The Clarks Fork of the Yellowstone in Montana and Wyoming is one of the nation's wildest rivers outside Alaska. It's also the most endangered, according to American Rivers. For three years running, it's topped the group's annual report, North America's Ten Most Endangered and Threatened Rivers. The reason: Plans for the proposed New World Mine include a tailings reservoir at the river's headwaters that could contaminate the river with acid waste. The group's top ten also listed five other Western rivers in peril: California's American River, the Northwest's Columbia River, Oregon's Elk River, Arizona's Pinto Creek and the Southwest's Animas River. "These rivers are in trouble not simply because of development, but because of reckless development practices - dams that block fish passage, irrigation practices that waste water, and careless grazing and logging along river banks," says Lorraine Bodi of American Rivers. For a copy of the report, send $10 to American Rivers, 1025 Vermont Ave., N.W., Suite 720, Washington, DC 20005 (202/547-6900). An electronic version is free on the world wide web: http://www.amrivers.org/amrivers/.