the Kettle Falls area contains rocks born both in that gentle marine world and in that continent altering collision.
 

250 Million Years Ago vs Today

A sandy beach at sunrise A sandy beach at sunrise

Left image
A modern day beach represents this area North America 250 million years ago.
Credit: NPS / VICTORIA STAUFFENBERG

Right image
Today, the Kettle Falls area is mountainous and forested.
Credit: NPS

 
How did this watery world end up as mountains in the middle of a continent?

To piece together these events, we first must understand plate tectonics. The Earth's crust is divided into distinct movable pieces called tectonic plates. These plates vary in thickness, with thinner plates underneath the ocean and much thicker plates making up the continents. These plates interact with each other to create the landforms that we see around us. At a convergent plate boundary, two plates meet. Around 200-250 million years ago, at the modern day position of Lake Roosevelt, a thinner oceanic plate was pitted against a thicker, more massive continental plate. The continent inevitably won, forcing the oceanic plate to subduct. In this collision, the sediments of the ocean floor were scraped off the subducting plate and shoved skyward. But this didn't happen just once. Over the years, the oceanic plate subducted further. Multiple landmasses riding on the oceanic plate attached to North America one after another, like items riding on a conveyor belt. This process is called accretion.

 
diagram of convergent plate boundary showing accreted terranes
A generalized diagram of a convergent boundary with accreted terranes.

Images above modified from “Parks and Plates: The Geology of our National Parks, Monuments and Seashores,” by Robert J. Lillie, New York, W. W. Norton and Company, 298 pp., 2005, www.amazon.com/dp/0134905172

As the North American continent grew, the sediments first laid down in the quiet offshore environment uplifted. In this slow, but violent process, they were metamorphosed, subjected to intense compression and heat. Volcanoes created by this subduction zone further altered the rocks. All of these preocess together created the mix of igneous, metamorphic, and sedimentary rocks that make up the mountains surrounding Kettle Falls and the north end of the lake.

Today, these mountains don’t rise as high as they once did, for they were heavily eroded during the last Ice Age. In a later chapter of our story, we’ll learn how glaciers further shaped the landscape surrounding Lake Roosevelt.

 
 

Floods of Lava

Lake Roosevelt marks the northern extent of one the most dramatic geologic regions in North America. Beginning approximately 17 million years ago, lava poured onto the landscape of what is now central Washington and northern Oregon. Wave after wave of lava travelled hundreds of miles from its origin, plowing through temperate forests and trapping the stories of that time beneath. Today, the dark, cooled lava dominates the landscape.

 

17 Million Years Ago vs Today

Lava pools and flows across a dark landscape Lava pools and flows across a dark landscape

Left image
Lava in Hawaii helps us imagine the lava flows of 17 million years ago.
Credit: USGS / R. FISKE

Right image
Cooled lava in the Lake Roosevelt area today forms unique patterns.
Credit: NPS

 
A map of Oregon, Washington and Idaho showing the extent of the Columbia River Basalts
A map of the extent of the Columbia River Basalt Group

USGS / BARRY ET AL.

Where did the lava come from?

The lava came from fissures, or cracks. The majority of these were along what is now the southern part of the Idaho/Washington border. These flows covered about 87,000 square miles of the Pacific Northwest. After 11 million years of flows, up to 150 separate flows with a combined depth of over 2 miles remain. Together, these layers of rock comprise the Columbia River Basalt Group.

The source of the lava was very likely a hot spot. This is a place in the Earth's mantle that has warmed significantly, casuing rising magma to spill through the crust and onto the surface. The exact origins of hot spots still remain a mystery, but their effects can be seen across the globe, in places like Hawaii, Iceland, and Yellowstone.

 
A landform with layers of dark rock in the foreground, more rock and farmland in the background
Moses Lake Coulee is a good example of the almost layer-cake quality of cooled lava flows.

Public domain

The lava cooled into a dark, distinct rock known as basalt. Easily recognizable patterns remain in these rocks, telling us about their formation. A hallmark of the cooled flood basalts are stocky, uniform hexagonal columns. Insulated by the top of the flow, this columnar basalt would have taken a long time to cool—up to several decades!

The current course of the Columbia River is also thanks to these lava flows. As the molten rock came to the surface, the Earth’s crust gradually sank into the space it left behind. The subsidence of the crust produced a large, slightly depressed lava plain now known as the Columbia Basin. The ancient Columbia River was forced into its present route by the northwesterly advancing lava.

 
 

Ice Age Floods


For years, the story of the Ice Age in the Lake Roosevelt area offered many more questions than answers. Today, we know enough to imagine a wall of icy floodwater thundering across the landscape. Multiple cataclysmic events moved colossal amounts of earth in a geologic heartbeat and left in their wake the quiet and dramatic vistas of the region today.
 

17 Thousand Years Ago vs Today

Icebergs the size of buildings float in muddy water past a person Icebergs the size of buildings float in muddy water past a person

Left image
Icebergs in Glacier Bay National Park give us an idea of the muddy and icy water of the floods thousands of years ago.
Credit: NPS

Right image
The result of massive floodwaters, the Grand Coulee sits quietly today.
Credit: NPS / JEFF AXEL

 
A map of Oregon, Washington, and Idaho showing the extent of the Cordilleran Ice Sheet and the location of the Channeled Scablands
The extent of the Cordilleran Ice Sheet and the location of the Channeled Scablands against modern day boundaries and landmarks.

USGS

Was Lake Roosevelt once covered in ice?

Yes and no. The area to the north and west of the lake were once under a continental ice sheet. Earth experienced an overall cooling trend beginning around 2.6 million years ago, within which were additional periods of cooling and warming. Over this period, whenever snowfall exceeded melting and evaporation, a great accumulation of snow covered part of North America, eventaully compacting to form extensive ice fields. In the most recent glaciation, which ended around 15,000 years ago, Washington, Idaho, and Montana were covered by the southern edge of the Cordilleran Ice Sheet. As this ice sheet advance and retreated, it scoured the mountains in the north Lake Roosevelt area, filing them down to their height today.

The southern half of the lake and the landscape beyond experienced something much different and much more unusual.

 
A satellite image of Lake Roosevelt and the area to the south
A satellite image of part of Lake Roosevelt and the area immediately to the south. The light brown channels are coulees and features etched by the Ice Age Floods.

NASA EARTH OBSERVATORY / JESSE ALLEN

As the ice sheet crept south, it blocked rivers and lakes across the region. In doing so, deep glacially dammed lakes formed. Lake Missoula, covering northwest Montana, was the most massive, at over 2,000 feet deep. Eventually, the ice dam weaken, burst, and sent as much as 500 cubic miles of water racing across northern Idaho and into eastern Washington.

With no modern analog, it's taken much study and imagination to envision these floods. An amount of water 1300 times the average daily discharge of the Columbia River raced across the landscape. This wall of water moved up to 65 mph, creating a roar that could be heard far in advance of its arrival. The water was muddy and churning, full of sediment, rocks, chunks of glacial ice, and anything else in its path. Floodwater and debris ripped away the thick basalt layers of the region, etching into the landscape what we now call coulees. These broad, box like canyons with steep cliffs cut across an otherwise fertile landscape. The region today is known as the Channeled Scabland. The waters continued their to journey all the way to the Pacific Ocean, dumping out of the mouth of the Columbia River near Astoria, Oregon.

This flooding didn't happen just once. The exact number is yet to be determined, but multiple ice dam failures led to multiple floods, each event bringing the landscape that much closer to the one we know today.

The unique story of the floods and their discovery led to the creation of the Ice Age Floods National Geologic Trail in 2009. This automobile route travels 3,380 miles (5,439 km) through Montana, Idaho, Washington, and Oregon. The National Park Service manages the trail in collaboration with other federal agencies, state parks, private land owners, and the Ice Age Floods Institute.

To discover how geologists untangled the mystery of the complex landscape around the lake, check out the booklet Lake Roosevelt and the Case of the Channeled Scablands.

 
 

The Landscape Today


With a moderation in climate, the ice slowly retreated north. The Grand Coulee and the network of other watercourses across the plateau were left high and dry several hundred feet above the Columbia River.

Today the traveler sees numerous coulees and small lakes, a scoured and potholed landscape, with boulders strewn about, far from any apparent source. These strange landforms presented a complex geologic puzzle. One of the first to seriously solve it was J Harlen Bretz, a geologist who first saw the region in 1922. His proposal of cataclysmic floods shook the study of geology.

Not only are there still many more questions to be answered about the Lake Roosevelt area geology, but it is a story that is still unfolding. The lake continues to be shaped by wind and water, by tectonic forces, and even by human forces.

Learn more about the geology of the Lake Roosevelt area!

General Geology

Columbia River Basalt Group

Ice Age Floods

Last updated: April 5, 2021

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Mailing Address:

Lake Roosevelt National Recreation Area
1008 Crest Drive

Coulee Dam, WA 99116

Phone:

509-754-7800
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