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<br />PARSONS
<br />various ways, including the downstream migration of bars, the gradual shifting of
<br />meanders, and the rapid alteration of course through cut -offs or channel diversions.
<br />Anthropogenic activities, such as clearing of a floodplain or construction of a dam, may
<br />bring rapid changes in stream type and behavior. However, as pointed out by Brice
<br />(1982), anthropogenic activities do not necessarily lead to a decrease in stability.
<br />In many circumstances, the landforms comprising current topography are an
<br />expression not only of current conditions — the interaction between the driving forces
<br />(climate, tectonics) and the geologic framework — but also contain relict features that
<br />resulted from past conditions (Brice, 1966; Schumm, 1968; ibid., 1974; Rinaldo et al.,
<br />1995). In the Great Plains as in much of the Northern Hemisphere, climate is the primary
<br />force acting to produce and shape landforms from the geologic framework (Wenzel et al.,
<br />1946; Leopold and Miller, 1954; Brakenridge, 1980; Sundborg and Jansson, 1991).
<br />During the past 15,000 to 18,000 years, substantial changes of the whole environment
<br />have occurred, usually described as a shifting of the climatic - vegetational zones as a
<br />consequence of the worldwide transition from glacial conditions (during the most recent
<br />Ice Age) to post - glacial conditions. These changes created new fluvial systems or
<br />initiated large -scale transformations of the basic parameters of existing hydrologic
<br />regimes, including river discharge, sediment load, basin area, channel length, and gradient
<br />(Starkel, 1991a). Changes in the hydrologic regime and sediment load in areas formerly
<br />affected by glacial climatic conditions caused a shift from bedload to mixed -load rivers,
<br />and then to suspended -load rivers ( Schumm, 1981). These paleogeographical changes in
<br />the temperate zone were superimposed on river valleys, which yet retain some
<br />characteristics associated with past conditions. Climatic fluctuations have continued up
<br />to the present, and consequently have produced a sequence of changes in the hydrology
<br />and geomorphology of numerous fluvial systems, which have been superimposed on, and
<br />have modified, the pre- existing systems ( Sundborg and Jansson, 1991; Starkel, 1991b).
<br />Thus, the current geomorphology of the Platte River basin consists of an Ice -Age
<br />topography, somewhat modified by the forces resulting from changing climates of the
<br />past 15,000 years.
<br />As has occurred in numerous other rivers worldwide, the configuration of the Platte
<br />River has changed in response to changes in water and sediment discharges, resulting
<br />from climatic change and other factors. However, the time scale for adjustment of a
<br />channel varies with the sensitivity of the channel to changing conditions. A significant
<br />adjustment in stable humid -zone rivers may require centuries; but in semi -arid areas (such
<br />as the Great Plains), the time scale for fluvial adjustments can be much shorter, close to
<br />the time scales required for the random response to a single catastrophic event (Chang,
<br />1986). The rapidity of the response of the fluvial system to changes in extrinsic
<br />conditions is a measure of the relative stability (or lack thereof) of the channel type, with
<br />respect to particular conditions.
<br />METHODS OF INVESTIGATION
<br />Investigation Task "Al" consisted of in -depth review of the available information
<br />regarding stable rivers, geomorphic processes, and extrinsic thresholds that can represent
<br />discontinuities between morphological patterns separating straight, meandering, and
<br />braided regimes. Information pertaining to the existence of geomorphic thresholds and
<br />the characteristics of braided and other streams was obtained from federal and state
<br />SAES \WP\PR0JECTS\3- StatesW 1 Final Tech Memo.doc
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