Sentinel Gap is a water gap formed by the Columbia River in the Saddle Mountains , near Mattawa in Washington state . The gap is " a water gap where erosion by the Columbia River was able to keep pace with folding , faulting and uplifting across the Saddle Mountain anticline " . During Ice Age floods in which waters from the Channeled Scablands found passage to the Pacific Ocean here and at Wallula Gap , this opening was " repeatedly reamed out , which probably widened and steepened the walls of the gap " . Strandlines from the floods can be seen on the basalt walls of the gap
Ice Age floods
Missoula Floods
Sentinel Gap