Laserfiche WebLink
• <br />PARSONS <br />2. The cyclicity of the interactions between climate and geomorphic processes is <br />characterized by sequences of incision, lateral erosion, deposition, and landform <br />stability, perhaps with significant soil development. These cycles are known as <br />"erosion- deposition - stability" (EDS) cycles. All stages of this sequence are <br />observable in different parts of the Great Plains. <br />3. In the Great Plains, EDS cycles were induced by climate changes, because <br />tectonic processes are negligible in this relatively stable region. <br />4. Four time - duration classes of EDS cycles appear to have operated in the past: <br />microcycles lasting 10 to 100 years; mesocycles, lasting 1,000 to 10,000 years; <br />macrocycles, approximately 100,000 years in length; and megacycles, 400,000 <br />to 500,000 years in length. <br />5. At least four megacycles, and probably the beginning of a fifth, are apparent in <br />the geologic history of the past 2 million years. The initiation of each <br />megacycle is indicated by a period of prolonged alluvial downcutting. The <br />Great Plains appears to be entering a fifth process megacycle, and Great Plains <br />rivers currently are downcutting their channels. <br />6. The fundamental climatic- and surficiaf- process controls of the EDS cycles, <br />including various feedback mechanisms, are poorly understood, especially the <br />extrinsic threshold controls of the longer cycles. Shorter cycles were driven <br />over low thresholds at short intervals by relatively small changes in process <br />intensity. The high thresholds of the megacycles required large changes in <br />process intensity, and were accompanied by major geomorphic - stratigraphic <br />changes. <br />The proximate causes and modes of evolution of a number of fluvial systems were <br />evaluated in conjunction with a project of worldwide scope -- the International <br />Geological Correlation Programme (IGCP) Project 158 (Starkel, 1991a). For example, <br />the Vistula River in central Europe had a narrow, anastomosing planform at the end of the <br />last Ice Age (Starkel, 1991c). By about 13,000 yr BP, the upper reaches of the Vistula <br />had become meandering, and the lower reaches were braided, primarily as a consequence <br />of the dramatic climatic changes (a threshold event) that occurred following the retreat of <br />glacial ice. Subsequently, the river channel became straighter, and a cycle of downcutting <br />was initiated. Most recently (within the past several hundred years), aggradation of the <br />channel has occurred, accompanied by a buildup of floodplains and valley floors by <br />channel deposits. <br />The Warta River in the lowlands of Poland was braided at the end of Ice Age time <br />(Kozarski, 1991), but evolved in successive stages to the meandering point -bar planform <br />of modern times. This evolution probably was a consequence of changes in climatic <br />conditions, accompanied by a concomitant increase in vegetation within the drainage <br />basin, which stabilized soils and reduced the amount of sediment potentially available for <br />transport. This general pattern of evolution was followed by most rivers during the <br />transition from Ice -Age to modern conditions, and fluvial metamorphosis has continued <br />in recent times with more subtle changes in flow characteristics, sediment types and loads <br />S:\ES\WP\PR0JECTS\3-States\A1 Final Tech Metno.doc <br />ME <br />