An eddy is a swirling of water or air in a direction contrary to that of the main ﬂow within a larger system. Eddies refer to the swirling currents within high- or low-pressure zones in the atmosphere as well as to river features that develop where the current encounters an obstruction and turns back on itself upstream of the main current. Those that form a completely circular system are called back eddies, and in the right conditions can form whirlpools. Ann Zwinger, in Downriver, describes an eddy in the Grand Canyon this way: “The bore of fast water from the rapid emerges and encounters the slower water along the edge of the river, a shear line forms that sets the slower water spinning the opposite direction; the eddy ﬂow is ﬁrmly upstream.” Water in eddies ﬂows more slowly than the main stream, averaging a foot per second, and in smaller streams the silt and sand thus transported settle into spits and bars, which in turn evolve into spawning beds for trout and salmon. Eddies are also signiﬁcant resting places for waterfowl. As a result, techniques for rebuilding streambeds to encourage the formation of eddies are important to artisans restoring wildlife habitat.