Harvest, Site Preparation, and Firebreak Effects on Hydrology and Sediment Transport in Coastal Plain Headwater Streams

Abstract

The Dry Creek paired watershed study was conducted to evaluate the effectiveness of modern southeastern forestry best management practices (BMPs) by observing hydrology and sediment transport in four southwest Georgia headwater streams during pre-harvest, post-harvest, and post-site preparation (post-prep) periods. After collecting two years of baseline data on all four streams, two of the watersheds were harvested in accordance with the Georgia Forestry Commission's 1999 BMPs. Downstream sections of streamside management zones (SMZs) were partially harvested to the maximum extent allowable by BMP guidelines of 11.5 m2 ha-1 (50 ft2 acre-1) of remaining basal area, while upstream sections remained unthinned. Treatment data collection consisted of a one-year post-harvest period and a two-year post-prep period. After harvest, water yields increased by 92 to 267 mm year-1 and groundwater tables rose, as evidenced by the immediate appearance of toe-slope seeps in the treatment watersheds. Peak flow behavior displayed large variation among storm events and between watersheds but did not significantly change after harvest. Total suspended solid (TSS) yields and inorganic suspended solid (ISS) concentrations were variable across watersheds and years but did not reveal significant effects as a result of harvest or site preparation. Firebreaks plowed around the SMZs were an area of concern for sediment production. Occurrences of concentrated overland flow entering SMZs and streams increased in the treatment watersheds immediately after harvest but decreased to pretreatment levels within two years following harvest. Modern forestry BMPs were effective in preventing sediment increases due to harvest and site preparation within the studied streams.