Origin of Coarse Sediment in Lodore Canyon

Robert Thompson and Brooke Eustis

After paddling and rowing through two days of Browns Park flat water, our first glimpse of the Gates of Lodore was a welcome sight. The transition from an alluvial restricted meander reach to a debris fan dominated canyon was a jolting one. Rolling hills and gentle sand islands suddenly became towering, jagged canyon walls and massive boulders. A riverbed of sand and mud has transformed into sand, gravel, cobbles and boulders.


Figure 1. Entering the Canyon of Lodore (Photo by Robert Thompson)

Where have all these rocks come from? One likely wonders this as they float from the Brown’s Park Valley into Lodore Canyon, since they were obviously not carried through the low gradient, sand dominated reach of Brown’s Park. The answer lies in the canyon walls, made of resistant red quartzite walls rising high above the river-carved canyon. The original sand of these quartzite walls was deposited in the Precambrian period (2500 to 570 million years ago) and subsequently compressed by overlying deposits and the same compressional forces that formed the Uinta Anticline. The sands were slightly metamorphosed forming the quartzite walls of the Uinta Mountain Group we see today.

The process of erosion of the quartzite creates magnificent scallops in the canyon walls (Figure 2). When pressure is released from the quartzite by erosion of overlying units, the quartzite expands slightly because it is no longer at a stable state at low pressures. As expansion occurs, joints form and sheets of quartzite fracture off parallel to the canyon wall. This process is called exfoliation. In Lodore Canyon, scallops are most impressive higher in the canyon wall where they have been exposed for longer and have less overriding pressure. Large amphitheater- scale scallops are magnificent to anyone who passes through Lodore Canyon.


Figure 2. Scalloped walls in Lodore Canyon (Photo by Brooke Eustis)

Erosion of these walls creates boulder, cobble, and gravel-sized sediment which can be transported to the base of the canyon walls and river channel via gravity as talus or carried as debris flows—large flows of unconsolidated sediment that can occur after periods of intense precipitation (Figure 3).


Figure 3. A large debris flow in the Canyon of Lodore (Photo by Julia Halverson)