Colorado's central and southern mountains still draw droves of skiers to the slopes. However, many of the historic runs and areas that were popular over the past century--some near the current resorts of Aspen, Breckenridge, Crested Butte, Purgatory, Telluride and Vail--no longer exist. Local hills like Whittaker Ranch near Eagle featured little more than a rope tow and warming hut. Now underneath Lake Dillon, Prestrud Jump hosted tournaments where...