Oregon Idaho
The Grand Canyon is the most famous canyon in North America, but it’s not the deepest. That distinction goes to Hells Canyon, a 7900-foot-deep chasm on the border between Oregon and Idaho. From high above, it looks like a narrow rip through layers of jagged, dark rock. The rocks that make up the walls of Hells Canyon have a diverse geologic history that includes tectonic crashes, large-scale volcanism, and catastrophic floods.
Between 130 million and 17 million years ago, a series of tectonically driven collisions with offshore landmasses and islands helped build the west coast of North America. This time period was also punctuated by vigorous volcanic activity in what is now the Pacific Northwest, including Oregon, Idaho, and Montana. The tremendous flood basalts produced by these widespread eruptions resulted in volcanic flows so thick, they formed a high plateau called the Columbia River Basalt Group. Around 6 million years ago, the Snake River changed course and began cutting down into this plateau, forming Hells Canyon. Originally, the river ran south, but at some point, uplifting tectonic forces tipped the flow of the river toward the north. Most of the canyon carving took place over the last 2 million years, a result of melting glaciers and most recently because of a single catastrophic event, the Lake Bonneville flood.
One of the best ways to see Hells Canyon is from the Snake River, which cut this deep canyon over the past 6 million years.
The Snake River lives up to its sinuous name through the deep gorge of Hells Canyon. The benches above the river were carved out all at once by the Lake Bonneville flood, around 14,500 years ago.
Some 30,000 years ago, Lake Bonneville covered a huge region in the vicinity of Utah’s Great Salt Lake. Around 14,500 years ago, geological activity caused the lake to take on waters from a redirected river, raising Lake Bonneville’s water level to the point that it topped a natural dam at Red Rock Pass in what is now eastern Idaho. The resulting megaflood is the second-largest flood in the geologic record; more than 15 million cubic feet per second of water poured over the Snake River Plain at speeds up to seventy miles per hour. When this wall of water reached the already deep and narrow Hells Canyon, the erosive hydrologic power significantly increased the size of the canyon, creating the impressive passage we see today.
The canyon’s oldest rocks are exposed in the northern stretches of the canyon, where the Snake River has cut deeper into the geologic layers, revealing 300-million-year-old volcanic rocks that originally erupted at the bottom of the Pacific Ocean. Younger coral reefs built on top of these volcanic layers are seen in the middle stretches of the canyon, where steep gray limestone walls line the river. Also exposed are remnants of the islands that had earlier collided with and helped build the west coast of North America.
Hells Canyon appears in the middle of this satellite image, running north to its junction with the Imnaha River.
The narrow gouge in the terrain that is Hells Canyon can be seen on flights to Lewiston or Boise, Idaho. The canyon creates a portion of the border between Idaho and Oregon.
