HISTORICAL FLOODPLAIN DEPOSITION AND DISTURBANCE ZONE EVOLUTION IN THE MIDDLE JAMES RIVER, SW MISSOURI

Floodplain deposits contain a record of geomorphic response to land disturbance over different timescales. The James River Basin (3,770 km²) in southwest Missouri has a record of settlement going back to the early 1800s. This study uses aerial photography, Zn mining-related tracers, and contemporary Cs-137 records to identify the temporal and spatial distribution of historical alluvium on the valley floor. Aerial photos show the channel planform position has remained relatively unchanged since the 1950s except at localized disturbance zones. Disturbance zones active since the 1950s tend to be located in areas of gravel bar deposition with little lateral, fine-grained floodplain development. The floodplain sedimentation record however shows that the lower floodplains were actively forming during historical time with Zn contamination profiles indicating periods of lateral migration that occurred prior to the historical photography record. Post-1950s channel migration is relatively limited, suggesting geomorphic recovery to earlier land use changes and that Ozarks rivers at this scale may be closer to equilibrium to current watershed conditions. Core analysis also shows 0.5-1 m of overbank sedimentation on the pre-settlement floodplain indicting significant fine-grain material storage. These results will be used to properly interpret channel morphology data used for stream restoration and aquatic habitat assessments.