Abstract

The controversy about which river flowed through Unaweep Canyon in western Colorado has been ongoing since the Hayden Survey of 1875. Physical evidence that the Gunnison River flowed through the canyon was established in 1966 (Cater, 1966), but evidence that the Colorado River also was present in the canyon has been lacking until now. This paper reviews the history of thought and presents evidence that clasts of red, fine-grained sandstones (Pennsylvanian Maroon Formation?) found on a high terrace at the mouth of Unaweep Canyon near Gateway, Colorado, are identical to clasts that are commonly found in Colorado River gravels. The provenance identification of the Colorado River clasts is based on general megascopic and microscopic appearance. The grain size, framework-grain mineralogy, and cementation in the rocks are indistinguishable. Clasts of similar appearance are nearly lacking in Gunnison River gravels upstream from its confluence with the Colorado River.

The abundance of the clasts on the high terrace near Gateway, Colorado is similar to their abundance downstream of the confluence of the modern Colorado and Gunnison rivers. This indicates that the combined Colorado and Gunnison rivers flowed through Unaweep Canyon for at least part of its history. A lower terrace in the same area contains only half the abundance of clasts compared to the high terrace, and is interpreted to represent material reworked by the Gunnison River after the Colorado River had been captured and abandoned Unaweep Canyon.