The Columbia River Gorge is a spectacular canyon that cuts a path through the Cascade Range, forming the boundary between Oregon and Washington. This dramatic 80-mile stretch of river valley is renowned for its sheer cliffs, dense forests, and countless waterfalls, serving as the only sea-level passage through the mountains. The gorge’s formation is the result of a complex, multi-stage geological history spanning millions of years. This iconic landscape was sculpted by ancient volcanism, relentless river erosion, tectonic forces, and the most powerful floods ever known on Earth.
Laying the Foundation: Volcanic Basalt Flows
The deep structure of the gorge began with an outpouring of molten rock during the Miocene Epoch. Starting around 17 million years ago, immense volumes of dark, fluid lava erupted from fissures far to the east, flooding the landscape. These ancient flows formed the Columbia River Basalt Group (CRBG), a vast igneous province that covered over 163,700 square kilometers of the Pacific Northwest.
Successive sheets of lava piled one upon the other, mostly between 16.7 and 15.9 million years ago. In the area of the gorge, these basalt layers accumulated to a thickness of more than 1.8 kilometers in some places, forming the bedrock plateau that the river would later incise. The CRBG flows were so extensive that they fundamentally altered the regional drainage, forcing the ancestral Columbia River into its present, westward-flowing course.
The Ancestral River and Tectonic Uplift
Once the basalt flows had cooled, the ancestral Columbia River established a course across the newly formed plateau. The gorge as we know it did not yet exist, with the landscape likely a broad, shallow valley. The next geological force was the slow, continuous rise of the Cascade Range, which began its modern uplift about 5 to 4 million years ago.
As the mountains slowly rose across the path of the river, the Columbia maintained its trajectory, a process known as antecedent drainage. The river’s erosive power was sufficient to cut down through the rising layers of basalt and older underlying rock at a rate equal to the uplift. This prolonged period of gradual downcutting created a deep, but relatively narrow, V-shaped canyon, setting the stage for the dramatic events of the Ice Age.
Catastrophic Carving: The Missoula Floods
The most significant and rapid carving of the Columbia River Gorge occurred during the Pleistocene Epoch, a time marked by repeated floods. This series of events began when the Cordilleran Ice Sheet advanced and blocked the Clark Fork River in Montana, creating a massive ice dam that impounded Glacial Lake Missoula. The lake held an immense volume of water.
Periodically, the ice dam would fail, releasing a flood of water that surged across eastern Washington and roared down the Columbia River Valley. Evidence suggests at least 25 of these megafloods occurred between 20,000 and 14,000 years ago, each one rapidly scouring the landscape. The largest of these floods achieved a flow rate approximately 300 times greater than the highest historical flow of the Columbia River.
As this volume of water was funneled through the narrow, antecedent canyon, it fractured the basalt and rapidly eroded the walls, widening and deepening the gorge in a geological instant. The floods filled the valley to elevations as high as 340 meters (1,150 feet) above the present river level, leaving behind features like stripped basalts and streamlined erosional forms. The large-scale erosion during these events transformed the river’s slow-cut V-shaped valley into the broad, U-shaped cross-section characteristic of the modern gorge.
Modern Shaping and Ongoing Features
Following the cessation of the Ice Age floods, the gorge continued to evolve through slower, but still powerful, geological processes. The steep, scoured walls of the canyon became highly susceptible to mass wasting. This activity, primarily in the form of landslides, has been a persistent force, continuously widening the canyon and pushing the river channel southward.
One notable event was the Bonneville Landslide, a large rockslide-debris avalanche that occurred approximately 550 years ago. This slide dammed the Columbia River, creating a temporary lake and later forming the dangerous Cascades rapids that early explorers encountered. The numerous waterfalls, such as Multnomah Falls, are a direct result of the river cutting against the continually uplifting Cascade Range, leaving the tributaries hanging high on the gorge walls.
The gorge’s evolution is ongoing, with significant landslides and debris flows still triggered by heavy rainfall or minor seismic activity. The combination of the river’s continued downcutting and the persistent slope instability ensures that the Columbia River Gorge remains a dynamic landscape.