Grand Coulee the Ice Age Floods. Archeological records date human presence back to nearly the end of the Ice Age, but the raging torrents erased the land of clear evidence, leaving us to question who, if anyone, may have survived. With the end of the last glacial advance, the Columbia settled into its present course. The river bed is about below the Grand Coulee. Walls of the coulee reach in height. Grand Coulee is the longest and deepest of eastern Washington canyons. Its unique characteristics include a lower floor at the head of the channel than at its outlet and the widest
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