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" - 9 - 1'1.) QJ ~ Channel Erosion and Sediment Transport If a stream is ephemeral, runoff in traversing the dry alluvial bed may be drastically reduced by transmission losses (absorption by channel alluvium). This decrease in the volume of flow results in a decreased potential to move sediment. Sediment may be deposited in the streambed from one or a series of relatively small flows only to be picked up and moved on ina subsequent larger flow. Sediment concentrations, determined from field measurements at consecutive stations, have generally been shoWn , to increase many fold for instances of no tributary inflow. Thus, although water ,yield ,per unit area will decrease with increasing d~ainage area, the s€diment yield per unit area may remain nearly constant or may even increase with increasing drainage area. In instances of convective precipitation in a watershed wit~ perennial flow, the role of transmission losses is not as significant as in watersheds with ephemeral flow, but other channel factors, such as the shape of the channel, may be important. For frontal storm runoff the flow durations are generally much longer than for convective storms, and runoff is often generated from the entire basin. , In such instances, sediment removed from the land surfaces is generally carried out of the area by the runoff. Stream channel degradation and/or aggradation must be considered in such cases, as well ,as bank scour. Because many of the stream beds in the Pacific Southwest are composed of fine-grained alluvium in well defined channels, the potential for sediment transport is limited only by the amount and durat:ion of runoff . Large volumes of sediment may thus be moved by these frontal storms because of the longer flow durations. The combination of frontal storms of long duration with high intensity and limited areal-extent convective activity will generally be in 'the ,highest class for sediment movement in the channels., Storms of this type generally produce both the high peak flows and the long durations necessary for max- imum sediment transport. Sediment yield may be substantially affected by the degree of channel develop- ment in a watershed. This development can be described by the channel cross sections, as well as by geomorphic parameters such as drainage density, channel gradients and width-depth ratio. The effect of these geomorphic parameters is difficult to evaluate primarily because of the scarcity of sedi- ment transport data in the Pacific Southwest. If the cross section of a stream is such as to keep the flow within defined banks, then the sediment from an upstream point is generally trans?orted to a downstream point without significant losses. Confinement of the flow within alluvial banks can result in a high erosional capability of a flood flow, especially the flows with long return periods. In most channels with wide floodplains, deposition on the floodplain during floods is often sig- nifiCant, and the transport is thus less than that for a within bank flow. The effect of this transport capability can be explained in terms of tractive