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PARSONS <br />of time since the end of the Ice Ages. The relatively rapid rates at which geomorphic <br />changes appear to have occurred suggest that most landform evolution in the Great Plains <br />region has occurred in response to threshold excursions. <br />SUMMARY AND CONCLUSIONS <br />Investigation Activities <br />The existence and nature of threshold geomorphic processes were investigated, and the <br />possibility of their past occurrence in the Great Plains was examined. The technical <br />literature regarding geomorphic processes and thresholds, and geomorphic changes in the <br />Great Plains region, was reviewed. Based on examination and evaluation of the compiled <br />literature, geomorphic trends and the possible existence of threshold conditions for the <br />study area were identified. <br />Thresholds and Uncertainties in the Platte River Basin <br />In 1990, the U.S. Department of Justice, representing the U.S. Forest Service and <br />acting on behalf of the United States, filed federal reserved water right claims for <br />channel - maintenance flows in the Colorado Water Division 1 trial (Gordon, 1995). These <br />claims to in- stream flows were challenged by the State of Colorado and by water <br />conservancy districts in northern Colorado that divert water from national forests. In <br />February 1993 the presiding judge ruled that although the reserved water rights of the <br />United States include water to be used for channel- maintenance purposes, the United <br />States had failed to demonstrate that the reserved water rights claimed were necessary to <br />preserve the timber and water characteristics which represent the primary purpose for <br />reservation of the national forests. During the course of the trial, the concept of <br />"geomorphic threshold" was used to examine the possibility that significant changes in <br />the configuration of mountain streams could occur, in relatively short periods of time, as <br />a consequence of anthropogenic activities within the stream basins. "Geomorphic <br />threshold" was defined in the court as a condition which, if exceeded, could cause <br />acceleration of natural processes such as increased rates of bank cutting or increased <br />deposition. Within its threshold limits, a stream could accommodate changes in flow <br />regime or sediment yields without experiencing major alterations to its morphology or <br />hydraulic regime. The concept of "sensitive" landscapes also was examined. "Sensitive" <br />landforms were defined to be those which- respond rapidly and dramatically to an external <br />influence of small magnitude, while "insensitive" landforms do not. Threshold <br />conditions separating streams having braided morphology from meandering streams also <br />were identified. A stream near the threshold separating the two morphologies would be <br />"sensitive," because it had the potential to change its morphology abruptly, while a <br />stream which is further from the threshold condition would be less sensitive, or "stable." <br />Technical testimony in the case largely focused on whether bankfull discharge was <br />necessary to maintain the morphology of mountain river channels. The United States <br />argued that bankfull discharges every 1 to 2 years were necessary; the State argued that <br />the channels were relatively unaffected by bankfull discharge, instead having been formed <br />by the relatively higher discharges that occurred during melting of glaciers in the southern <br />Rocky Mountains at the end of Ice Age time, and by rare, large floods such as the 1976 <br />flood in Big Thompson canyon (threshold events). The assertions of both sides in the <br />-25- <br />SAES \WP\PR0JECrs\3- States\A1 Final Tech Memo.doc <br />