A study of morphology and texture of natural levees—Cumberland Marshes, Saskatchewan, Canada

Field investigations of natural levees were conducted in the developing avulsion belt of the lower Saskatchewan River. Surveyed transects show that levees adjoining main-thread and crevasse channels vary considerably in size, shape, and slope away from the channel. A slope value of 0.01 was used to define the boundary between levees and adjoining backswamps. Morphometric analyses show that, in general, narrower levees tend to be steeper, and levees adjoining recently developed crevasse channels are considerably steeper and narrower than those adjoining main-thread active or abandoned channels. Levees along main-thread active channels display the largest variety of shapes and slopes. Levee deposits become finer grained from the channel towards the backswamp. This variation can be expressed quantitatively either as an exponential decrease of median diameter or as a linear increase of the percentage of sediment finer than 0.016 mm (fine silt to clay). ation between topographic inflections and lateral variations in the percentage of coarse sediment (mainly sand-sized), and relationships between the slope of the levee and percentage of proximally deposited sand, suggest that non-uniform deposition of coarse overbank sediment is primarily responsible for the shape and slope of the levee. Initially, banks along newly formed channels (e.g., crevasse channels) have low relief and are readily submerged by floods. Coarse sediment transferred out of the channel is deposited within a short distance from the channel margin and leads to formation of initially narrow and steep levees. As overbank deposition continues, the banks become higher, and it becomes increasingly difficult for the coarser fraction to be transferred out of channel. In contrast, finer suspended sediment is more easily transferred over the banks and across the entire floodplain; deposition in distal portions of the levee reduces the slope. Following abandonment, the channel no longer receives significant amounts of sediment; distal portions of the levee, however, continue to receive finer sediment supplied by nearby active channels during floods, and slope is further reduced. Thus, as overbank sedimentation proceeds, natural levees tend to become wider with more gentle slopes because of the different transport mechanisms and settling velocities of coarse and fine suspended sediment.