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.

The recognition of large fluvial channels in the geological record is of great importance for regional palaeohydraulic and palaeogeographical reconstructions, inputs to reservoir modelling, and estimating the input of sediment to sedimentary basins, with consequent larger-scale implications for modelling basin fill. However, available criteria for the interpretation of the scale of ancient fluvial systems are still poorly tested, particularly the widelyadopted assumption that the abundance of large-scale dunes in some deep channels implies that abundant large-scale cross-strata sets will be preserved in similar palaeochannels. To test this hypothesis, high-resolution multibeam echo-sounding imaging of two reaches in the Amazon River where large dunes are common were investigated, yielding an extensive dataset concerning dune geometry, position within the channel and, most importantly, the presence and distribution of smaller superimposed dunes on their lee sides. These results show that despite 90% of the bedforms at water depths >20 m being constituted by up to 12Á2 m high compound dunes, 94% of the lee sides of these dunes are covered by smaller superimposed dunes. These results suggest that steep avalanche foresets that are several metres in height may be rare in the preserved stratigraphic record of these large channels, which are instead more commonly represented by decimetrescale cross-stratified cosets formed by superimposed dunes migrating down the lee side of the large-scale host bedforms. This observation thus suggests that the recognition of compound dune cosets is key to the interpretation of river-channel scale, since compound dunes are the principal bedform in most large river channels. Consequently, successions dominated by decimetre-scale thick cross-strata sets, but that show rarer preservation of outsized metre-scale avalanche foresets, and abundant similar-sized cosets near the base of fining-upward cycles are probably the most common bedform record of large-river channels. 2388

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Alluvial rivers are the most important agents of sediment transport in continental basins, whose fluvial deposits enclose information related to the time when rivers were active. In order to extract the most information from fluvial deposits in the sedimentary record, it is imperative to quantify the natural variability of channel patterns at the global scale, explore what controls may influence their development, and investigate whether channel pattern information is preserved in the alluvial plains in order to develop tools for recognizing them in the sedimentary record. By surveying 361 reaches of modern alluvial rivers with available water discharge data at a global scale, we present a quantitative channel pattern classification based on sinuosity and channel count index applicable to the recognition in the rock record. A continuum of channel patterns ranging from high-sinuosity single channel to lowsinuosity multichannels is documented, along with the proportion of depositional elements in their alluvial plains and their conditions of occurrence. Preserved barforms in the alluvial plains of these rivers are used to infer and quantify paleoflow directions at the channel-belt scale and result in ranges of paleocurrent circular variance that may lead to channel pattern identification in the rock record. Data from this work indicate that the recognition of channel patterns may be used to predict paleogeographic features such as the scale of drainage basin area and discharge, slope, and annual discharge regimes.