A STUDY OF MORPHOLOGY AND TEXTURE OF NATURAL LEVEES - CUMBERLAND MARSHES, SASKATCHEWAN, CANADA

Field investigations of natural levees were conducted in the developin g avulsion belt of the lower Saskatchewan River. Sun eyed transects sh ow that levees adjoining main-thread and crevasse channels vary consid erably 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 b ackswamps. Morphometric analyses show that, in general, narrower levee s tend to be steeper, and levees adjoining recently developed crevasse channels are considerably steeper and narrower than those adjoining m ain-thread active or abandoned channels. Levees along main-thread acti ve channels display the largest variety of shapes and slopes. Levee de posits become finer grained from the channel towards the backswamp. Th is 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). Correlation betwe en topographic inflections and lateral variations in the percentage of coarse sediment (mainly sand-sized), and relationships between the sl ope of the levee and percentage of proximally deposited sand, suggest that non-uniform deposition of coarse overbank sediment is primarily r esponsible for the shape and slope of the levee, initially, banks alon g newly formed channels (e.g., crevasse channels) have low relief and are readily submerged by floods. Coarse sediment transferred out of th e channel is deposited within a short distance from the channel margin and leads to formation of initially narrow and steep levees. As overb ank deposition continues, the banks become higher, and it becomes incr easingly difficult for the coarser fraction to be transferred out of c hannel. In contrast, finer suspended sediment is more easily transferr ed over the banks and across the entire floodplain; deposition in dist al portions of the levee reduces the slope. Following abandonment, the channel no longer receives significant amounts of sediment; distal po rtions of the levee, however, continue to receive finer sediment suppl ied by nearby active channels during floods, and slope is further redu ced. Thus, as overbank sedimentation proceeds, natural levees tend to become wider with more gentle slopes because of the different transpor t mechanisms and settling velocities of coarse and fine suspended sedi ment. (C) 1998 Elsevier Science B.V. All rights reserved.