Laserfiche WebLink
PARSONS <br />catalyst that induces change at a particular time, because the existence of geomorphic <br />thresholds, and the complex feedback mechanisms of geomorphic systems, permit events <br />of large magnitude to play a major role in landscape evolution. According to Schumm <br />(1974), these concepts are not in conflict with the concept of dynamic equilibrium, but <br />rather supplement it. <br />Schumm (1980) also found that in general, stream gradients and floodplain <br />configurations do not change progressively (in linear fashion) through time; rather, <br />relatively brief periods of instability and incision are separated by long periods of relative <br />stability, when the system is in equilibrium and "at grade ". Therefore, a landscape having <br />a very complex evolutionary history may be the norm, in the geomorphic sense. <br />Patton and Schumm (1975) examined the causal factors of development of gullies and <br />arroyos in stream valleys in northwestern Colorado, and determined that the slope along <br />the longitudinal axes of valleys in the region represented an intrinsic threshold. If the <br />slope exceeded some critical value in a localized segment of a valley, a discontinuous <br />gully was likely to develop in that segment. Patton and Schumm (1975) considered the <br />exceedance of a critical slope value to represent a condition of valley instability, which <br />could result in rapid alteration of the drainage basin by erosion. <br />Episodic occurrences of fluvial erosion and deposition in the American Midwest were <br />examined by Brakenridge (1980). Stratigraphic analyses were combined with <br />radiocarbon dating of paleosols and terrace surfaces to develop an alluvial chronology of <br />the Pomme de Terre River in central Missouri (Figure A1-2), which was related by <br />Brakenridge (1980) to changes in climatic conditions. Periods of slow or fast floodplain <br />aggradation, stability, and degradation are immediately evident, and produced, at various <br />times, floodplains having surface elevations ranging from about 12 feet (4 meters) below <br />the elevation of the current floodplain, to more than 12 feet (4 meters) above the current <br />floodplain. One facet of the fluvial history that is particularly apparent is that conditions <br />of "grade" or stability have prevailed for only limited periods of time since the end of the <br />Ice Ages (Figure Al -2). Instead, <br />• a major aggradational event occurred between 10,500 and 8,100 years before the <br />present time (yr BP); <br />• following a brief erosional event between 8,100 and 7,500 yr BP, the river rapidly <br />re- adjusted its grade; <br />• floodplain stability prevailed from 7,500 to about 5,000 yr BP; <br />• by 4,600 yr BP a major change in long -term stream regime had occurred; and <br />• since that time, rapid aggradation has alternated with closely- spaced intervals of <br />erosion. This last period, characterized by lack of long -term stability, continues to <br />the present. <br />Brakenridge (1980) related these dramatic geomorphic changes to regional long -term <br />climatic trends, derived on the basis of the regional vegetational history as deduced from <br />SAES \WP\PR0JECTS\3- States\A1 Final Tech Memo.doc <br />-14- <br />