Structure-from-motion (SfM) photogrammetry has become widely used for topographic data collection in field and laboratory studies. However, the relative performance of SfM against other methods of topographic measurement in a laboratory flume environment has not been systematically

River channels commonly exhibit downstream variations in channel width, which can lead to the development of alternating shallow and deep areas known as riffle-pool sequences. The response of these channels to variations in sediment supply remains largely unexplored. Here we investigate the morphodynamic response of a variable-width channel to changes in sediment supply through laboratory experiments conducted in a straight flume in which we imposed sinusoidal variations in width. We first developed equilibrium conditions under a constant sediment supply and then eliminated the sediment feed to create a degraded, armored bed. This sediment-starved bed was subjected to two types of sediment supply increases: a return to the initial constant supply, and the introduction of a well-sorted sediment pulse (analogous to gravel augmentation). Riffles and pools formed in wide and narrow areas, respectively, and the location of and relief between riffles and pools remained the same throughout all experimental runs, regardless of the sediment supply. The primary channel response to changes in supply was adjustment of the overall slope. The sediment pulse evolved primarily through dispersion rather than translation, which contrasts with prior gravel augmentation experiments conducted in constant-width channels and suggests that width variation and resulting riffle-pool topography enhances pulse dispersion. Our results indicate that width variation is a primary control on the location and relief of riffles and pools in straight channels, and sediment supply changes are unlikely to affect riffle-pool morphology when bank geometry is fixed and water discharge is steady.

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Gravel‐bed rivers characteristically exhibit shallow riffles in wide sections and deeper pools where the channel becomes constricted and narrow. While rivers can adjust to changing flow and sediment supply through some combination of adjustments of channel slope, bed‐surface sorting, and channel shape, the degree to which riffle‐pools may adopt these changes in response to changing flows and sediment supplies remains unclear. This article presents results from a flume experiment investigating how constant‐ and variable‐width channels adjust their morphology in response to changing flow and increased sediment supply. Two flume geometries were used: (1) constant‐width and (2) variable‐width, characterized by a sinusoidal pattern with a mean width equal to that of the first channel. The variable‐width channel developed bed undulations in phase with the width, representing riffle‐pools. The experiment consisted of three phases for each flume geometry: (1) steady flow, constant sediment supply; (2) unsteady flow, constant sediment supply; and (3) unsteady flow, doubled sediment supply. Unsteady flow was implemented in the form of repeated symmetrical stepped hydrographs, with a mean discharge equal to that in the steady flow phase. In all phases the bed and sediment supply were composed of a sand/gravel mixture ranging from 1 to 8 mm. In both the straight and variable‐width channels, transitioning from steady flow to repeated hydrographs did not result in significant changes in bed morphology. The two channel geometries had different responses to increased sediment supply: the slope of the straight channel increased nearly 40%, while the variable‐width channel reduced the relief between bars and pools and decreased the variability in cross‐sectional elevation with a slight slope increase. Bar‐pool relief varied with repeat discharge hydrographs. Pool elevation changed twice the distance of bar elevations, emphasizing the relevance of pool scour for riffle‐pool self‐maintenance in channels with width variations.