The Geomorphic Influence of Agricultural Land Use on Stream Hydraulics and Biological Function

Abstract

Agricultural land use near streams frequently results in long-term disturbance to woody riparian vegetation and an alteration of reach scale geomorphic structure. Such disturbances often result in increased fine sediment input to the stream along with direct changes in channel structure. The study described here was designed to quantify stream geomorphic changes associated with agriculture and their influence on reach scale transient storage hydraulics and sediment biological function. Six small streams in the Appalachian Mountains of western North Carolina were selected to compare 3 reaches with active near-stream agriculture to 3 forested reference reaches. The study site categories differed significantly in many structural and hydraulic properties including slope, sinuosity, sediment size, and transient storage extent. However, differences cannot be attributed to land use alone. Distinct disparity in slope suggests that many of the categorical differences between stream types may also reflect valley scale structure. Despite these larger scale controls, the abundance of suspendable fines varied substantially among agricultural stream substrates, possibly due to varied land-use practices. Suspendable fine sediments and valley slope explained 91 % of variability in transient storage exchange, and abundance of inorganic fine sediments explained 77 % of variability in sediment microcosm nitrate production. This study supports conclusions that reach-scale influence of fine sediments occurred within the context of larger-scale valley structure, with implications on stream hydraulics and biogeochemistry.