Predicting the (active) width of gravel‐bed braided rivers

The bankfull width of single-thread gravel-bed rivers (Wbf) can be effectively predicted by empirical and theoretical equations. However, applying these width predictors to multithread rivers is more challenging, and channel width is often underpredicted. Field and laboratory studies indicate that in braided rivers, only a small portion of the wetted width—referred to as the active width—is involved in sediment transport or shows morphological change, over short time periods like that of a single flood event. In this study, we compile results from various laboratory experiments to test the hypothesis that single-thread bankfull width predictors better estimate the active width rather than the total channel width in braided rivers. Our dataset, which spans a range of stream power and degrees of flow confinement, confirms that active width does not depend on the available river corridor space and the width estimators align closely with active width measurements. Prior research has shown that for gravel-bed braided rivers active width is typically 15% to 40% of the wetted width suggesting that a minimum corridor width about three times Wbf is necessary to sustain a braided morphology for stream power conditions under which braiding could occur. We further tested these findings taking advantage of a recently published dataset of historic trajectories of braided rivers in the Canterbury region, Aotearoa New Zealand. On average, braided reaches exhibit channel widths greater than three times Wbf. In contrast, wandering reaches have widths roughly twice Wbf, and single-thread reaches correspond closely to the estimated bankfull width. These findings offer a robust physical and theoretical basis for channel corridor design criteria that have been applied in restoration and conservation efforts for braided rivers in this region and are relevant to controls on the occurrence, and to conservation and restoration, of braided morphology more generally.