Canoeing in Nebraska? Isn't it terminally flat, dry and boring in the Cornhusker State? Prepare to stand corrected while exploring one of America's largely undiscovered ecological Brigadoons: the clear, cool waters of the Niobrara National Scenic River. It riffles beneath towering bluffs through a pine-banked valley along the northern edge of Nebraska's vast, awesomely empty Sandhills region. The river is only 2 to 3 feet deep on average, and only knee- to hip-high during summer. The Niobrara (the word means "running water" in the Sioux language) is one of 180-odd National Scenic Rivers across the United States. Originating in the High Plains of eastern Wyoming, it's little more than a creek for most of its 435-mile journey to northeastern Nebraska and its confluence with the Missouri. The National Scenic River stretch, designated as such in 1991, extends 76 miles east of Valentine and includes nine miles within the Fort Niobrara National Wildlife Refuge, where the river valley remains its most pristine. The primary reason the Niobrara was awarded its protected status stems from its unique biology and ecology. The Niobrara also remains remarkably unpolluted and almost all free-flowing--only three dams on the whole river--somehow having escaped the dam-builders and developers who have straitjacketed so many of America's once-unfettered waterways. Another Niobrara notable: waterfalls. There are said to be 230 (most unnamed) in one 30-mile stretch, and new ones still are being discovered. They're fed by clear waters that filter through the region's sandy soil and rock (there's little soil to sop it up in these Sandhills).