STRATIGRAPHIC RECORD OF LEGACY SEDIMENTS IN FLOODPLAIN DEPOSITS ALONG BIG RIVER, MISSOURI OZARKS

Floodplains function as long-term storages of fluvial sediments and therefore are often targeted for stratigraphic analyses to evaluate environmental disturbances. In addition, human activities can increase flood frequency and sediment loads in watersheds that produce legacy deposits on floodplains. This study evaluates the sedimentary and geochemical characteristics of legacy floodplain deposits along the Big River watershed (2,500 km²) which drains the Ozark Highlands in southeast Missouri. Upland runoff and soil erosion rates peaked during the historical agricultural settlement period between 1850 and 1910. In addition, Big River received large volumes of mine tailings generated from the Old Lead Belt District which was a world leader in lead production from the early 1900s to 1972. Previous research indicates that mining-contaminated legacy deposits from 1 to 4 m thick occur on floodplains of Big River. However, this study will be first to link floodplain stratigraphy and deposition rates to watershed disturbance events including settlement, mining, large floods, and, possibly, mill dams. This study focuses on the analyses of 15 overbank floodplain cores collected along a transect across the inside of a channel bend. This study focuses on high resolution core analyses at 3 cm intervals to assess vertical trends in sediment properties, metals analyses, Cs-137, organic content, and magnetic susceptibility to link watershed factors to stratigraphic indicators along the lower segment of Big River near the USGS gage at Byrnesville, Missouri. Results include: (i) review of watershed settlement and mining history; (ii) summary of previous floodplain data; and (iii) evaluation of the historical trends of floodplain deposition rates. Preliminary results indicate average overbank deposition rates of 2 cm/yr over the past 150 years and that Pb profiles tend to correlate with production history by the Old Lead Belt.