Mouth Bar

a ledge that is located at the floor of a tributary valley of a mountain river and rises above the floor of a main valley that has been scoured by a glacier. Mouth bars form because glacial erosion in the main valley was more rapid than in the tributary valley. They reach a relative elevation of 200–300 m. Streams that flow over a mouth bar form rapids or waterfalls. Mouth bars are frequently subjected to the postglacial erosion of streams, and they appear as terraces on the slopes of the mouths of tributary valleys.