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The term WMsh is commonly used to denote an alluvial valley floor. Such a valley can be thought of as an alluvial fan which is restricted in extent by the proximity of rock walls. Washes are found both in the section of ravine immediately above the apex of a fan and as separate formations where the channel draining the mountain watershed remains confined until it reaches a large river. Hence, washes are typically long. narrow formations. Contour lines on a wash are typically linear and perpendicular to the confining canyon walls. A series of fans often form along the front of steep mountain ranges where numerous small watersheds are drained by individual streams. As these fans expand out onto the valley floor. the edges, or toes. of the fans coalesce into an alluvial aoron. This apron area is characterized by nearly linear contour lines and a series of parallel ravines or arroyos. which drain the apron. Figure 3.1 shows two fans and an apron in the Wenatchee, Washington area and illustrates the major features of these landforms. Further discussions of alluvial landforms can be found in Ritter (1973) and Scott (1973). flood flows on the three landforms behave quite differently, due to the obvious differences in morphology. Floods on washes are confined by the canyon walls and attain high velocities and depths of flow. The path of floods on a wash tends to be stable and predictable, so that the flood plain is often well defined. If a wash is very wide however, braiding and meandering of the channel may occur in a manner similar to that in river valleys. Sediment deposition tends to occur uniformly across the wash and permanent incision of one channel is rare. Flooding on alluvial aprons is arroyos which drain the apron. generally limited to the The characteristics of flow 25