The confluence of two rivers in west-central Nebraska, the North Platte and the South Platte, is revealed near the center of this west-looking, low-oblique photograph. Lake McConaughy on the North Platte River is seen near the northwest corner of the photograph. The river is 680 miles (1095 kilometers) long and originates in the Park Range of northern Colorado. Its waters are used for crop irrigation and grazing livestock in the area from near Guernsey, Wyoming, to below Bridgeport, Nebraska. North of Lake McConaughy and the North Platte River are the Nebraska Sand Hills (visible in the photograph), a maze of grass-covered dunes formed during the last glacial epoch. The South Platte River originates in the Rocky Mountains of Colorado northwest of Denver. After leaving the mountains and reaching the Great Plains, the river becomes a broad, shallow stream. It supplies water for cattle grazing and irrigation of agricultural crops (irrigated field patterns lie south of the river in the photograph). At the confluence of the two rivers near the city of North Platte, the river becomes the Platte and flows eastward 310 miles (500 kilometers) before joining the Missouri River south of Omaha. Much of the Platte River is used for irrigation and the production of hydroelectric power.