The Black Canyon of the Gunnison needs a good scrubbing. Ever since 1965, when the first of three dams blocked the Gunnison River upstream of the western Colorado national park, the 2,000-foot-deep canyon has missed the regular spring floods that are necessary for its ecological health. Tamarisks and other plants have taken over sandbars that were once swept clean every year or two. "You can actually date some of the trees down there to right when one of the dams was put in," says park ranger Danguole Bockus. But more regular spring cleanings are on the way. Thanks to a mostly finalized agreement between the National Park Service, the state of Colorado and environmental groups, the Black Canyon will get an annual peak-flow event -- a one-day manmade flood -- plus 85 days of high flow timed to coincide with spring runoff. The rest of the year, the river