Map showing bathymetric survey of the Missouri River channel in the vicinity of structures 435-105-11.97 (235) and 435-105-11.98 (240) on Interstate 435 near Kansas City, Missouri ...

Dunes are present in all the worlds' big rivers and form critical agents of bedload transport, constitute appreciable sources of bed roughness and flow resistance, and generate stratification that is the most common depositional element of ancient alluvium. Yet our current models of dunes are conditioned by the geometry of bedforms observed in small rivers and laboratory experiments, and in which the downstream leeside angle is often assumed to be at the angle-of-repose. Here we show, using high-resolution bathymetry from a range of the worlds great rivers, that dunes are instead characterized predominantly by low-angle leeside slopes (<10 • ), complex leeside shapes where the steepest portion is near the base of the leeside slope, a mean wavelength:height ratio greater than 100, and a height that is often only 10% of the local flow depth. This radically different shape of dunes in the world's big rivers demands that we incorporate such geometries into predictions of flow resistance and water levels, rethink the scaling relationship of dunes when reconstructing alluvial palaeoflow depths, and calls for a fundamental reappraisal of the character, and origin, of low-angle cross-stratification within ancient alluvial sediments.

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with footing exposure. Detailed hydraulic assessment (Level 2) scour depths were conservatively estimated by the rapid estimation method (Level 1+) for abutment scour, but there was substantial variability in the estimates and several substantial underestimations.