It rises on the north slope of the De Long Mountains, at the western end of the Brooks Range, north of the continental divide in the southwestern corner of the National Petroleum Reserve. It flows initially north, then generally east through the foothills on the north side of the range, broadening as it receives the inflow of many tributaries that descend from the middle Brooks Range. Along its middle course it forms the southeastern border of the National Petroleum Reserve. At the Iñupiat village of Umiat it turns north to flow across the Arctic plain, entering the western Beaufort Sea in a broad delta near Nuiqsut, approximately 120 mi (190 km) west of Prudhoe Bay.