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PARSONS <br />phenomenon, that occurs primarily by dissection of transverse bars. Transverse bars form <br />by aggrading to a profile of equilibrium and expand laterally by additions of sediment to <br />avalanche faces. Bars formed during the high annual spring discharges are the first to <br />become exposed. Those that escape complete removal by waning currents following <br />high- discharge events are soon overgrown by vegetation to become semi - permanent <br />features unless destroyed by subsequent high- discharge events. <br />Several factors determine the shape of an evolving transverse bar (Smith, 1971), <br />including cross - sectional shape of the bar mouth, proximity to stable banks, direction and <br />power of adjacent currents, steadiness of flow, and depth distribution of the floor over <br />which the bar is growing. The ideal bar pattern is a bilaterally symmetrical lobe shape <br />with currents radially distributed from the mouth over the surface. Such lobate forms are <br />fairly common during high discharges in the Platte River and during early stages of bar <br />growth during lower discharges. Ordinarily, however, the bars take on a wide variety of <br />asymmetric and irregular patterns soon after_ their initiation, as a result of one or more of <br />the listed factors. <br />Braiding begins when flow passing through the bar mouth decreases to the point where <br />it is unable to sustain sediment transport over the entire bar surface (Smith, 1970). The <br />flow then becomes to one or more channels which begin to dissect the bar surface. <br />During waning stages of flow, smaller transverse bars may merge with or override the <br />original bar. These new bars and their accompanying smaller -scale bedforms, combined <br />with both lateral and downward dissection by surface and adjacent currents, produce a <br />complex depositional and erosional history for the original bar area after flows have <br />diminished or stopped completely. Thus, according to Smith (1971), most exposed bars, <br />especially the larger ones, separating anabranches in the Platte River during periods of <br />low discharge, are not simple exposed transverse bars at all, but rather are complex <br />depositional and erosional features which merely began as transverse bars. <br />Schumm (1974) examined the occurrence of geomorphic threshold conditions in <br />natural systems, and found that in a complex system, one event can trigger a complex <br />morphologic reaction as the components of the system respond progressively to change. <br />Schumm (1974) felt that this observation provided an explanation of the complexities of <br />alluvial chronologies, and suggested that infrequently - occurring events, though <br />performing little of the total work within a drainage system, may in fact be the catalysts <br />that cause the crossing of geomorphic thresholds and the triggering of complex sequences <br />of events that will produce significant modifications to landscapes. <br />In Schumm's evaluation, landscape discontinuities, or what appear to be abrupt <br />changes in the morphologic evolution of drainage systems, are not always a consequence <br />of external influences; and the evolution of landforms, at least in semiarid and arid <br />regions, need not be progressive in the sense of constant and orderly development — in <br />fact, change may occur progressively and also by rapid shifts from one state of dynamic <br />equilibrium to a new one. It appeared very possible to Schumm (1974) that, without the <br />influence of external variables and over long time spans, progressive development of a <br />landscape will be interrupted by periods of rapid readjustment, as geomorphic thresholds <br />(extrinsic and intrinsic) are exceeded. Readjustment of the system will be complex as <br />morphology and sediment yields change with time. The timing of these changes may be <br />related to events of low frequency but large magnitude; but such events may only be the <br />-13- <br />S:\ES \WP\PR0JECTS\3- States\Al Final Tech Memo.doc <br />