Sediment Yield along an Actively Managed Streamside Management Zone

Abstract

In this study, we aim to regenerate a mature streamside management zone (SMZ) and create an uneven-aged forest with multiple canopy tiers and a dense understory using single tree selection (based on the Proportional-B method). We observed the effects of this partial cutting on sedimentation by comparing a treatment watershed with an unharvested reference site. In addition to determining partial cutting effects on sediment yield, we also evaluated the effects of different land uses and a recent clearcut on sedimentation, quantifying the effects of forest cover on sediment, and determining the efficacy of the SMZ at reducing sediment yield from the clearcut. The study was conducted on the Mary Olive Thomas Demonstration Forest which is owned and managed by the School of Forestry and Wildlife Sciences. Sediment water quality data were sampled from April 2009 to April 2010. Water stage measurements were monitored using pressure transducers installed at each monitoring station. In addition to continuous water stage measurements by transducers, stream discharge measurements were recorded, and water samples collected during storm events. Continuous discharge data were created using rating curves between water levels and discharge data. Total suspended sediment (TSS) was determined from water samples and continuous sediment data were estimated using the LOADEST software. Pre-harvest data from two watersheds were calibrated to determine the effects of the partial cutting on sedimentation. During the calibration period, forest cover caused a decrease in sediment yield while the current clearcut and forest road increased sediment yield because the SMZ was not sufficient to trap all of the sediment originating from these disturbances. During the treatment period, in comparison to the reference watershed, the partial cut within the SMZ caused an increase in both water and sediment yield on the harvested sections. During pre-harvest period, upstream sections (pasture and urban on the treatment and control watersheds, respectively) generated much more sediment yield than downstream sections. However, following harvest there was a significant increase in sediment load from downstream sections of the treatment. Our data suggest that undisturbed forest cover seems to be effective at reducing sediment yield. It may be suggested that forest operations can cause an increase in sediment load if forest roads and SMZs are not managed properly. If effective forest road best management practices (BMPs) are not in place, then simply focusing on SMZs to reduce sediment yield is not sufficient. The study also shows the importance factoring in upstream land use/cover conditions in designing SMZs for sediment trapping as well as the importance management of a watershed as a whole to increase the efficacy of BMP’s.