The cover article in this month's GSA Today [right] by UA geosciences prof Jon Pelletier, concludes that 1.5 km of erosion from the middle Miocene to the present could have been caused primarily by an increase in the intensity of snowmelt flooding. Snowmelt became a larger fraction of river discharge due to climate cooling. Jon found that a 4-fold increase in sediment flux is reasonable in mid- to high-elevations, and is consistent with deposition of the Ogallalla Fm to the east, one of the great aquifers of the world.
The full article can be read online.
Ref: Pelletier, Jon D., 2009, The impact of snowmelt on the late Cenozoic landscape of the southern Rocky Mountains, USA, GSA Today, V19, #7, pp4-10, doi: 10.1130/GSATG44A.1