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• PARSONS (Baker, 1991). Cyclic changes in hydrologic regime occurred over similar periods throughout the temperate zone. Martin (1992) and May (1992) examined the paleohydrology of the Republican River and South Loup River, respectively — two stream systems in the near vicinity of the Platte River. Apparently, two episodes of incision (degradation) of stream channels, one beginning about 4,200 yr BP, and the other occurring after 1,100 yr BP, were widespread across the central Great Plains (Martin, 1992). The earlier episode was associated with a shift to relatively wetter conditions (a threshold event), whereas the later episode was preceded and followed by dry conditions. Three episodes of floodplain aggradation have been identified in the South Loup River valley (May, 1992). The first episode of aggradation occurred beginning prior to about 3,500 yr BP and continuing until at least 3,000 yr BP. This followed a period of deep incision of the valley, perhaps corresponding to the earlier episode of incision identified by Martin (1992). The second interval of slow aggradation on the floodplain, accompanied by formation of soil on terrace surfaces, occurred between 1,800 and 1,050 yr BP. A third, rapid episode of valley- bottom aggradation occurred sometime after 1,050 yr BP. Episodes of floodplain erosion during high- magnitude floods occurred before, between and after these intervals of aggradation. The intervals of aggradation documented for the South Loup River valley appear to have been synchronous throughout the Loup River basin (May, 1992). Rinaldo et al. (1995) developed a mathematical model of geomorphic processes and used the model to simulate the development and evolution of various landforms under conditions corresponding to tectonic uplift and climatic variability. On the basis of the results of a series of numerical simulations, Rinaldo et al. (1995) concluded that significant time lags can occur between climate change and adjustment of the resulting landforms to changing conditions. In particular, extreme climate excursions that cause accelerated threshold- limited erosion seem likely to occur and to leave very long -lived morphological evidence. Threshold events that have affected the fluvial geomorphology of rivers originating in the Front Range of Colorado were examined by Wohl (2001), who noted that a threshold event separates two distinct modes of operation of a river system. Often, there appears to be a lag period before a river responds to a change in water or sediment discharge; and this lag may be caused by the existence of a threshold. As an example, Wohl (2001) notes that a severe forest fire may destroy the vegetative cover that tends to stabilize hillslope soils. The first heavy rainfall following the forest fire may destabilize the hillslope and cause a landslide that introduces large quantities of sediment to the river. Because the river discharge is not capable of transporting all of the newly- introduced sediment downstream immediately, over a period of months to years the morphology of the river may change from a- meandering to a braided pattern. This new pattern may persist for decades to centuries until sufficient sediment has been removed and the river once again crosses a threshold and assumes a single- channel planform. However, the response of a river to some threshold process or external change may not be synchronous along all reaches of the river. Downstream parts of a drainage basin may be affected by changes upstream, and vice versa. It thus becomes important to consider how changing conditions in one part of the basin may affect the remainder of the basin. Wohl (2001) describes and characterizes the changes in Colorado streams and rivers that have occurred S:\ES \WP\PROJECTS\3- States\Al Final Tech Memo.doc -19-