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PARSONS <br />hydrologic effects associated with changing climatic conditions as unimportant in the <br />functioning of the Platte River system (e.g., Murphy and Randle, 2001 a). <br />Thresholds Intrinsic to Hydraulics of Fluvial Systems <br />In addition to thresholds arising as a consequence of changes in extrinsic conditions <br />(e.g., climate), several researchers also identify geomorphic thresholds that are inherent in <br />the physics of fluvial hydraulics. In particular, Chang (1985 and 1986) contends that in <br />considering the relationships among channel pattern, channel geometry, hydraulic slope, <br />and bankfull discharge (Figure A1-3), at least four thresholds can be identified; and that <br />these thresholds occur as a direct consequence of such factors as the critical slope for bed <br />load movement. These intrinsic hydraulic thresholds divide the universe of channel <br />regimes into 4 hydraulic "regions ", within which natural river channels will adopt <br />characteristic morphologies (Figure Al -3): <br />Region 1 — Equiwidth point -bar streams <br />Region 2 — Straight braided streams <br />Region 3 — Braided point -bar and wide -bend point -bar streams <br />Region 4 — Steep braided streams <br />As a consequence of the configuration of its channel (wide and shallow) and its <br />relatively steep hydraulic gradient (greater than 0.001), the Platte River is generally <br />considered to be a "steep braided stream" (Region 4) (Murphy and Randle, 2001a). <br />However, Chang (1985) notes that another possible stable channel geometry can occupy <br />Region 4 of channel regimes, and that this configuration Is not braided, consisting of <br />alternating riffle and pool sections, similar to the configuration of wide -bend point -bar <br />streams (Region 3). <br />Moreover, changes in morphology of a channel that is hydraulically near an intrinsic <br />threshold can readily be induced by relatively minor changes in extrinsic conditions. <br />Consider a channel that has formed in response to hydraulic conditions that place it in <br />Region 3. The anticipated configuration of this channel is that of a braided point -bar or <br />wide -bend point -bar stream (Figure A1-4). Now suppose that some extrinsic change <br />causes the local base level of the stream to be slightly lowered, leading to a slight increase <br />in hydraulic gradient. (Such a lowering of base levels occurred during the last Ice Age, <br />when lowered sea levels caused a general increase in stream gradients throughout the <br />northern hemisphere.) In response to even a slight increase in gradient, the hydraulic <br />condition of the stream crosses an intrinsic threshold (Figure A14), and the channel <br />morphology changes to that of a steep braided stream (Region 4). If the hydraulic slope <br />of the river varies in response to the sediment contributions from tributaries, or due to <br />variations in deposition during the geologic past, then the river also will reflect these <br />changes in slope by changing channel pattern (Schumm, 1974). <br />SAES \WP\PR0JECTS\3- StatesW I Final Tech Memo.doc <br />-21- <br />