Plunging along the boundary between two colliding slabs of the earth's crust, one of Asia's mightiest river systems has yielded up the last of its hidden jewels - a waterfall that has tantalized Western explorers for much of this century. Written off by earlier expeditions as an object of religious myth, the waterfall on the Tsangpo River, which becomes the Brahmaputra once it crosses into India, is a toddler by global standards. It drops only 100 to 115 feet. The find, however, confirms the cataract's existence and closes a five-mile gap in the West's exploration of the world's deepest and perhaps most biologically diverse gorge. The gorge's discovery marks the end of a five-year quest for expedition leader and Buddhist scholar Ian Baker. He, colleague Hamid Sardar, and Ken Storm Jr. had undertaken an initial search in 1993, but came up short. Last year, after several more tries, Mr. Baker, Mr. Sardar, and Mr. Storm marshaled their efforts for another attempt. On Nov. 8, the trio and their guides found the falls. The discovery, announced last Friday by the National Geographic Society, which funded the trip, required team members to steep themselves in Tibetan lore. Over the centuries, says Storm, Tibetan monks had evolved sacred guidebooks that mentioned the falls, one of 70 cataracts along the river, each hosting its own deity. The inaccessible region long has attracted pilgrims and local hunters near the roof of the world. As last year's expedition was about to step off, the team members visited a local monastery to ask a blessing. "The monk told us he had seen us on several previous visits and knew that we were pilgrims, traveling as Tibetans travel, so he began to tell us these wonderful stories relating to the great waterfalls," Storm says. Clues also came from records of the last major Western expedition before World War II, led by British botanist Francis Kingdon-Ward. He discovered Rainbow Falls, a 70-foot waterfall only a quarter of a mile upstream from what Baker and his team have named Hidden Falls. But Mr. Kingdon-Ward's view downriver was obscured by a spur jutting out from the 4,000-foot cliffs - ramparts that rendered Hidden Falls inaccessible to Kingdon-Ward's group. Storm says he hopes the botanical riches that drew Kingdom-Ward to the gorge in 1924 will spur moves to preserve this region. Although only 20 to 25 miles long as the crow flies, the cleft hosts a range of vegetation, from rhododendron forests to stands of hemlock, fir, and spruce that cling precariously to the cliffs, which in places vault more than 16,000 feet.