Christo and his late wife, Jeanne-Claude, first achieved worldwide fame with "Valley Curtain," a project they carried out in Colorado in the 1970s. For that massive work, the pair suspended an enormous drapery across Rifle Gap in the western portion of the state. Thirty years later, they again looked to Colorado for what will surely be one of their last pieces, "Over the River." To explicate and promote this proposal, the Museum of Contemporary Art Denver presented a spectacular show of Christo's stunning presentation drawings. If the project comes to fruition (the final decision is due this spring), "Over the River" would comprise canopies of silvery fabric stretched across the Arkansas River in southern Colorado. And judging from the Over the River show, Colorado could be in for one of the best art events in its history, one that would rank right up there with "Valley Curtain."