Ancestral Upper Colorado River System

Stream capture may account for the fact that the upper Colorado River east of the Grand Canyon appears to have once flowed in the opposite direction, probably fed by the Little Colorado River. The Little Colorado most likely drained mountain highlands in eastern Arizona, ultimately draining into an inland lake in the vicinity of the present Rocky Mountains.

According to the stream capture theory, around 70 million years ago what is now the upper Colorado River east of the Grand Canyon actually flowed in the opposite direction, fed by the Little Colorado River. The Little Colorado drained mountain highlands in eastern Arizona, ultimately draining into an inland lake in the vicinity of the present Rocky Mountains.

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