Laserfiche WebLink
PARSONS <br />analysis of pollen (palynology), and concluded that if climatic conditions typical of the <br />recent past continue, they will be accompanied by widespread episodes of stream erosion. <br />- Peak flood discharges of elevated magnitude and low frequency were regarded as <br />potential threshold events by Knox (1983), who also noted that severe droughts, which <br />reduce vegetative cover on a basin -wide scale, tend to increase surface runoff, thereby <br />causing larger floods from a precipitation event of a given intensity. <br />• <br />Chang (1985) examined the regime geometry and channel patterns of sand -bed rivers, <br />using an energy approach, and identified four general morphologic types of streams, on <br />the basis of distinct characteristics related to morphologic balance achieved between flow <br />resistance and stream power. The four morphologic types — equiwidth point -bar streams, <br />straight braided streams, braided point -bar and wide -bend point -bar streams, and steep <br />braided streams — are separated by energy thresholds that result from differences in the <br />stream energy gradient and bankfull discharge characteristics of each stream type (Figure <br />A1-3). According to Chang (1985), the formation of braided channels reflects in part a <br />river's adjustment in expenditure of stream power, which in turn affects the channels <br />stability. For streams having the typical longitudinal riffle- and -pool configuration, a non - <br />braided channel is more stable for "pool" sections, but the braided and unbraided channel <br />types are approximately equal in stability in the "riffle" sections. Chang (1985) noted that <br />because non - braided reaches appear to be more stable than braided reaches under most <br />circumstances, wide streams in reality usually are braided as a consequence of high <br />sediment loads, bank erosion, and physical heterogeneities, rather than representing a <br />stable configuration that has resulted from lowered expenditure of stream power. <br />Chang (1986) also used energy and stream power to develop a method of predicting a <br />river channel's adjustments of equilibrium to changing conditions of discharge, slope, <br />sediment size, and channel width and depth. The evolution of fluvial systems was <br />compared with a feedback system in which the effect (river channel formation) and the <br />cause (stream discharge) are inter - related and interdependent. Chang (1986) found that <br />streams having a braided configuration were most sensitive to changes in slope or <br />discharge or both, so that a change in slope or discharge could be associated with large <br />changes in channel width. <br />Cyclic occurrence of geomorphic processes in the Great Plains was described by <br />Osterkamp et al. (1987), who determined that the morphology of the Great Plains is a <br />product of the dramatic climatic changes that have characterized the period of time <br />following the conclusion of the Ice Ages. In particular, because of the relationship <br />between climatic variation and the resulting geomorphic processes, landforms on the <br />Great Plains appear to have developed in a complex cyclic manner: <br />1. Stratigraphic and geomorphic sequences in the Great Plains have many small to <br />large discontinuities that record repeated changes in the rates of surficial <br />processes (fluvial, aeolian, pedogenic). Commonly, the change in rate of one <br />process as compared with another became large enough to change the type of <br />surficial process that was dominant during a given episode. <br />S:\ES \WP\PROJECTS\3- States\Al Final Tech Memo.doc <br />-16- <br />