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Forests, floods and channel processes : illuminating links between forest harvesting, the flood regime and channel response in snowmelt headwater streams Green, Kim Cordelia


A meta-analysis of four snowmelt catchments with moderate harvest levels (30% to 40%) utilizing a frequency-based approach demonstrates, despite a century’s-worth of studies to the contrary, how harvesting increases the magnitude and frequency of all floods on record including the largest floods (R.I. = 1:50 yrs.) and how such effects increase unchecked with increasing return period as a consequence of changes to both the mean and standard deviation of the flood frequency distribution. Additionally, meta-analysis outcomes reveal up to three-fold increases in the number and duration of peak flows with return periods ranging from 0.6*Q₁.₅ to Q₁₀, which includes floods capable of mobilizing bedload and altering the form of gravel-bed streams. A frequency-based meta-analysis provides new insights concerning the physical processes governing the relation between forests and floods in snowmelt environments that were previously unrecognized using traditional chronological-pairing methods. The dominant process responsible for flood regime changes following harvesting is the increase in basin-average snowmelt rates that are amplified or mitigated by physical characteristics such as aspect distribution, elevation range, slope gradient and amount of alpine area. The outcomes of a high-resolution, nested-monitoring investigation of hydrologic and geomorphic controls on bedload mobility indicate that flow regime changes from harvesting in snowmelt streams can alter rates of bedload transport, a first-order determinant of channel form in fluvial systems. Regression analysis shows that annual sediment yield is controlled by the number of peaks-over-threshold discharge. During peak events, repeated destabilization of channel armor and re-mobilization of sediment temporarily stored behind LWD generates bedload transport across the entire snowmelt season. However, the potential for channel response to changes in the flow regime depends on characteristics of channel morphology. Study results indicate that patterns of bedload entrainment and mobility are influenced by flow resistance associated with channel form, grain size and LWD while the value of the critical dimensionless shear stress varies with channel gradient, relative and absolute grain effects and flow resistance. Differences in bed texture, gradient and channel form contributes to variations in rates and characteristics of bed mobility, hence differences in potential for alteration due to changes in the flow regime.

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