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• <br />PARSONS <br />of discrete, interconnected channels which are separated by bedrock or by expanses of <br />stable alluvium. While braided channels are primarily depositional forms, anastomosing <br />channels are essentially erosional in nature, since material between the channels is <br />resistant to transport, except by exceptional flows (Summerfield, 1991). <br />Four major fluvial types — equiwidth with point bars; wide -bend with point bars; <br />braided with point bars; and braided, no point bars — can be distinguished primarily on the <br />basis of variability of channel width, nature of point bars, and degree of braiding (Figure <br />Al -1, after Brice [1982]). Any of these stream types may be anabranched, locally or <br />generally (Brice, 1982). A stream is unlikely to be of the same type throughout its length. <br />Most streams change pattern in a downstream direction, and some change from place to <br />place along their courses. For example, any meandering stream is likely to include some <br />locally straight or nearly straight reaches. <br />The properties of each stream type, as indicated by the dimensional characteristics of <br />width, depth, and sinuosity (Figure A1-1), change gradually from one to the next, and <br />some properties have a definite trend. For a stream of a particular size, as measured by <br />bankfull discharge, effective discharge, or other hydraulic metric, channel width tends to <br />increase from equiwidth to braided streams, and sinuosity tends to decrease. The <br />dimensional properties associated with each stream type also have a relative degree of <br />stability associated with them. For example, the stability of certain features associated <br />with the equiwidth point -bar type of stream varies from "low" (channel width) to <br />moderately high (lateral stability), indicating that the channel width of an equiwidth <br />point -bar stream is particularly subject to change, but that the axis of the stream channel <br />is unlikely migrate great distances laterally. According to this classification system <br />(Brice, 1982), the dimensional stability of most features of braided streams range from <br />"moderate" to "low" (Figure AI-I). <br />Because channel properties are gradational from one stream type to the next, a <br />particular stream may lie at the boundary between two types. For example, it may be <br />difficult to decide whether a stream is of the equiwidth or wide -bend type if channel <br />width is greater at some bends than at others, or if the width at bends does not clearly <br />exceed the width at straight reaches. A borderline stream probably is gradational in <br />stability characteristics between two categories. <br />Although many alluvial channels can _be described as "stable," in that they currently <br />are not experiencing a dramatic change in form, some change is an inevitable element of <br />the behavior of all alluvial channels, since they are (at least partly) composed of material <br />which is eroded or deposited as the hydraulic stress exerted on the channel bed and banks <br />by the flowing water changes over time (Summerfield, 1991). A number of interrelated <br />variables, including discharge, flow velocity, sediment concentration and size, and <br />channel gradient, any or all of which can change in response to extrinsic factors (climatic, <br />tectonic or anthropogenic), or intrinsic factors (e.g., changes in erosion or drainage <br />patterns within the basin), will promote changes in channel pattern. Therefore, the <br />morphology of many or most streams may change with time. Such changes can occur in <br />SAES \WP\PROJECTS\3- States\Al Final Tech Memo.doc <br />-7- <br />