| Metal mining and ore processing have impacted river systems in much of the world. Many of these activities have occurred in alpine settings with gravel and cobble bed rivers. Much research into the transport and deposition of trace elements in river systems has focused on in-stream sediments, with correspondingly little research regarding trace elements in floodplains.; The distribution of trace elements in floodplain sediments of the upper Blackfoot River in western Montana was investigated regarding distance from potential sources, geomorphic setting, textural classification and soil organic matter. Sediment/soil samples were collected along 20 floodplain transects in the upper 105 river kilometer of the river. The samples were collected at multiple depth intervals, and analyzed for total As, Cd, Cu, Mn, Pb and Zn by ICP-AES. Samples were also analyzed for pH, soil organic matter and particle size distribution. Finally, geostatistical and nonlinear regression models were developed.; Arsenic levels declined to background concentrations 14 km from the Blackfoot headwaters, followed by Cd and Pb (20 km), Mn and Zn (36 km) and Cu (67 km). Floodplain trace elements were most concentrated in the upper 15cm of the soil profile, and at distances of 10 m or more from the channel bankfull stage. River kilometers (distance from mine areas) and stream power were found to be the best predictors of soil trace element concentrations. Soil organic matter was strongly correlated with soil element levels, while silt and clay-sized soil textures were only weakly correlated with levels of Cu and Mn.; This study provides several conclusions regarding the transport and storage of trace elements in intermontane gravel and cobble bed streams. First, trace element levels are negatively correlated with distance from the major source areas. Second, stream power per unit area is not a good predictor of element levels. Third, valley width has a negative correlation with element levels. Fourth, soil trace elements are positively correlated with percent organic matter, and poorly correlated with percent silt and clay. Finally, trace elements are not transported directly through the fluvial system, but may stored in the floodplain, and reactivated at a later time. |
