Hydrologic and sediment transport response to forest harvest and site preparation in headwater streams

Abstract

Best management practices (BMPs) are used in silviculture to reduce the adverse environmental effects of forest harvesting and site preparation. States first developed BMP guidance around 1980, and since then BMPs have evolved in response to new science. Without BMPs, forestry activities can detrimentally alter downstream water quality by introducing undesirable quantities of sediment, nutrients, and light, destabilizing stream channels, and reducing organic and woody debris inputs. The Dry Creek paired-watershed study was conducted to evaluate current Georgia forestry BMPs by observing the hydrology and sediment transport in four Southwest Georgia headwater streams during pre-harvest, post harvest and post site preparation periods. The treatment watersheds were clearcut harvested with rubber-tired skidders, and all activities were conducted in compliance with existing Georgia BMPs, including 40 and 70 foot streamside management zones (SMZs) depending on side-slope. SMZs were bisected into an upstream and downstream section. Downstream SMZ sections underwent a partial harvest, while the upstream sections remained intact. Our data included two years for watershed calibration before harvest, one year of post harvest data, and two years of post site preparation data. In treatment watersheds, water yield increased as a result of harvest by 30 to 316%. Storm event peakflows significantly increased for one pair, but decreased significantly for the other pair after harvest. Natural variance in sediment transport was high and a statistically significant response to harvesting and site preparation was not observed. Evidence of concentrated overland flow entering SMZs and streams increased in the treatment watersheds immediately after harvest, but was reduced within two years following harvest. KEY WORDS: Paired Watershed, Best Management Practices, Streamside Management Zones, Partial Harvesting, Water Quality, Nonpoint Source Pollution, Water Yield, Peakflow, Hysteresis.