This caught my attention because I remember the famous Ansel Adams Snake River picture.

It seems geologists are still debating what formed the river. The eastern and western halves are apparently very different geologically. Although, from what is said in this article it seems that it's from a boundary fault forming the western basin and the hot spot (which is now Yellowstone) moved NE and lifted the eastern section.
Source(s):
http://imnh.isu.edu/digitalatlas/geo/snkrvpln/snkrvpln.htm

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