Moses Coulee course southward past the Frenchman Hills and turning west to run along the north face of the Saddle Mountains and rejoin the previous and modern course of the Columbia River just above the main water gap in the Saddle Mountains, Sentinel Gap. Although the Columbia River flowed only for a short period through Moses Coulee, this period included one or more of the tremendous flows from the Missoula Floods. As the Okanogan lobe melted, the upper portions of Moses Coulee were littered with clear evidence of its passing in the Withrow Moraine. The glacier left behind a blanket of glacial
Columbia River known as the Grand Coulee. After the floods, the river found its present course, and the Grand Coulee was left dry. The construction of the Grand Coulee Dam in the mid-20th century impounded the river, forming Lake Roosevelt, from which water was pumped into the dry coulee, forming the reservoir of Banks Lake. The river flows past The Gorge Amphitheatre, a prominent concert venue in the Northwest, then through Priest Rapids Dam, and then through the Hanford Nuclear Reservation. Entirely within the reservation is Hanford Reach, the only US stretch of the river that is completely free-flowing, unimpeded by dams