It rises in southern Summit County, on the western side of the continental divide in the Ten Mile Range, near Quandary Peak. It flows north past Blue River and Breckenridge, then through the Dillon Reservoir near Dillon. The west portal for the Roberts Tunnel is at the base of Dillon Reservoir. The Roberts Tunnel is a trans-basin diversion, built by Denver Water in 1962, that diverts water under the Continental Divide from the Colorado River basin into the South Platte River Basin. The east portal is approximately one mile upstream of Grants, Colorado.