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" . - 10 - force which signifies the hydraulic stress exerted by the flow on the bed ~ of the stream. This average bed-shear stress is obtained as the product ~ of the specific weight of the fluid, hydraulic radius, and energy gradient ~ slope. Thus, greater depth results in a greater bed shear and a greater ~ ~ potential 'for moving sediment. By the same reasoning, steep slopes (the energy slope and bed 'slope are assumed to be equivalent) also result in high bed-shear stress. . The boundary between sediment yield classifications in much of the Pacific Southwest maybe at the mountain front, with the highest yield designation on the alluvial plain if there is extensive channel eros,ion. In contrast, , many mountain streams'emerge from canyon reaches and then spread over fans or valley flats. Here water depths can decrease from many feet to only a few inches in short 'distances with a resultant loss of the capacity to'trans- port sediment. Sediment yield of the highest, classification can, thus drop to the lowest in such a transition from a confined channel to one that has no definition. Channel bank and bed composition may greatly influence the sediment yield of a watershed. In many areas within the Pacific Southwest, the channels in valleys dissect unconsolidated material which may contribute significantly to the stream ,sediment load. Bank 'sloughing during periods of flm., ,as well as during dry periods, piping, and bank scour generally add. greatly to the sediment load of the stream and often change upward the sediment yield class- ification of the watershed. Field examination for areas of head cutting, aggradation or degradation, and bank cutting are generally necessary prior to classification of the transport expectancy of a stream. Geology plays a significant role in such an evaluation. Geologic controls in channels can greatly affect the stream regimen by limiting degradation and headcuts. Thus, the transport capacity may be present, but the supply of sediment from this source is limited. Man-made structures can also. greatly affect the transport characteristics of the stream. For example, channel straightening can temporarily upset the channel equilibrium' and cause an increase in channel gradient and an increase in the stream velocity and the shear stress. Thus, the sediment transport capacity of the stream may be temporarily increased. Structures such as debris dams, lined channels, drop spillways, and detention dams may drasti,- cally reduce the sediment transport.