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PARSONS <br />5. The morphology of the Great Plains is a product of the dramatic climatic <br />changes that have characterized the period of time following the conclusion of <br />the Ice Ages. In particular, because of the relationship between climatic <br />variation and the resulting geomorphic processes, landforms on the Great Plains <br />appear to have developed in a complex manner. <br />6. In addition to thresholds arising as a consequence of changes in extrinsic <br />conditions, intrinsic geomorphic thresholds may be inherent in the physics of <br />fluvial hydraulics. Changes in morphology of a channel that is hydraulically <br />near an intrinsic threshold may be induced by relatively minor changes in <br />extrinsic conditions; and the magnitude of the change in extrinsic conditions <br />required to cross a threshold may be sufficiently small that it may not be <br />possible to distinguish the proximate cause of the change in morphology from <br />among all possible causes. <br />7. The Platte River has crossed intrinsic and extrinsic thresholds in the geologic <br />past, probably on numerous occasions; and that these threshold excursions <br />likely have left geomorphic signatures of their occurrence. However, the details <br />of threshold excursions in the Platte River system remain largely unknown. <br />Within a complex natural system, a single event or change in condition can trigger a <br />complex reaction (morphologic and/or stratigraphic) as the components of the system <br />respond progressively to change (Schurnm, 1974). This principle provides an explanation <br />of the complexities of alluvial morphologies and chronologies, and suggests that an <br />infrequent event, although performing little of the total work within a drainage system, <br />is may in fact be the catalyst that causes a fluvial system to cross a geomorphic threshold, <br />with a consequent triggering of a complex sequence of events leading to significant <br />landscape modification. <br />Recommendations for Additional Investigation Activities <br />Additional investigation activities could be conducted to clarify some of the <br />uncertainties identified regarding the occurrence of threshold geomorphic conditions and <br />their association with changes in the planform of particular features specific to the Platte <br />River basin. Possible investigations that should be considered include the following: <br />• A detailed geomorphic assessment of several reaches of the Platte River below <br />Kingsley Dam could be conducted to collect and examine evidence of past changes <br />in channel morphology. The evaluation would include aerial - photographic <br />reconnaissance to identify any relict channel features that may be present, in <br />conjunction with a drilling or trenching program to examine stratigraphic features <br />within the current Platte River channel, and in relict or paleochannel areas. <br />Detailed evaluation of stratigraphic and depositional features of the current channel <br />and paleochannels (if any), together with dating and correlation of prominent <br />stratigraphic features (e.g., terrace surfaces) would enable the nature and timing of <br />changes in the morphology of the current Platte River channel to be assessed. In <br />particular, the occurrence of threshold events probably would be indicated by <br />areally widespread changes in the nature of fluvial deposits of the Platte River and <br />its tributaries prior to, and subsequent to the threshold event, and could provide <br />-27- <br />S:\ES \WP\PROJECTS\3- States\WI Final Tech Memo.doc <br />