The lower Dolores River may have once been the original course of the Colorado River, which flowed through the now dry Unaweep Canyon, currently occupied by West Creek, a small tributary of the Dolores. When the Uncompahgre Plateau was formed it diverted the larger Colorado northwards through what is now the Grand Valley, looping around through Westwater Canyon to the confluence with the Dolores in eastern Utah and leaving Unaweep Canyon as a huge dry gap across the plateau. However, some geologists contend that the Colorado never flowed through Unaweep and the lower Dolores River, as the erosive force of the river should have created a water gap here; instead, the canyon may have been formed by glaciation during the Paleozoic.