Laserfiche WebLink
PARSONS <br />agencies, independent researchers, and the published literature, and is described in the <br />• following sections. <br />Initially, basic hydrologic information and bibliographic references regarding <br />geomorphic conditions and processes within the Platte River basin were obtained from <br />the EIS team. During our initial review, it became apparent that conditions affecting the <br />stability of the braided planform in general, and the possible existence of extrinsic <br />threshold conditions, had not been considered by the EIS team, during development of <br />their description of the hydrology and geomorphology of the Platte River system. <br />Therefore, after reviewing the EIS team's data and documents, the Parsons team then <br />expanded our review to include other sources of published data and reports relating to the <br />geomorphic and climatic processes affecting the Great Plains, with particular emphasis on <br />braided fluvial systems and the Platte River corridor. <br />• <br />SYNOPSIS OF LITERATURE REVIEW — GEOMORPHOLOGY, HYDROLOGY, <br />AND THRESHOLDS <br />Technical literature and other information regarding hydrologic and geomorphic <br />conditions in the Great Plains and similar areas was reviewed by the Parsons team as part <br />of Task Al. Literature reviewed in conjunction _with Task Al has been cited in the <br />bibliography, and is summarized in this section. <br />Wenzel et al. (1946) completed an evaluation of the geology and groundwater <br />resources of the Platte River valley near Scottsbluff, Nebraska, which included an <br />assessment of the geomorphology of the North Platte River and nearby areas. Wenzel et <br />al. (1946) identified nine terraces bordering the river, having elevation differences <br />between the lowest and highest terrace of nearly 1,000 feet. Several of the highest <br />terraces were developed during Ice Age time; but later in its history, the North Platte <br />River apparently incised channels to depths greater than 200 feet below its present <br />floodplain elevation; these channels later were backfilled by the river. The terraces, <br />erosion surfaces, and channel deposits represent the visible results of alternating periods <br />of erosion and deposition, occurring from the end of the Ice Ages through the present <br />time, which Wenzel et al. (1946) ascribe to threshold excursions, including changing <br />climatic conditions, tectonic uplift, or both. <br />Leopold and Miller (1954) examined the geomorphology of several alluvial valleys in <br />eastern Wyoming, including the North Platte River, in an attempt to reconstruct the <br />sequence of events that produced alluvial terraces bordering the valleys at different levels. <br />On the basis of stratigraphic evaluation and comparison of the relative elevations of <br />terrace deposits, channel -fill deposits, and erosion surfaces, Leopold and Miller (1954) <br />identified several periods of alternating deposition and erosion in eastern Wyoming, <br />beginning at the end of Ice -Age time. Erosional processes resulted in channel incision to <br />depths of several tens of feet, which was followed by a cycle of deposition, in which the <br />incised channels were backfilled with sediments to nearly their pre- erosion level. <br />Leopold and Miller (1954) related these cyclic geomorphic occurrences to climatic <br />changes that have occurred since the end of the Ice Ages. The authors concluded that <br />while some of the recently - occurring channel incision and erosion observed in eastern <br />Wyoming was a consequence of threshold conditions associated with changing land use <br />(overgrazing resulting in removal of vegetation), much of the modern erosion also could <br />S:\ES \WP\PROJECTS\3- States\Wl Final Tech Memo.doc <br />IR111 <br />