Channel planform as a non-controlling factor in fluvial sedimentology: the case of the squamish river floodplain, British Columbia

Abstract

Channel planform styles demonstrate a continuum of variability. Whereas end member situations may exhibit diagnostic sedimentologic characteristics, non-end member situations may be exceedingly difficult to reliably differentiate. The relevance of channel planform-based sedimentologic differentiation of fluvial depositional units has been questioned for some time (e.g. Jackson, 1978; Bridge, 1985; Brierley, 1989a). Despite reservations outlined in these papers, facies models and interpretations of depositional suites in the rock record continue to be applied in terms of channel planform style. This paper describes a test of this principle in one particular depositional setting. The Squamish River, in southwestern British Columbia is a high-energy, gravel-based river in a fjord setting. In a 20-km reach, this river displays a distinct downstream gradation in channel planform type, from braided through wandering gravel-bed to meandering. Assemblages of bedform-scale facies units on bar surfaces in this river do not differ by planform type (Brierley, 1989a); rather, facies associations relate directly to the pattern of morphostratigraphic units on bar surfaces (Brierley, 1991a). When analysed in floodplain exposures, sediment sequences which are equivalent to morphostratigraphic units are termed elements. These elements are defined in terms of their geometry, bounding surface and sediment characteristics. Detailed analysis of floodplain deposits in the braided, wandering gravel-bed and meandering reaches of the Squamish River demonstrates that just as assemblages of bedform-scale facies cannot be differentiated by planform style, neither can assemblages of floodplain elements. Indeed, for the Squamish River, the type, character, and spatial association of these elements cannot be differentiated reliably at the channel planform scale. This indicates the limited relevance of planform differentiation of sedimentary environments under transitional planform situations such as those of high-energy, gravel-bed rivers in confined valleys. In such situations, sediment inventory at the element scale provides greater insight into mechanisms of floodplain evolution than can be provided by interpretation at either the channel bedform or channel planform scales.