Quantification of the active width in gravel-bed rivers: The effect of morphology, flood magnitude and survey frequency

Active width is a key indicator of bedload dynamics in gravel-bed rivers. While theoretical models typically refer to instantaneous bedload active width, its direct measurement in the field remains difficult due to high spatio-temporal variability. Consequently, morphological active width, derived from detectable bed changes after flood events, is often used as a proxy, though its relationship to bedload transport dynamics across different morphologies, flood magnitude, and survey frequencies remains poorly understood. This study aims to (a) quantify active width in relation to dimensionless stream power and morphology; (b) assess differences between bedload and morphological active widths across time intervals; and (c) evaluate the Exner timescale framework for linking transport dynamics to spatio-temporal morphological evolution. A physical model replicating braided, wandering, and alternate bar morphologies under controlled flow conditions, combining time-lapse imagery and DEM differencing, was used to address these objectives. Results were compared with field data from the Tagliamento (Italy), Rees (New Zealand Aotearoa), and Sunwapta (Canada) rivers. Findings show a strong correlation between active width and dimensionless stream power and reveal a large effect of the timespan, ranging from 0.4 to 3 times the Exner timescale. Morphological active width was consistently lower than the time-integrated bedload active width, with larger differences for single-thread/wandering morphologies, reaching 33%, compared to braiding (about 10%) and remaining constant with increasing timespan. Comparison with repeated DEMs of Difference (DoDs) in the field confirmed the validity of the approach, with both dimensionless stream power and the timespan between surveys controlling the morphological active width. • Flume experiments map bedload and morphological change dynamics at high resolution. • Instantaneous bedload active width correlates with dimensionless stream power. • Braided rivers show higher lateral mobility than other systems at similar timescale. • In braided rivers, morphological width proxies bedload width at short timescale. • The Exner timescale links bedload transport and flood properties across conditions.