A NEW SURVEY OF THE GEOMORPHOLOGY OF BUCKSKIN GULCH, THE LONGEST SLOT CANYON IN NORTH AMERICA

Slot canyons are narrow bedrock canyons that are significantly deeper than they are wide and are cut by the erosive action of water. Due to the often-remote locations and difficult access, the dynamics and shape of these landscape features are poorly understood. Further, our understanding of how elevation decreases in the downstream direction is limited because of the inability of remotely-sensed digital elevation models to ‘see’ the canyon floor. That being the case, these stream segments must be studied entirely by means of fieldwork.

The main objectives of this study are 1) to perform the first survey of the longitudinal profile of the Buckskin Gulch slot canyon in southern Utah, 2) to investigate whether the knickpoints within the canyon are a result of headward erosion due to base level fall of the Paria and Colorado Rivers or from the presence of large boulders from the collapsing canyon walls, and 3) to establish an accurate baseline for the documentation of future changes in the canyon’s geometry due to floods that pass through the channel. The roughly 12 mile long canyon was surveyed over a two-day period from the confluence of Wire Pass and Buckskin Gulch to the confluence of Buckskin Gulch and the Paria River. A laser rangefinder, abney level, and a stadia rod were utilized to measure distance and elevation drop of over nearly 500 measurements in the canyon. With each survey segment, sediment size and canyon width were recorded.

Several knickpoints along the profile are results of boulders ranging in sizes from small vans to semi trucks that effectively block the stream channel. The significant knickpoints are located where these large sandstone blocks have collapsed into the canyon. Observations suggest that the elevations of the channel bed within the canyon rise and fall dramatically as rockfalls dam the channel and are then slowly cut out by the bedrock erosion processes. The lower 3 miles of the profile reveal a steeper, concave up trend that interrupts the smoother trend of the previous 9 miles upstream. This feature may be related to base level fall of the Paria and Colorado Rivers, which would result in headward erosion (although the aforementioned rockfalls likely isolate the upper watersheds from these ephemeral waves of incision).