HISTORICAL LAND USE INFLUENCE ON FINE-GRAINED SEDIMENTATION IN CHANNEL AND FLOODPLAIN DEPOSITS IN A FORESTED MISSOURI OZARK WATERSHED

Hydrologic disturbances due to land use and climate effects can disrupt river form and increase sediment transport. Ozark streams have been experiencing the effects of accelerated channel erosion on coarse sediment delivery and gravel bar deposition since the onset of early European settlement in the late 1800’s. Little attention has focused on understanding the fate of fine-grained sediment released by upland soil and headwater channel erosion and the potential for storage as legacy deposits on floodplains. Legacy deposits are attributed to human disturbances as the result of land clearing and agriculture that increase runoff, soil erosion, flooding, and sediment supply in watersheds. Big Barren Creek (BBC) watershed (191 km²) drains the Salem Plateau in the Ozark Highlands in south eastern Missouri. The watershed was heavily logged between 1880 and 1920 and stream channelization practices on farmland in the area began as early as 1950. Today about ¾ of the watershed area is within the Mark Twain National Forest. This study assesses the occurrence of fine-grained alluvial deposits along BBC and its tributaries and characterizes the spatial distribution and history of legacy sedimentation on alluvial landforms. There were four conclusions: (i) Fine-grained legacy deposits occur in BBC and are distributed non-uniformly upon channel, floodplains, and terrace landforms; (ii) Rates of post-settlement deposition from 1890-1950 were highest in upper BBC (~0.45 cm/yr) where the effects of historical timber harvest were most prevalent and decreased downstream to 0.29 cm/yr in middle BBC; (iii) Rates of post-1950 sedimentation were highest in lower BBC (~0.80 cm/yr) due to increased sediment supply from upstream head-cutting, channelization, and lateral bank erosion in disturbance zones; and (iv) In response to human-induced watershed disturbance, BBC has generally undergone a transition from a multi-threaded channel to a single channel form over the past century.