If a rifﬂe is a murmur and a cataract a roar, rapids are all the river voices in between. Rapids begin at a nickpoint, a place where the gradient of a river tips more steeply downward and the current becomes swift, turbulent, and broken. Whitewater rafters learn to look for rapids where side canyons debouch into the main channel, for these are likely places for ﬂoods to have washed heavy debris into the river, damming it slightly and creating a nickpoint. The intensity and character of rapids vary greatly with water level. Some rapids become more dangerous as ﬂows increase; others ﬂood out and become calmer. Anyone who has experienced the thrill and fear of shooting powerful rapids will feel the anticipation in these words of John Wesley Powell: “At last we ﬁnd ourselves above a long, broken fall, with ledges and pinnacles of rocks obstructing the river. . . . The rushing waters break into great waves on the rocks, and lash themselves into a mad, white foam.” Powell was describing a run of rapids in the Grand Canyon that he and his men appropriately named Sockdolager. In the vernacular of his day, a sockdolager was a roundhouse, knockout punch.