How Do Braided Rivers Grow Channel Belts?

River channels often develop channel belts—wider corridors imprinted by past channel occupation—that record past landscape dynamics and form widespread aquifers. Controls on channel belt width for braided rivers have been inferred through physical experiments that evolve an initially straight channel and yield an interpreted, quasi‐equilibrium form. Yet uncertainties remain regarding the independent effects of discharge, slope, and time on channel belt dimensions and the morphodynamic phenomena that coincide with channel belt development. To address these uncertainties, I conducted four experiments that independently varied discharge and bed slope for flow over a bed of medium sand (D50 = 0.42 mm). The laboratory basin (37 m long, 2.7 m wide) was 200 times longer and nine times wider than the width of the largest initial channel, which reduced boundary effects and provided a baseline to measure downstream variability in channel belt width. Transitions in channel belt growth rate occurred in concert with three phases captured by time‐resolved topography data: (1) meandering; (2) braiding, with logarithmic growth of the channel belt; and (3) maturity, with slowing, localized growth due to decreasing flow depth. A dimensionless framework involving discharge, slope, and sediment grain size collapses the growth trajectories of the channel belts to a common trend. In comparison, for natural cases, discharge is the dominant predictor of channel belt width (wcb ~ Q0.55). The time dependence of channel belt width in the experiments suggests that the widths of isolated, ancient channel sand bodies are wider than their formative channels. Key Points Physical experiments tested how initially straight channels in cohesionless sediment evolve to form braided rivers and channel belts Channel belt width varied systematically with discharge, bed slope, and time, consistent with a heuristic model Discharge is a significant predictor of channel belt width for a data set of 11 natural rivers