A study of morphology and texture of natural levees—Cumberland Marshes, Saskatchewan, Canada
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.
Section snippets
Study area
The Cumberland Marshes are part of a large wetland region situated along the Saskatchewan River near the Saskatchewan–Manitoba border, Canada (Fig. 1A). The area occupies the western margin of former glacial Lake Agassiz which covered the region approximately 10,000 yr BP (Schreiner, 1983). Since the retreat of Lake Agassiz, the wetland terrain has been continuously modified by fluvial processes. Regional aggradation has forced major river channels to periodically shift routes by avulsion, and
Methods
The reaches selected for this study include active and abandoned main-thread channels and an active crevasse channel. The levees bordering these channels differ in size, shape, and steepness. Eight cross-channel transects (A through H, Fig. 1C) were surveyed, and sediment samples were collected along these transects. Four transects were located on major active channels: one on the New Channel (A) and three on the Centre Angling Channel (B, C, and D). Two other transects were located on
Morphology
Topographic profiles of the eight transects show a large variability in the morphology of levees, reflected by differences in shape, width, and slope (Fig. 2). Morphometric analysis requires a precise definition of the geometry of the levee; however, it is difficult to precisely measure the width of a levee because levees form a continuum with the floodbasin, commonly without a clear demarcation between the two. Examination of topographic profiles and lateral variation in slope reveals that
Texture
Textural variations along levee transects are shown in Fig. 6 along with normalized topography. A large variation in texture can be observed, with most of the variation occurring close to the channel margin. The most common size fractions are in the range of very fine sand–coarse silt. At those locations where both levees of a pair were sampled (B, D, F, G, H), the grain-size distributions differ from one side to the other (Fig. 6).
It is commonly believed that the mean grain size of levee
Discussion
Because elevation and textural characteristics vary with distance from the channel, it is reasonable to look for a connection between these two variables. Fig. 6 shows that at many locations, topographic inflections of the levee surface correspond to abrupt changes in the proportion of coarse sediment. For example, in B-right, C-right, D-left, D-right, E-right, F-right, and H-left, the elevation decreases sharply as the percentage of sand decreases. In B-left and F-left, the elevation decreases
Conclusions
1. For levees of this study, it is possible to define the widths based on local variation of slope, with a slope value of 0.01 corresponding to the transition between the levee and backswamp. Levees along an active crevasse channel are relatively narrow and steep whereas those along abandoned channels are wide and gentle. Levees developed along main-thread active channels are, in general, intermediate in character and show the largest variety of slopes and sizes.
2. The median grain sizes of
Acknowledgements
We would like to thank Marta Pérez-Arlucea for her assistance in preparation of Fig. 1. Also, special thanks to Stella Hocksbergen, our hostess at Cumberland House. This paper is based on part of the senior author's M.S. thesis (Cazanacli, 1997). Our work was supported by National Science Foundation Grant EAR-9304104.
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