 The cutthroat trout is thought to have evolved over the past two million years from other Oncorhynchus species which migrated up the Columbia and Snake river basins to the Green and Yellowstone river basins.Within the past 20,000 years, a population which crossed the Continental Divide during the most recent ice age gave rise to the greenback cutthroat.The greenback cutthroat trout today is found east of the Continental Divide in the cold, clear foothill and mountain waters of the Arkansas and South Platte Rivers.