DOWNSTREAM CHANGES IN HISTORICAL FLOODPLAIN SEDIMENTATION ALONG THE BIG RIVER, MERAMEC BASIN, MISSOURI OZARKS

In 1987 focusing on Galena Watershed in Wisconsin, Dr. Knox published one of the first studies to use mining-metal dating to interpret watershed-scale trends in channel morphology and floodplain sedimentation in response to historical climate and land use changes. The present study utilizes the same approach to describe historical floodplain deposits and sedimentation patterns along the Big River (2,500 km²). The Big River is a major tributary to the Meramec River which drains the southern portion of the Upper Mississippi Basin within the Missouri Ozarks. The sources of mining-related metal contamination to the Big River were tailings releases from several large mines in the Old Lead Belt Pb-Zn mining district that peaked between from 1910 to 1940. Geochemical profiles were examined in over 70 cores collected from main stem sites at cut-bank exposures and across the valley floor with a push-tube coring rig. In general, there are two overbank floodplain features present: (i) historical floodplains that functioned as tailings sinks during the active mining periods and (ii) recent floodplain benches that have formed since the end of the mining period at an elevation of 1-2 meters below the historical floodplain. In historical floodplains, peak lead concentrations of >5,000 ppm extend over 171 km from Leadwood (uppermost mine influence) to the confluence with the Meramec River. The depth of contamination in over bank deposits ranges from 1 to 3 m, with the contaminated unit overlying uncontaminated pre-settlement material and sometimes a buried A-horizon. Recent floodplain benches are contaminated throughout (up to 7 m thick), but not at such high levels as found in the older floodplains. Results of sediment and metal profile analysis, cross-section surveys, and Cs-137 dating will be used to determine timing and rates of historical sedimentation, describe downstream changes in geomorphic response, and compare Big River trends to the Knox (1987) model for the Galena River.