At the end of August I was invited to go on a private river trip through Hell’s Canyon on the Snake River. This trip had become visible on my radar just a couple of years earlier and so I jumped at the opportunity to something completely different than the American Southwest. Enjoy the ride downstream with me!
The trip began right below Hell’s Canyon Dam with a height of 310 feet. A much larger dam (710 feet high) had been proposed to inundate much of the scenery seen on this river trip. But a compromise with conservationists included this dam and two others immediately upstream in order to preserve the most scenic sections of the 652,488 acre Hell’s Canyon National Recreation Area.
A view of the put-in below the dam. It is quite tight bringing a vehicle down from the USFS Visitor Center and the ramp is steep and narrow.
Location on the border between Idaho and Oregon.
Two other large rivers parallel the Snake River, the Imnaha to the west (shown left of the words “Hells Canyon) and the Salmon to the east (far right). Later in this point I will show the confluence of the Salmon with the Snake.
Legend for the map above.
Here is a more obvious view on river right a bit further downstream. This terrace formed during the Bonneville flood, when Pleistocene Lake Bonneville spilled over its lowest rim and in a matter of a few days was lowered 300 feet. All of the runoff came through Hell’s Canyon!
Map of Lake Bonneville and the path of the Bonneville Flood during the Pleistocene, some 14,500 years ago. Lake Bonneville began to overtop its rim in Red Rock Pass in southern Idaho and spilled northward toward present-day Pocatello Idaho where it entered the Snake River. Map taken from Fallschirmjäger – based on a map by Laura DeGrey, Myles Miller and Paul Link of Idaho State University, Dept. of Geosciences. https://commons.wikimedia.org/w/index.php?curid=25056130.
The Columbia River Basalts began to appear just before we neared Pittsburgh Landing. This area marks the end of the Wild portion of the Snake River. The Columbia River Basalt Group records an epic effusion of lava flows covering 81,000 square miles in the American Northwest. Eruptions were mostly between 17 and 14 Ma but smaller eruptions continued until about 6 Ma. These layered lavas sat on top of the basement rocks we encountered along the rivers edge.
Near Pittsburgh Landing, we saw a Cascade Range ash bed in the riverside alluvium. I am unaware of the provenance of this light-colored ash bed but our river guidebooks mentioned that ash from Mt. Mazama (Crater Lake) was present in the area.
Below Pittsburgh Landing the canyon became narrow once again as the Seven Devils Group was exposed. See an early USGS Professional Paper about this package of rocks here. These volcanic, volcaniclastic and marine rocks are Late Permian to Triassic in age and have been interpreted as being formed in the Panthalassic Ocean (early eastern Pacific Ocean), perhaps to within 18º of the equator (north or south). These rocks were then transported tectonically to the north, ultimately colliding with the ancient western edge of North America. Some geologists ascribe these rocks as being part of the Wrangellia Superterrane.
In the distance looking downstream, note how far down the Columbia River basalts are. I wondered if maybe they have been erupted onto paleotopography? Another likely scenario is that the rocks were faulted low.
This is the confluence with the Salmon River, seen coming in from the east (left). The Snake River is shown on the right.
A large fire had scared the Idaho side of the river all the way from the confluence with the Salmon River and the take-out, some 17 miles of river.