Effectiveness of Forestry Best Management Practices in minimizing harvesting impacts on streamflow and sediment loading in low-gradient headwaters of the Gulf Coastal Plain

Abstract

Few studies have examined the effectiveness of timber harvesting Best Management Practices (BMPs) in water quality protection of widely-spread, low-gradient, and highly intermittent headwaters on the Gulf Coastal Plain. Also, a spatial disparity exists between the plot-scale water quality benefits afforded by BMP implementation and the watershed-scale with which most watershed stewardship programs are managed. In this thesis research, paired-watershed and Before-After-Control-Impact (BACI) designs were utilized to quantify plot- and watershed-scale changes in streamflow, as well as baseflow and stormflow Total Suspended Sediment (TSS) concentration and yield for 27 months after clearcut harvesting with and without BMPs in a low-gradient, forested, 3rd-order watershed of north-central Louisiana. Based on analyses of post-harvest baseflow, stormflow, stage-discharge relationships, TSS concentration, and sediment yield, low-intensity (2-8% disturbance of sub-watershed drainage area), clearcut harvesting adjacent to streams and with BMP implementation did not impact streamflow or sediment transport at the plot- or watershed-scale. No difference was found between treatment periods for monthly baseflow discharge measurements or in peak water level response to storm events. Flow duration curve analysis showed that baseflow decreased during the post-harvest period, possibly due to differences in the timing of precipitation between treatment periods. Changes in the stage-discharge relationship were observed downstream of harvesting without BMPs, indicating harvest-induced changes to stream geomorphology. Baseflow and stormflow TSS concentration (mg L-1) and yield (kg ha-1 mo.-1) were similar between treatment periods and were on the lower end of published results for Coastal Plain sites. Post-harvest TSS yield increased downstream of harvesting without BMP implementation when high flow events were included in yield calculations. These results indicate that current Louisiana BMPs for timber harvesting are effective in mitigating sediment runoff at the plot- and watershed-scale for conditions similar to the monitored sites, which include an abundance of beaver/debris dams and highly intermittent streamflow. These natural conditions may have further improved sediment reduction from BMP implementation through ponding and reduction of flow rate and carrying capacity. The potential exists for future studies to determine the intermittency and beaver dam impacts to streamflow and sediment transport as forest disturbance increases throughout the watershed.