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

Abstract

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).

Correlation 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.

Introduction

Natural levees, wedge-shape aggradational features that border many alluvial channels, are particularly common along many low-gradient, laterally stable channels (Nanson and Croke, 1992, Table I). Generally, the elevation of levees is highest close to the channel margin and decreases towards the adjoining floodbasin. Although levees are frequently mentioned in the sedimentological and geomorphological literature (Brierley et al., 1997), little research has documented the detailed characteristics and the variability of levees. Only few studies provide detailed descriptions of levees, and these descriptions are usually placed into larger contexts of floodplain facies or overbank sedimentation. Fisk (1947)provides information on the morphology and some general textural characteristics of levees. Allen (1965)mentions the progressive decrease in grain size away from the channel. Wierts and Bierkens (1993)and Tornqvist et al. (1996)show spatial positions of levee deposits within overbank suites, and other workers provide information on sedimentary structures, mostly from cut-bank exposures (Coleman, 1969; Singh, 1972). Some attempts have been made to distinguish levee deposits from other floodplain facies in ancient alluvial sequences (Bridge, 1984; Fielding, 1986; Kraus, 1987; Farrell, 1987; Platt and Keller, 1992; Fielding et al., 1993; Kraus, 1996; Sánchez-Moya et al., 1996). Detailed investigations that include such information as lateral variations in texture of levees, however, are uncommon (Guccione, 1993). Similarly, geometry of levees are, in general, poorly described (Brierley et al., 1997) and little has been published concerning the morphology of levees or the magnitudes and lateral variations of the slopes. Brierley et al. (1997)point out the paucity and ambiguity of existing descriptions of levees in the rock record, noting that “the primary factor inhibiting interpretation of levees in the rock record is the limited number of detailed geomorphological and sedimentological studies from modern depositional environments”. Details of the geometry and the grain-size characteristics of levees can provide useful insights into the formation and evolution, and can contribute to better identification and delineation of levee deposits within alluvial sequences.

To obtain such information on the morphology and texture of levees, a field study was carried out in a portion of the recent avulsion belt of the lower Saskatchewan River (Smith et al., 1989). One goal of the study was to examine geometric characteristics and the variations in shape of levees. Another goal was to examine the surficial grain-size distributions of levee deposits to determine lateral variations from the channel margin to the floodbasin. The relationships between the topography and lateral grain-size variations were then assessed in the context of the development and evolution of levees.