Featured in this northeast-looking, low-oblique photograph are the Kingsley Dam and Lake C. W. McConaughy on the North Platte River, which separates the agricultural south from the Sand Hills to the north. The lake provides water for public water systems, irrigation, and recreation. The Sand Hills, formed during the ice ages and covering 20 000 square miles (50 000 square kilometers), is the largest sand dune complex in the Western Hemisphere. Most are transverse dunes—some as high as 300 feet (90 meters) and 10 miles (16 kilometers) long. Many Sand Hills valleys distant from major streams contain wet meadows or small, shallow lakes, some of which rise and fall with the fluctuating water table. The flat valleys of both the North Platte River and the South Platte River (bottom center) were followed by pioneers moving west from the eastern United States. The first transcontinental railroad also followed this route.