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is <br />PARSONS <br />case were hampered by the limited information regarding channel dynamics available for <br />mountain rivers. <br />Similar uncertainties arise in considering the occurrence and timing of threshold <br />conditions affecting the geomorphology of the Platte River system. Examination of the <br />terrace geomorphology and stratigraphy of streams in the Great Plains, including <br />tributaries of the Platte River (Wenzel et al., 1946; Leopold and Miller, 1954; May 1992) <br />leaves little doubt that the Platte River has crossed intrinsic and extrinsic thresholds in the <br />geologic past, probably on numerous occasions; and that these threshold excursions likely <br />have left geomorphic signatures of their occurrence. However, the literature dealing with <br />the geomorphology and geologic history of the main stem of the Platte River proper is <br />limited; and specific threshold events on the Platte River, and their resulting geomorphic <br />consequences, have yet to be identified. Therefore, although it is certain that threshold <br />geomorphic events have occurred throughout the Platte River system, the exact nature <br />and timing of these events currently cannot be identified. <br />Despite these uncertainties, the following summary statements can be made regarding <br />threshold events and geomorphic responses in the Platte River system. <br />1. Geomorphic changes usually occur in response to changes in threshold <br />conditions. Geomorphic thresholds may be extrinsic (occurring in response to <br />an external change) or intrinsic (inherent in the system). Threshold conditions <br />may occur rapidly, or may develop in response to gradual, often imperceptible, <br />changes within the system. Similarly, the geomorphic response to threshold <br />conditions may be gradual, or abrupt. <br />2. A major hydrologic event may be of major or minor importance in landscape <br />modification, depending upon the existence of threshold conditions. Some <br />landscapes or components of landscapes apparently have evolved to a condition <br />of geomorphic instability, and these landforms fail — that is, depending on their <br />development, they will be significantly modified by a large, infrequent event <br />while other landforms will be unaffected. Therefore, there will be different <br />responses to the same conditions, even within the same region (Schumm, 1974). <br />3. Dramatic geomorphic changes have occurred in the Great Plains region within <br />the past 1,000 to 2,000 years. These have included episodes of aggradation and <br />degradation, which have resulted in the addition (or removal) of tens to <br />hundreds of feet of material to (from) channel systems. These changes are <br />recorded in the stratigraphy of alluvial deposits, and in the morphology of <br />fluvial terrace systems throughout the Great Plains. <br />4. Climatic change appears to be the most significant external factor driving <br />geomorphic change. Significant time lags can occur between climate change <br />and adjustment of the resulting landforms to changing conditions. Extreme <br />climate excursions that cause accelerated threshold - limited erosion or <br />deposition seem likely to occur and to leave very long -lived morphological <br />evidence. <br />SAES \WP\PR0JECTs\3- StatesW 1 Final Tech Metno.doc <br />-26- <br />