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PARSONS <br />Following failure of the slope, a long period of preparation ensues before failure can <br />occur again. Such intrinsic thresholds probably are common in natural systems <br />(Schumm, 1974). <br />These types of geomorphic thresholds may be inherent in the development of <br />landforms. Until the system has evolved to a threshold condition, adjustment of the <br />system will not occur. It is when thresholds are exceeded that things begin to happen, and <br />many apparently deleterious events may be nothing more than nature's way of <br />reestablishing a geomorphic equilibrium. Considerable experimental work and numerous <br />field observations support the concept of geomorphic thresholds, which has been used to <br />explain the development of arroyos, channel aggradation and degradation, and variations <br />in channel patterns in particular fluvial systems (Schumm, 1974). However, it may not <br />always be clear whether the system is responding to geomorphic thresholds or to an <br />external influence. <br />Implicit in the concept of threshold are the ingredients of cause and effect in nature, a <br />dual relationship that is basic to geologic thinking. Cause and effect are essential <br />components of geologic history, where the effects are commonly preserved in rocks, <br />sediments, or landforms, and the cause becomes the object of investigation and <br />interpretation. <br />In the braided planform characteristic of some fluvial systems, streamflow through a <br />particular channel cross - section occurs in multiple channels, divided by islands or bars <br />that consist of exposed accumulations of sediment (Brice, 1982; Summerfield, 1991). <br />Islands are usually vegetated, and are relatively long -lived features, whereas bars are less <br />stable, being composed of unvegetated sand or gravel. Bars within the channel have no <br />consistent relation to bank stability (Brice, 1982). Bars visible at normal stage indicate <br />that bedload, either sand or gravel, is a prominent part of the stream's total load. The <br />degree of braiding of a channel increases with the frequency of mid - channel bars. <br />Because of the shifting of these bars, and of the channel braids between them, a braided <br />stream has an unstable bed, but the banks are not necessarily unstable (Brice, 1982). <br />The development of braided channels is caused by several factors — a steep channel <br />gradient, a large proportion of sediment being transported as bed load, and readily - <br />erodible bank material which enables channels to shift with relative ease ( Summerfield, <br />1991). Once formed, bars in braided channels can become vegetated rapidly, thereby <br />being stabilized as islands. <br />If a channel contains islands whose width is greater than three times the total width of <br />wetted channel at mean discharge, the stream is described as anabranching. The <br />anabranches, or individual channels, are more widely and distinctly separated and more <br />fixed in position than the channels of a braided stream. An anabranch does not <br />necessarily transmit flow at normal stage, but is an active and well - defined channel, not <br />blocked by vegetation (Brice, 1982). <br />A third type of channel pattern is termed anastomosing. Anastomosing channels <br />consist of distributaries which branch and rejoin, superficially resembling a braided <br />pattern. Braided channels, however, are single - channel forms in which the flow is <br />diverted around obstructions in the channel itself, whereas anastomosing patterns consist <br />-6- <br />S:\ES\WP\PR0JECTs\3-States\A1 Final Tech Memo.doc <br />