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• <br />PARSONS <br />in historic time as a consequence of anthropogenic activities, including beaver trapping, <br />deforestation, mining, and the introduction and expansion of irrigated agriculture. <br />RESULTS OF EVALUATION <br />Geomorphic Processes and Thresholds <br />Analysis of the morphology and behavior of contemporary and paleorivers shows that <br />there are a number of identifiable and quantifiable fluvial states (Schumm, 1968, 1974, <br />and 1981; Thornes and Gregory, 1991). The principal morphological taxonomy of rivers <br />identifies straight, meandering, and braided channels; the principal hydraulic taxonomy <br />identifies wide, shallow channels and narrow, deep channels; and the principal behavioral <br />taxonomy identifies aggrading, graded (regime) and degrading (eroding) channels. There <br />is ample evidence to show that different reaches of the same river can (and commonly do) <br />occupy different morphological, hydraulic, and behavioral states (Thornes and Gregory, <br />1991), but that a particular reach tends to spend relatively long periods of time in the <br />same state. However, if certain conditions change, the response of the river, or of a <br />particular reach, can be sudden and dramatic (Schumm and Lichty, 1963; Schumm, <br />1968). <br />In hydrology, changes in state usually are thought to indicate changes in the controlling <br />variables, and most commonly are taken to indicate extrinsic changes, such as changes in <br />the hydrologic regime. In turn, these may be attributed, for example, to changes in <br />vegetation cover, snowmelt, or precipitation. Various authors have demonstrated that <br />changes in state also may come about without alteration of the external conditions as the <br />controlling variables pass through internal (structural or intrinsic) thresholds in the <br />systems (Schumm, 1974; Chang, 1986). The prevailing view is that under conditions of <br />dynamic equilibrium, river channels adjust the values of state variables (which describe <br />the condition of the fluvial system) more or less continuously to the slope, and to <br />sediment and water supplied to the channel (Thornes and Gregory, 1991). <br />Numerous hydrologic investigations illustrate three types of behavior of fluvial <br />systems in the geologic and historic record: <br />• stability, indicated by the dominance of a particular state over long periods of time <br />and exemplified by the widespread occurrence of meandering morphology <br />throughout most of recent time; <br />• rapid change, as indicated by switches in morphology over a relatively short period <br />from meandering to braided and back, or from aggrading to degrading conditions, <br />with consequent formation of terraces; and <br />• oscillations between states. <br />These three behaviors usually are ascribed to changes in external conditions (primarily <br />climate), which, in light of the known changes in global climatic conditions and their <br />oscillations, seems perfectly reasonable (Thornes and Gregory, 1991). This view is in <br />contrast to the approach used by the EIS team, who apparently regarded the possible <br />S:\ES \WP\PROJECTS\3- States\Al Final Tech Memo.doc <br />-20- <br />