Spatial patterns of historical overbank sedimentation and floodplain evolution, Blue river, Wisconsin

Abstract

Increased soil erosion and overbank flooding associated with land use changes following European settlement have buried pre-settlement floodplain surfaces in many Driftless Area watersheds with as much as several meters of alluvium. This paper examines the processes influencing spatial variations in the storage of post-settlement vertical accretion alluvium and the impact on floodplain evolution in the Blue River watershed, Wisconsin. The cross-sectional area of historical vertical accretion sediments stored on floodplains was measured at 41 cross-valley transects in the 208 km 2 watershed. The storage of historical overbank sediments is influenced by valley width, stream power, and the development of meander belts. Post-settlement alluvial storage increases with valley width and decreases with cross-sectional stream power. In a multivariate regression model, valley width and cross-sectional stream power explain 67% of the variance in the amount of post-settlement alluvium. The development of meander belts, enlarged channels confined between alluvial terraces, by accelerated lateral channel migration influences historical floodplain storage by decreasing the influence of valley width and modifying stream power characteristics for flows, greater than bankfull. Meander belts are located in headwater and mid-basin valley reaches where high stream power produces rapid lateral channel migration, whereas low gradient channels in downstream valleys lack the power necessary to migrate laterally and develop meander belts. Meander belts influence the spatial pattern of historical sedimentation by: (1) containing large magnitude floods and preventing overbank flow, (2) increasing sediment-transport capacities, (3) initiating the erosion of previously deposited historical sediments, (4) reducing the attenuation of flood peaks, and (5) increasing the amount of sediment routed downstream to wide, low gradient valley bottoms lacking meander belts. Continued high rates of lateral channel migration will eventually be damped as widening of the meander belt decreases flow depths and stream power.