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Stream channel adjustments following logging road removal in Redwood National Park
The excavation of road fill from stream crossings on former logging roads is a major focus of the watershed restoration program in Redwood National Park. Channel adjustments in response to winter stormflows were measured on 24 newly excavated stream crossings to identify the relative importance of on site independent variables. Two dependent variables, channel erosion and surficial channel bed armor, were regressed on sets of independent variables related to stream hydraulics and stream bank material properties in two series of regression analyses. In the channel erosion series, the final regression equation incorporated as significant predictors (in order of decreasing predictive importance) total stream power, the percent boulder and cobble content of streambank materials, and the percent stream elevation drop composed of organic debris steps. Of primary importance in explaining the variability in channel erosion was total stream power of the peak flow of the study period, which was directly related to channel erosion. The boulder and cobble content of stream bank materials, inversely related to channel erosion, was second most important among predictors. Because the best fit was obtained using the square of this quantity, it is inferred that the potential for erosion decreases as a power function of increasing coarse fragment content of eroding materials, all other factors being equal. The percent of stream elevation drop composed of organic debris steps was inversely related to channel erosion because it provided for energy dissipation and hydraulic resistance. In the channel armor regression series, the final regression equation incorporated (in order of decreasing predictive importance) total stream power and percent of the stream elevation drop composed of organic debris steps as significant predictors. Being inversely related to channel armor, the presence of organic debris steps partially offset the channel armoring demands of total stream power. To minimize channel erosion following stream crossing excavation, consideration must be given to the relative magnitudes of driving forces (stream power) and resisting forces (coarse fragment content of bank material and level of organic debris control of the stream profile). In proposed excavations where resistance to erosion is anticipated to be low and stream power is expected to be high, consideration should be given to channel protective measures such as installation of check dams, importation of rock armor, or emplacement of organic debris.