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~ Geomorphically Effective Floods John E. Costa and Jim E. O'Connor U_S_ Geological Survey, Vancouver, Washington Investigations of the hydrology and geomorphology of recent floods from the rapid failure of two small upland dams document the unusually large peak boundary shear stress and peak stream power per unit area for each flood. Downstream consequences to alluvial channels and floodplains, however, were minimal. Lack of geomorphic change is attributed to the short duration of the floods, which lasted about six. and sixteen minutes each. Distribution of stream power over hydrographs of eight exceptional floods is constructed from channel geometry, discharge rating curves, and flood hydrographs; the resulting curve is defmed as a stream-power graph. A stream- power graph gives a better portrayal of the potential for a flood to be geomorphically effective than simple statements of flow magnitude. From stream-power graphs, total energy expended over a flood hydrograph can be computed. Total flood energy may not be a sensitive measure of geomorphic effectiveness without consideration of channel and floodplain resistance. A conceptual model combining flow duration, peak stream power per unit area, flood energy. and alluvial and bedrock thresholds may represent the effectiveness of floods and can distinguish among such cases as (a) floods of long duration, moderate to large energy expenditure. but low peak stream power per unit area. These floods are ineffective in causing significant landform changes in alluvial or bedrock channels; (b) floods of medium to long duration, with medium to large total energy expenditure. and large peak str~am power per unit area. These are believed to be the most effective geomorphic floods in any kind of channel because of the optimal combination of peak flood power. duration, and total energy expenditure; and (c) floods of very short duration, low total energy expenditure. but large peak stream power. These floods are also ineffective agents of geomorphic change in spite of record values of peak stream power per unit area because of their short duration. and resulting low energy expenditures. 1. INTRODUCTION One fundamental underpinning of the science of geomorphology is that the form of the earth's surface is the consequence of past and present geophysical forces acting on the earth's landforms. In fluvial geomorphology, this notion led to the classic question of whether valleys and channels were primarily shaped by frequently-occurring moderate flows, and resulting small forces, or by rare and cataclysmic flows with corresponding large forces [Wolman and Miller, 1960]. The maximum discharge of a flood is commonly used as Natural and Anthropogenic Influences in Fluvial Geomorphology Geophysical Monograph 89 This paper is not subject to U. S. copyright. Published in 1995 by the American Geophysical Union a measure of the potential of a flow to be an effective geomorphic agent, primarily because maximum discharge is routinely measured or computed and published for large floods. In general, the larger the discharge, sometimes indexed by drainage area or recurrence interval, the more change that is anticipated in the channel and valley. A dilemma for geomorphologists is the observation that floods of similar magnitude and frequency sometimes produce surprisingly dissimilar geomorphic results. Unfortunately, few quantitative hydraulic data on large floods have been presented to assess disparities in landform response. Recently, channel boundary shear stress and stream power per unit boundary area have been shown to be more useful concepts than discharge alone in assessing the potential of flood flows to affect landscapes [Baker and Costa, 1987]_ The concept of "geomorphic work" is difficult to define precisely, partly because the issue is clouded by semantics. ' 45