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<br />perceptible change in alluvial or bedrock channels, even
<br />though instantaneous peak stream power per unit area may
<br />be comparable to other floods that eroded and shaped
<br />bedrock channels. These kinds of stream-power graphs are
<br />representative of flash floods in small basins that rise
<br />quickly and are gone in a matter of minutes. Such floods
<br />are generally caused by cloudburst rainstorms or the rapid
<br />failure of natural or constructed dams such as Reservoir
<br />No.3 in Centralia, Washington, or the Porter Hill dam in
<br />Oregon_
<br />
<br />5. CONCLUSIONS
<br />
<br />Floods are a fascinating phenomenon that mayor may
<br />not be effective agent in shaping the channels and flood.
<br />plains through which they flow. We have attempted to
<br />demonstrate that it is possible to quantify approximately the
<br />amount of energy in a flood available for geomorphic
<br />work. Construction of stream-power graphs from channel
<br />geometry and flood hydrographs shows how stream power
<br />varies at a cross-section throughout a hydro graph, and
<br />allows computation of the total geomorphic energy
<br />expended by a flood. A conceptual model combining flood
<br />duration, stream power per unit area, and thresholds for
<br />alluvial and bedrock channel erosion can predict geo-
<br />morphic effectiveness and distinguishes between cases
<br />where (a) some floods with long duration and large total
<br />energy expenditure along alluvial channels may not be
<br />effective channel or floodplain-disrupting events; (b) some
<br />floods with very large peak instantaneous stream power per
<br />unit area, but low total energy expenditure, may also not
<br />be effective channel or floodplain-disrupting events; and (c)
<br />floods with a combination of high peak instantaneous
<br />stream power, sufficient flood.flow duration, and large
<br />total energy expenditure are able to alter significantly the
<br />land surface, and become geomorphically effective floods.
<br />While we believe the preceding analysis is a valuable
<br />way to conceptualize and perhaps predict channel and
<br />floodplain changes, it only addresses half of the force
<br />versus resistance equation. We suspect that quantifying
<br />landscape resistance and erosion thresholds will prove to be
<br />much more difficult than quantifying the hydraulic forces.
<br />One conclusion is sure: floods will continue to provide a
<br />bounty of questions and opportunities for generations of
<br />present and future students.
<br />
<br />Acknowledgments. The inspiration and example of Reds Wolman
<br />helped focus our thinking ahout fluvial processes. especially
<br />floods. Questions he formulated nearly four decades ago continue
<br />to challenge us_ We also thank Andy Miller for careful and
<br />thoughtful comments that helped reshape an early draft. and Robb
<br />
<br />COSTA AND O'CONNOR 55
<br />
<br />Jacobson, Andy Miller, Michael Church, and Peter Wilcock for
<br />their insightful reviews. Jasper Hardison assisted in the
<br />calculation of hydraulic parameters.
<br />
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