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PARSONS Similarly, the morphology of a steep braided stream (Region 4) may change in response to extrinsic conditions that cause even slight reductions in discharge, which can shift the hydraulic condition of the stream across an intrinsic threshold into Region 3 (braided point -bar and wide -bend point -bar streams) (Figure Al -5). Such reductions can occur as a consequence of numerous factors, including anthropogenic activities, or climatic change — in particular, droughts. Note that the magnitude of the change in extrinsic conditions required to cross a threshold is sufficiently small that it may not be possible to distinguish the proximate cause of the change in morphology from among all possible causes. Geomorphic Thresholds and Landform Responses Fluvial terraces are evidence for naturally - occurring episodes of channel degradation, and most alluvial streams, including streams in the Great Plains, are bordered by terraces. A terrace is an abandoned floodplain, the surface of which is rarely inundated because the stream channel has gradually cut below it. Most terraces probably are formed at rates of degradation too slow to be observed at human time scales; nevertheless, low terraces along a stream are noteworthy, because they suggest that the stream has been vertically unstable in the past, and that such instability may readily be induced (Brice, 1982). Floodplain deposits are evidence for naturally - occurring episodes of channel aggradation; and depending upon the geologic history and geomorphology of a particular region, deposits associated with terraces also may record periods of aggradation. Review of the technical literature demonstrates that alternating periods of incision (degradation or erosion) and aggradation have affected numerous, and perhaps all streams in the Great Plains region (Wenzel et al., 1946; Leopold and Miller, 1954; Brakenridge, 1980; Osterkamp et al., 1987; Martin, 1992; May 1992) during the past several thousand years. These episodic occurrences have resulted in the addition (or removal) of material totaling tens to hundreds of feet in thickness to (or from) floodplains in the region. During the same period, fluvial systems in the temperate zone throughout the world have undergone large -scale changes in hydraulic and hydrologic characteristics, and in planform, changing from braided to meandering planforms (Kozarski, 1991; Baker, 1991; Starkel, 1991b), and sometimes from meandering to braided planforms (Schumm and Lichty, 1963). These transformations may require periods approaching geologic time (Schumm, 1968), but can occur in rapid and dramatic fashion (Chang, 1986; Schumm and Lichty, 1963; Schumm, 1974). Generally speaking, these geomorphic changes appear to occur in response to climatic changes (Thomes and Gregory, 1991). Although climate can function as an extrinsic factor, driving geomorphic processes that eventually produce changes in the configuration and planform of drainage systems, the nature of threshold occurrences is such that even minor changes in extrinsic conditions may cause a system to cross one or more intrinsic thresholds, triggering geomorphic changes of large magnitude (preceding section). Stratigraphic and geomorphic sequences in the Great Plains have many small to large discontinuities that record repeated changes in the types and rates of surficial processes ( Osterkamp et al., 1987). The changes in interactions between climate and geomorphic processes is characterized by sequences of incision, lateral erosion, deposition, and landform stability, perhaps with significant soil development. Periods of slow or rapid aggradation, stability, and degradation are evident in the stratigraphic record; but it is apparent is that conditions of "grade" or stability have prevailed for only limited periods -23- SAES \WP\PROIECTS\3- States\AI Final Tech Memo.doc