Geologic History of the Columbia River Gorge
From: U.S. Army Corps of Engineers, Portland District, and the U.S. Department of the Interior, U.S. Geological Survey, The Geologic History of the Columbia River Gorge: Information Brochure.
40-20 million years ago ( Eocene to Miocene)
Thousands of volcanic eruptions piled layers of volcanic ash, lava, and mudflows over the region, creating the Ohanapecosh Formation. These rocks weathered into slippery red clay and greenish rocks visible near Stevenson, Washington.
Millions of years later, mudflows poured off volcanoes, covering the land with hundreds of feet of ash, boulders, and cobbles, creating the Eagle Creek Formation. You can see this beige formation on cliffs north of Bonneville Dam and along I-84 near exit 41. Beacon Rock, just downstream from the dam, is the ancient core of one of these volcanoes.
Trees buried in the Eagle Creek Formation petrified and their leaves fossilized. If you have a sharp eye, you might spot an ancient trunk in a rock outcropping along a trail. A petrified log from this formation lies in from of the Bradford Island Visitor Center.
Lava, lava everywhere!
17-12 million years ago (Miocene)
During this period, unusual volcanoes, called basalt floods, erupted in eastern Washington and Oregon. These volcanoes were cracks in the earth’s crust, several miles long, which poured out floods of liquid molten rock. 41,000 cubic miles (170,000 cubic kilometers) of this lava spread to cover large parts of Oregon and Washington. Out of 270 lava flows that spread across the region, 21 poured through the Gorge forming layers of rock up to 2,000 feet (600 meters) deep. Look at the cliffs in the Gorge. Can you see these layers?
As the lava cooled it formed a dark gray rock called basalt. Many of these lava flows cooled into columnar basalt; the lava cracks, forming six-sided columns. As you look for lava layers, notice that some contain columnar basalt.
If you look closely at a columnar layer, you might notice it is divided into two parts. At the bottom, the lava cooled slowly forming regular, widely spaced columns. Higher up, it cooled rapidly creating a jumbled looking mass of irregular, closely spaced columns.
The Birth of the Gorge
2 million to 700,000 years ago (Pleistocene)
Hundreds of volcanoes erupted in the Cascade mountain range. You can still see the 14 major peaks and hundreds of smaller peaks and cinder cones that form the range. Near Hood River, Oregon, you see dramatic views of Mount Adams and Mount Hood. Both are dormant volcanoes that could erupt within the next 50 years.
During this period, the Cascades began to uplift. As the mountains rose, the Columbia River carved out a deep gorge. This is the only near sea-level passage through the Cascades.
The Missoula Floods
16,000-14,000 years ago (Pleistocene)
Did you know that the largest floods to occur on the planet happened here? During the last ice age, ice sheets covered much of Canada. One lobe of ice grew southward, blocking the Clark Fork Valley in Idaho. This 2,000 foot (600 meters) high ice dam blocked the river, creating a lake that stretched for hundreds of miles. When the lake was full, it contained 600 cubic miles (2,500 cubic kilometers) of water. How much is that? Imagine a block of water a mile high (as high as the mountains around Bonneville Dam), a miles wide, and stretching from Bonneville Dam to San Francisco!
Eventually, water traveled under the ice dam. The water drained out of the lake in two or three days, flooding eastern Washington. The flood, moving up to sixty miles per hour, scoured out hundreds of miles of canyons called coulees, created the largest waterfall to ever exist, and left 300 foot (90 meter) high gravel bars. At Bonneville, the water crested at 650 feet (200 meters). If you look on the cliffs southeast of the dam, you will see a transmission tower (the one with three poles) that is 200 feet (60 meters) above the high water mark.
During a period of 2,500 years as many as 100 of these floods scoured the Gorge.
Sliding into History
500 years ago
Near Bonneville, the lava layers making up Table Mountain slid into the Gorge. This series of four landslides, covering five square miles, blocked the Columbia River. The Second Powerhouse butts against this landslide. If you look north of the dam, you can see cliffs exposed after the mountain gave way.
Original inhabitants of the area may have marveled at the 200 foot (60 meters) high landslide blocking the Columbia. They could have crossed on foot, possibly giving rise to a story about “The Bridge of the Gods”. This natural dam created a lake that stretched almost seventy miles (up to the present day John Day Dam). After a few months, the Columbia rose high enough to wash through the southern side of the landslide creating a flood of water that was 100 feet (30 meters) deep at Troutdale.
Things returned to normal, except the river was displaced a mile to the south and a set of rapids, the Cascades, had formed. In 1938, the rapids disappeared under water rising behind Bonneville Dam. The only hints of their existence are the remnants of a navigation lock at Cascades Locks built in 1896 to allow boats around the rapids.
The Gorge is still changing. In the winter of 1996, landslides similar to the Bridge of the Gods landslide destroyed homes in Warrendale. At milepost 35 on I-84 you can see this damage.
Video: Forged Through Time, The Creation of the Columbia River Gorge
Geologic History of the Columbia River Gorge, As Interpreted from the Historic Columbia River Scenic Highway (Link Coming Soon)
The Magnificent Gateway: A Layman’s Guide to the Geology of the Columbia River Gorge