Variations in natural levee morphology in anastomosed channel flood plain complexes

Abstract

Natural levees are common features of alluvial river systems, yet their origin and evolution are poorly understood. In this paper, we present morphologic and sedimentologic data from two anastomosed rivers and offer a hypothesis of natural levee growth in these systems based on mechanisms of sediment transport. In settings where floodbasins fill at the same rate as the channel, levees form by turbulent diffusion of suspended sediment away from a high-velocity thread into a floodbasin of relatively stagnant water. The theory of diffusive transport suggests that these levees should be narrow, steep, and display abrupt decreases in grain size due to rapid decreases in carrying capacity with distance into the floodbasin. In environments where an appreciable water surface slope is maintained between the main channel and the floodbasin, levees form by advection of sediment out of the channel and into the floodbasin. Advective transport theory indicates that these levees should be broad and gently sloped, with grain sizes gradually decreasing away from the main channel. Natural levees occurring in the anastomosed reach of the upper Columbia River in SE British Columbia display significantly different morphology from levees in the Cumberland Marshes region of the lower Saskatchewan River in east-central Saskatchewan. At the upper Columbia site, the rise in stage of the floodbasin water nearly keeps up with that of the channel water because of good communication between channel and floodbasins through crevasses. This inhibits the establishment of a water surface slope, and the bulk of the escaping sediment is deposited close to the channel. In contrast, the Cumberland Marshes region is characterized by wide and volumetrically large floodbasins. These conditions keep floodbasin water surface elevations relatively low and maintain an appreciable elevation head between the channel and its floodbasin, fostering levee growth by advective transport.