ScienceDaily (Oct. 4, 2010) — Geologists have found evidence that some 55 million years ago a river as big as the modern Colorado flowed through Arizona into Utah in the opposite direction from the present-day river. Writing in the October issue of the journal Geology, they have named this ancient northeastward-flowing river the California River, after its inferred source in the Mojave region of southern California.
http://www.sciencedaily.com/releases/2010/10/101004112147.htm