ABSTRACT: The character of channel planform control of the morphology and sedimentology of the gravel-bed Squamish River floodplain, British Columbia

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Abstract

The gravel based, high energy Squamish River exhibits a distinct downstream sequence of river channel planform styles. In the 20km study reach, the river changes from a braided through a wandering gravel-bed to a meandering planform type. To assess the sedimentologic distinctiveness of these planform styles, sediment sequences were examined in a series of holes, regularly spaced in a grid-like fashion on 10 channel bar surfaces, in 9 trenches, dug perpendicular to the main channel at the bar/floodplain margin of 5 bars, and in 13 longitudinal bank exposures, divided equally among the three planform styles. A specific facies coding scheme was developed, based on channel bedform features. An alternative, broader scale depositional unit (termed elements), based on geomorphic criteria, also was employed in the trench and bank exposure data sets. Sedimentologic differentiation of the three channel planform styles was examined in terms of facies and element abundance, their composition and character, their spatial organization (vertically, laterally and longitudinally), particle size trends, and basal surface characteristics. Markov-derived planform facies models indicate complex sediment associations in each planform reach. Locale spatial association varies by planform, but this variability is not reflected in floodplain deposits, as channel bar sediments are reworked by chute channels prior to being incorporated into the floodplain (i.e. bar platform deposits have extremely low preservation potentials). In both the trench and bank exposure data sets, facies are extremely laterally discontinuous, and vertical sediment sequences typically are characterized by upward transitions from channel framework gravels to lower energy depositional units, irrespective of channel planform type. When analysed in vertical sequence, locale type is evaluated in elemental terms. The established floodplain locale is divided into flood cycle and sand sheet elements. These vertically accreted, top stratum deposits, are the dominant component of the Squamish River floodplain. In the older sediment sequences of bank exposures, distal overbank deposits occasionally are observed. In these instances, the preserved proportion of bar platform (bottom stratum) sands is greater, as chute channel reworking of sediments is minimized by rapid shifting or avulsion of the main channel.

Bibliography of Canadian Geomorphology