River planform facies models: the sedimentology of braided, wandering and meandering reaches of the Squamish River, British Columbia

The Squamish River, in common with other large, high-energy gravel-bed rivers in the Coast Mountain Ranges of British Columbia, exhibits a distinct downstream sequence of planform types. Using consistent field and analytical techniques, river planform facies models were derived for each of three contiguous reaches of the Squamish: braided, wandering and meandering. Sediment zones less than thirty years old were sampled systematically upon bars in each reach by digging holes to channel gravel depth or the water table. Sediment sequences were classified using a facies coding scheme based upon particle size, bedding properties and structural characteristics. The internal organization of facies types varies considerably in the facies models derived for each planform reach using Markov analysis. These planform facies models are shown to be unrepresentative of local sediment organization, as many one- and two-step upward facies transitions predicted for a given planform type are found with greater abundance in one or both of the other models. Predicted transitions can be viewed as statistical artifacts and are not representative of sediment associations observed in the field. Examination of the internal organization of small-scale structural units using summary statistical data indicates no consistent variation with channel planform. It is concluded that differentiation of fluvial depositional suites using one-dimensional data at the scale of channel planform is inapplicable.