A waterfall in a slot canyon filled with boulders, caught between the walls of a sedimentary drainage, a slot canyon.? Not the place to be when something large enough to dislodge these comes along.? Such defiles claim lives too regularly in the Canyonlands of Utah, where storms over the horizon can send torrents of water 'out of nowhere'. As the Colorado River descends from an elevation of about 3100 feet at Lee's Ferry, some 280 miles to Lake Mead (1200 feet); it is also a journey back in tim