During most of the Pliocene Epoch the ancestral Colorado River did not
flow past what is now Grand Junction; instead, it joined with the
ancestral Gunnison River about 10 miles southeast of the present city,
and the combined streams flowed southwestward across the slowly rising
Uncompahgre arch through what was later to be called Unaweep Canyon
(fig. 36). Southwest of the canyon, near the site of the present town of
Gateway, the ancestral Colorado River was joined by the combined flows
of the ancestral San Miguel River and the previously diverted ancestral
Dolores River, then it flowed northwestward to what is now the mainstem
of the Colorado River.
I have attempted to show my ideas of this ancient drainage system as it
may have existed in middle to late Pliocene time in figure 34. But the
stage was set for more spectacular drainage changes to follow.