Float a serene river, all attention on the bank, on indigo buntings in emerald willows. Suddenly, the river lifts into a long, silken comb that ripples, falls, then rises and falls all over again. The tawny transverse waves migrate upstream, defying the downstream ﬂow, passing with the rushing sound of the river breathing. Sand waves are creatures of transient, underwater dunes. They are seen on high-volume, rapid-ﬂow, sand-laden waters, whose currents shape their load sediments into dunes on the channel bottom. As the dunes shift, grow taller, and move upstream, waves build above them, some with eight-foot-high crests. The sand waves reach a climax, then, as the dunes collapse, drop into a smooth sheet of river again. From ﬂatwater to full-amplitude waves to washout, the cycle lasts minutes. Sand waves are uncommon on most rivers, but renowned on the San Juan, Dirty Devil, and other southwestern rivers.