THE ROLE OF SEDIMENT SUPPLY IN THE MORPHOLOGY AND DYNAMICS OF A SAND BED ALLUVIAL RIVER

Sand bed alluvial rivers adjust their width, depth, and slope to convey the supply of water and sediment. However, field scale ‘natural experiments’ that allow us to study the influences of changes in flow and sediment supply independently have been exceptionally rare. We explore one such case in the lower 175 km of the mainstem Minnesota River. The upper two thirds of this stretch of river experiences high sediment supply, but only a small fraction of the bed load is conveyed down to the lower third of the study reach. Flow increases only slightly in the downstream direction, but has increased considerably throughout the study reach over the past 30 years. We utilize an extensive bedload and suspended load monitoring database, longitudinal measurements of bed grain size, over 230 km of bathymetry data, and measurements of channel width and meander migration from historical air photos to document stark differences in the morphology, dynamics and modes of adjustment of the river channel. The high sediment supply reach exhibits considerably more complexity in bed topography and stronger asymmetry in channel cross section. This establishes hydraulic conditions that are amenable to relatively higher rates of lateral migration and therefore high capacity to adjust channel width in response to increases in flow over the past several decades. The lower, sediment starved reach, in contrast, exhibits considerably less topographic complexity and therefore exhibits limited ability to migrate laterally or adjust channel width in response to the significant increases in flow observed over the past few decades. We utilize a stochastic, reduced complexity model that routes a series of annual peak discharges and associated sediment supply through a simplified reach-averaged channel-floodplain cross section to further explore adjustments in channel width, depth, and slope in response to changes in peak discharge and sediment supply.