The headwaters of the Marias River drain slopes of the Continental Divide in Glacier National Park and Lewis and Clark National Forest before flowing eastward through the broad, rolling plains of north central Montana. The mainstem forms at the junction of Two Medicine and Cut Bank creeks, 12 miles north of Valier, Montana. Approximately 50 miles east and south downstream, the Marias discharges into Tiber Reservoir, a long earth-core impoundment completed in 1956 for flood control, irrigation, recreation, and municipal purposes. Below the dam, the stream meanders through a floodplain from 1/2 to 1 1/4 miles wide, flanked by steep buttes ranging to 200 feet high. The channel in many places is shallow, with small cobble in swift-current areas. Wide meanders and a shallow gradient characterize the river to Dugout Coulee. Twenty-four miles farther on, at Sheep Coulee, the Marias River begins a journey through a 21-mile-long, gorge-like river valley, 300 to 400 feet below the average elevation of the adjacent plains. Its varied and scenic badlands and breaks areas range from 1 to 3 miles in width. The river is approximately 150 feet wide through this section. A mile down from the mouth of the Teton River, the Marias joins the Missouri River, near Loma, Montana, over 170 miles southeast of its sources.