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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The Amazonian landscape evolution is the result of the combined effect of Andean tectonism, climate and the Earth’s interior dynamics. To reconstruct the landscape evolution and its influence on paleoenvironmental variations within Amazonia since the Oligocene, we conducted numerical experiments that incorporate different surface and geodynamic processes, reproducing many paleogeographic features as inferred from the sedimentary record. We show that the evolution of the drainage pattern gradually reduced the area of sedimentation derived from the Guiana and Brazilian shields while expanded the Andean derived deposits during the Miocene, affecting the nutrient availability. First order biotic habitats were inferred from these paleogeographical reconstructions, showing an eastward expansion of várzea and terra firme forests and consequent retraction of igapó forests, with a millennial-scale reconfiguration of a mosaic of habitats in the lowlands. We conclude that this dynamism probably guided the observed patterns of speciation in the most biodiverse biome on Earth.

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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

The interpretation of fluvial styles from the rock record is based for a significant part on the identification of different types of fluvial bars, characterized by the geometric relationship between structures indicative of palaeocurrent and surfaces interpreted as indicative of bar form and bar accretion direction. These surfaces of bar accretion are the boundaries of flood‐related bar increment elements, which are typically less abundant in outcrops than what would be desirable, particularly in large river deposits in which each flood mobilizes large volumes of sediment, causing flood‐increment boundary surfaces to be widely spaced. Cross‐strata set boundaries, on the other hand, are abundant and indirectly reflect the process of unit bar accretion, inclined due to the combined effect of the unit bar surface inclination and the individual bedform climbing angle, in turn controlled by changes in flow structure caused by local bar‐scale morphology. This work presents a new method to deduce the geometry of unit bar surfaces from measured pairs of cross‐strata and cross‐strata set boundaries. The method can be used in the absence of abundant flood‐increment bounding surfaces; the study of real cases shows that, for both downstream and laterally accreting bars, the reconstructed planes are very similar to measured bar increment surfaces.