Impacts of Three Silvicultural Prescriptions on Sediment Mobility and Water Quality in Headwater Streams of Forested Watersheds in the Upper Gulf Coastal Plain of Mississippi, USA

Abstract

Headwater streams are very important contributors of water, sediment, and nutrients to the downstream fluvial environment because their source areas compose more than 50-80% of a catchment. We investi- gated the influence of headwater areas (ephemeral and intermittent) on sediment mobility and downstream water quality using selected water quality parameters (temperature, dissolved oxygen, conductivity, and pH) within three first-order catchments in the Upper Gulf Coastal Plain of Mississippi, USA. This study included one year of pre-harvest and two years of post-harvest observations for ephemeral-intermittent streams. Four treatments including two Best Management Practices (BMPs) for ephemeral streams were applied: standard BMP (SBMP) - removal of all merchantable stems while leaving understory intact with minimum surface soil disturbance; modified BMP (MBMP) - same as SBMP with the addition of logging debris to the drainage channel; Clearcut (CC) - total harvest with no BMPs applied; Reference (REF) - left uncut as a con- trol. Forest clearcutting in ephemeral-intermittent streams resulted in substantial impacts to net erosion/ deposition for both channel and hillslope positions and significantly increased total suspend sediment (TSS) within ephemeral-intermittent stream channels. There were no significant differences among BMPs and REF in net erosion/deposition and TSS. However, perennial stream water quality did not change significantly among treatments following harvest. Results imply that the impacts of timber harvesting were ameliorated by implementing two ephemeral-intermittent stream BMPs during harvesting activities. These findings support the use of either SBMP or MBMP for conserving ephemeral-intermittent streams. While the addi- tion of logging debris in the channel of MBMP did not substantially reduce erosion and sediment flux when compared to SBMP, there may be operational benefits to permitting using a natural sediment baffle because it reduces operational constraints allowing for a greater timber yield.