Ancient drainage changes in and south of Unaweep Canyon, southwestern Colorado
S. W. Lohman
On the first day of the field trip our route takes us through spectacular Unaweep Canyon athwart the high Uncompahgre Plateau —the now "high and dry" abandoned gorge that I believe once carried the combined flows of the ancestral Colorado and Gunnison rivers. From Gateway, at the southwestern end of the canyon, we will go up the Dolores and San Miguel rivers to Uravan and Vancorum, then traverse Paradox Valley along most of its northwest-ward trending longitudinal axis. At about the middle of the valley, near Bedrock, Colorado, we will cross the Dolores River, which "paradoxically" cuts across the roughly canoe-shaped valley instead of following the longitudinal axis, as do most normal streams, hence the name Paradox Valley.
Evidence to be presented strongly suggests: that the Dolores River once joined the ancestral San Juan River to the southwest but was diverted northward from about the present town of Dolores, Colorado, to join the ancestral San Miguel River; that the combined flows of these two ancestral rivers once joined with the ancestral Colorado River at the southwestern end of ancestral Unaweep Canyon, near the present town of Gateway, Colorado; and that the ancestral upper Colorado River then joined the present Colorado River southwest of the present confluence of the Dolores and Colorado Rivers near Dewey Bridge, in eastern Utah.
Although most of the details of these suggested drainage changes have already been published, it is hoped that this brief summary, which includes some new findings, will be of interest to most participants on the field trip. As these possible drainage changes probably span more than 50 million years, they will be discussed in approximate chronological order.
As will be noted below, there is not universal agreement concerning some of these suggested drainage changes, and a new and quite different interpretation of the changes in and near Unaweep Canyon is given in this guidebook by Sinnock.
- Lohman, S. W., 1981, Ancient drainage changes in and south of Unaweep Canyon, southwestern Colorado, in: Western slope Colorado--western Colorado and eastern Utah, Epis, Rudy C.; Callender, Jonathan F., New Mexico Geological Society, Guidebook, 32nd Field Conference, pp. 137-144.