From The Mountain Town News (Allen Best):
Transmountain diversions from Colorado’s Western Slope to cities and farms east of the Continental Divide range from 450,000 to 600,000 acre-feet a year
How much water is that? It represents just 4.8 percent to 6.9 percent of all waters from the Western Slope, including not just the Colorado River but tributaries whose waters flow into the river outside Colorado: the Yampa, the Dolores and San Miguel, the Animas, and the San Juan.
But on the main stream of the Colorado River, upstream from Grand Junction (and excluding the Gunnison), the dent is much more severe. Half of all water available for consumption is taken from the basin in diversions ranging from Grand Lake to Aspen.
More transmountain/transbasin diversions coverage here.