COMPARATIVE GEOMORPHOLOGY OF UNAWEEP CANYON AND THE BLACK CANYON OF THE GUNNISON, WESTERN COLORADO

The Black Canyon of the Gunnison is an unusually deep and narrow gorge cut into crystalline Precambrian rock by the Gunnison River, whereas the abandoned Unaweep Canyon through the Uncompahgre Plateau is of unknown and controversial origin. Unaweep is larger and more U-shaped with a central drainage divide between East and West creeks, but evidence of a deep sediment fill suggests that the Black Canyon may be a suitable analog for comparison. Similarities include deep incision into Precambrian, and the fact that Unaweep is believed by many to have been carved by the Gunnison River, although the Colorado may have played a part as well.

The Black Canyon system was used as a model to better understand the morphological history of Unaweep Canyon. River gravel composition data was collected from a variety of locations and elevations from both canyons, revealing no evidence of the Colorado assisting the Gunnison in its incision of Unaweep. Ancient river gravel locations as well as modern river gradients allowed us to reconstruct the gradient of the Gunnison in Unaweep and the depth of the sediment fill. The volume of Precambrian rock eroded from the Unaweep is greater than 4.5 trillion cubic feet, which is slightly more than a four times the amount removed from the Black Canyon.

Based on a University of Oklahoma core hole and the modern gradient of the Gunnison in Black Canyon, we determined that Unaweep Seep may not represent the lowest point of sediment fill in the canyon. Fill may exist at lower depths but be dammed by the fault bounding the Ute Creek Graben. This projection suggests nearly 20% more fill, and potentially groundwater, than previously believed.

Neotectonic uplift of the graben may have been responsible for damming the Gunnison in Unaweep, causing it to pond and eventually be captured. A narrow channel in Precambrian rock is associated with a dramatic increase in the gradient of West Creek, an underfit stream in modern Unaweep. It suggests a spillway from the ancient lake that formed when the Gunnison ponded and deposited much of the canyon's sediment fill.