EVIDENCE THAT THE COLORADO RIVER FLOWED THROUGH UNAWEEP CANYON

The controversy over which river carved Unaweep Canyon, an enigmatic wind gap in western Colorado, began with the 1875 Hayden Survey. Peale thought that it was the Gunnison River whereas Gannett said that it was the Colorado River. Cater (1966) offered the first physical evidence that the Gunnison River flowed through Unaweep when he attributed gravels located downstream from Unaweep to the Gunnison River. He based his identification on the presence of abundant intermediate-composition volcanic clasts (common in the modern Gunnison River) and the scarcity of vesicular basalt. Recently Kaplan (2005, 2006) identified several additional outcrops of ancient river gravel downstream from Unaweep Canyon. These gravels also contain abundant intermediate volcanic rocks and only a small amount of vesicular basalt. She used these criteria to conclude that only the Gunnison River flowed through the canyon.

However, Kaplan's highest terrace contains clasts of dark red siltstone that look identical to clasts of dark red siltstone found in the Colorado River and which are nearly absent in the Gunnison River. A count of 511 clasts identified 13 of dark red siltstone, or 2.5 percent. This is well above the content of dark red siltstone in the modern Gunnison River (0.3 percent or less). Gravels at Cactus Park at the upstream end of Unaweep Canyon are generally considered to be of Gunnison River origin and represent the last material that the river was transporting through Unaweep Canyon. This gravel contains 0.25 percent dark red siltstone and could not have been the source of the more abundant dark red siltstone clasts in Kaplan's highest terrace. In comparison, clast counts in the Colorado River below its confluence with the Gunnison indicate the gravel contains between 3 and 4 percent dark red siltstone, values similar to those of the high terrace.

These results are consistent with the interpretation that the uppermost terrace below Unaweep Canyon contains a significant component of Colorado River gravel and the Colorado River must have flowed through Unaweep Canyon for part of its history.