HISTORICAL LAND USE INFLUENCE ON LEGACY FLOODPLAIN DEPOSITS IN FORESTED WATERSHEDS IN THE OZARK HIGHLANDS

Hydrologic disturbances due to land use and climate effects can disrupt river form and increase sediment transport. Stream channels in the Ozark Highlands have been affected by accelerated sediment delivery and gravel bar deposition since early European settlement in the mid 1800’s. The purpose of this study is to investigate the alluvial storage of fine-grained sediment released by historical soil and channel erosion and the spatial distribution of legacy floodplain deposits in an Ozark watershed. Big Barren Creek watershed (191 km²) drains the Salem Plateau in Mark Twain National Forest of southeastern Missouri. It is hypothesized that episodes of legacy deposition occurred during two different disturbance periods: widespread logging of pine forests (1880-1920) and stream channelization for flood control along some bottomland segments since 1960. Stratigraphic analysis was used to evaluate the origin and age of floodplain deposits. Floodplain cores collected at 24 locations along the length of Big Barren Creek were analyzed at 3-10 cm intervals for sediment texture, organic matter, Cs-137, magnetic susceptibility and buried soil and root crown elevations. Cs-137 profiles indicate recent (post-1963) sedimentation rates averaging 0.28 cm/yr in upper segments (15.5 km²) and 0.5 cm/yr in middle segments (20-50 km²). Post-settlement (1890-1963) sedimentation rates based on buried soils assumed to represent the pre-settlement surface ranged from 0.31 cm/yr in upper segments and 0.5 to 1.0 cm/yr in middle segments. These forest floodplain sedimentation rates are similar to those reported for legacy deposits in other watersheds affected by row-cropping in the Midwest and Eastern USA.