Spatial and temporal variations in the grain-size characteristics of historical flood plain deposits, Blue River, Wisconsin, USA

This study examined vertical, lateral, and downstream variations in the grain-size characteristics of historical (post-1830) overbank deposits in a watershed that has experienced high rates of accelerated flood plain sedimentation. More than 800 samples were collected from 53 cores along nine flood plain transects. Overbank deposits exhibit a coarsening-upward sequence attributed to historical changes in the sand content of source materials. The erosion of loess-capped soils increased the exposure, erosion, and transport of sandy parent materials. The average sand content of near-channel cores increases moderately downstream along two of the reaches because sandy source materials are increasingly exposed in larger main valleys in the northern part of the watershed. The two northernmost reaches are coarser overall, but do not display significant downstream trends. The sand content of surface and early historical overbank deposits generally decreases laterally as an exponential function of distance from the channel, suggesting transport by turbulent diffusion. The presence of sand throughout the transects and lateral coarsening at two of the transects, however, suggests that sediment transport by convection is also important.