The morphology of riffle‐pool sequences

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Abstract
The riffle‐pool sequence has not been subjected to the same level of intensive research as the meandering planform, although riffles and pools may be a fundamental prerequisite for meandering. The pseudo‐cyclic oscillation of the bed in a riffle‐pool stream suggests the application of a variety of techniques of spatial series analysis, which provide objective measures of riffle wavelength, and suggest processes capable of explaining riffles and pools and their relationship with meanders.The second‐order autoregressive process is suggested as a stochastic process which models the bed‐profile oscillation. Velocity pulsations associated with large scale turbulent eddies are probably responsible for accretions and erosions which interact with the flow to maintain these perturbations, so that sections lagged by distances of 2πw are positively correlated.The effect of the riffle‐pool sequence on flow geometry is far more significant than the effects of plan geometry or of downstream variations, which supports the view that this feature is a fundamental aspect of channel morphometry. There is a tendency, however, for curved reaches to exhibit reduced variance of roughness, velocity, and water surface slope, which reinforces the minimization hypothesis. The extreme temporal variation between riffle and pool flow characteristics demands that any classificatory scheme uses scale‐free and stable measures, and a discriminant analysis using hydraulic exponents represents a convenient summary of the field data.

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