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50 GEOMORPHlCALL Y EFFECTIVE FLOODS Fig. 5 _ Photograph of breach and slump failure of the Porter Hill dam. near Roseburg, Oregon. change. We hypothesize that other fl60ds in the same basins with smaller peak stream power values, but longer duration, could precipitate significant and perhaps permanent changes in channels and floodplains_ --- 3.1 Changing srream power owr a flood hydro graph If flow duration is believed to be an important factor in the llbility of floods to alter landforms, then it is important to know the temporal and spatial distribution of stream power, throughout a flood. For flooded locations not at gaging stations, indirect discharge methods are commonly used to calculate peak discharge associated with high-water marks. With data derived from these investigations, only instantaneous peak stream power for the flood can be ascertained and reported [Costa, 1987]. Peak stream power is useful in evaluating flood competence [Costa, 1983; Williams, 1983], but is not the sole factor in evaluating whether a flood may be geomorphically effective. Time- integrated flood power, computed over a hydro graph and combined with some quantitative measures of landscape resistance, such as shear strength of river channel banks and floodplains, may be more useful to evaluate potential for geomorphic effectiveness. There are no widely applicable procedures to quantify landscape modification accomplished by a given flood. Yaluable qualitative descriptions have been used in some studies [Kochel, 1988; Miller, 1990; Miller and Parkinson, 1993], but data requirements for a more rigorous quantitative analysis of landform modification preclude our use of anything but a simple two-class scheme at this time. The amount of geomorphic alteration is assigned a qualitative value of small or extreme. Small disruption represents sites where floodplains are inundated, but with little or no erosion of the floodplain surface_ Channel scour and erosion are local, drainage patterns remain similar (e.g. a meandering stream still meanders), deposition and \ sedimentation are restricted to small, local areas, and the valley floor and channel have minimal changes. Extreme disruptions occur in areas where the entire floodplain and channel are substantially affected by erosion or deposition. New channels may be formed or the floodplain may be entirely eroded. Extensive areas of deposition may occur on uneroded floodplain or in newly eroded areas during the flood recession. Bedrock, if present, may have been eroded, and the stream channel and stream pattern may be completely realigned. We recognize that deposition of sediment from large floods can significantly alter alluvial ---