Whitewater rapids break a river’s smooth ﬂow into descent and chaos, then restore a civilized ﬂow line again. Waves express the energy of motion through each stage of this passage. From silken tongue to runout, you can follow this sequence through a rapid: lateral, or diagonal waves, when the river constricts and its energy is poised between slow and swift ﬂow; a roiling, erratic turbulence in the rapid’s frothing heart; then stationary, standing waves formed as the river’s current slows over deep water. A rollercoaster series of standing waves is known as a wave train. Elongated standing waves, with spray peeling off their crests, are called rooster-tails. Large, symmetrical standing waves are haystacks. In big water their slopes can rise steeply enough to block out the downriver view and your boat will slide right off their face.