by Claudine Zap The artist Christo " the Swiss conceptual designer who put 7,000 gates in Central Park and wrapped the German parliament building in silver-colored fabric " has his sights on the Arkansas River for his last big project. The 75-year-old would like to temporarily suspend silvery fabric above a 42-mile section of the river in southern Colorado. For two weeks, drivers could follow the silver streak in their cars, and those in rafts could float underneath it,