Pre- and post-forest management investigations of factors affecting sediment movement in riparian areas in Northwestern Ontario
McCormick, Darren James
Master of Science
DisciplineForestry and the Forest Environment
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The principal objectives of this study were to measure the impacts of timber harvesting on sediment transport rates (mineral and organic) associated with two clearcut areas in northwestern Ontario and to evaluate the sediment controlling effectiveness of 30 m wide riparian reserves that were prescribed in accordance with Ontario’s Timber Management Guidelines For The Protection Of Fish Habitat Guidelines (OMNR, 1988). A goal of this study was to provide a practical means for assessing potential changes to sediment transport rates resulting from impacts of forest management activities in Ontario. Prior to road building and full tree logging with feller bunchers and grapple skidders, a sediment sampler was installed in each of 16 sub-catchments. Samplers were situated at a distance of either 0, 10, 20, or 30 m, measured into the reserve areas from the boundaries of planned clearcuts. Mineral and organic sediment collected in the samplers were monitored for one year (late spring to late fall) before, and two years (late spring to late fall) after impacts, and data were standardized with the amount of precipitation that fell during the respective sampling year. Indices of change were calculated to quantify differences in mineral and organic collection rates for each sampler in the first or second post-impact year compared to those in the pre-impact year. Field and GIS data (including: sampler position in the reserve, sub-catchment area, distance from road, presence of surface runoff, occurrence of trees thrown by wind, crown closure, thickness of soil organic layers (LFH), terrain slope, and a topographic index (TI) derived from GIS data describing upstream contributing area and slope) were collected in an attempt to quantify the capacity of the reserve areas to impede (or encourage) sediment collection in each sampler. The results clearly demonstrate that sediment movement in riparian reserve areas does not increase universally following forest management. Sediment attenuation through the reserve areas was variable, indicating that factors in addition to the width of a filter strip can function to control the distance to which eroded sediment is transported. Catchment area was not related to the rates that sediment was collected in samplers. Sediment collection rates were higher in samplers located closer to the road; however, the results can not be used to support categorically the accepted model whereby areas closer to roads are subject to higher erosion rates than areas further away from roads. The occurrence of surface runoff and windthrow, especially when combined, were predominant factors that contributed to increased sediment collection rates in samplers. The amount of crown closure and the thickness of LFH layers influenced rates of erosion, but the magnitudes of their influences were marginal compared to those of flow and windthrow. Steeper slopes did not consistently generate higher sediment transport rates, but the evaluation of the effects of slope on sediment transport rates was limited by the narrow range of slope conditions that were evaluated by this study.