This paper presents an overview of tropical river systems around the world and identifies major knowledge gaps. We focus particularly on the rivers draining the wet and wet-dry tropics with annual rainfall of more than 700 mm/year. The size of the analyzed river basins varies from 10 4 to 6 Â 10 6 km 2 . The tropical rivers across the globe drain a variety of geologicgeomorphologic settings: (a) orogenic mountains belts, (b) sedimentary and basaltic plateau/platforms, (c) cratonic areas, (d) lowland plains in sedimentary basins and (e) mixed terrain. All of them show clearly high but variable peak discharges during the rainy season and a period of low flow when rainfall decreases. Some tropical rivers show two flood peaks, a principal and a secondary one, during the year. We computed the intensity of floods and discharge variability in tropical rivers. The relationship between sediment yield and average water discharge for orogenic continental rivers of South America and Asia was also plotted. Insular Asian rivers show lower values of sediment yield related to mean annual discharge than continental orogenic rivers of Asia and South America. Rivers draining platforms or cratonic areas in savanna and wet tropical climates are characterized by low sediment yields. Tropical rivers exhibit a large variety of channel form. In most cases, and particularly in large basins, rivers exhibit a transition from one form to another so that traditional definitions of straight, meandering and braided may be difficult to apply. In general, it is more useful to apply the terminology of single and multi-channel systems or complex anabranching systems at least for selected regional segments.Present-day knowledge of tropical systems and its potential application to improve interpretation of older alluvial sequences and facies models are briefly discussed. Human impact and river management issues including land use changes, mining, dams, interbasin water transference as well as flood hazards are some of the daunting problems in tropical river basins today. D

1We present an extensive river sediment dataset covering the Ganga basin from the Himalayan 2 front downstream to the Ganga mainstream in Bangladesh. These sediments were mainly 3 collected over several monsoon seasons and include depth profiles of suspended particles in 4 the river water column. Mineral sorting is the first order control on the chemical composition 5 of river sediments. Taking into account this variability we show that sediments become 6 significantly depleted in mobile elements during their transit through the floodplain.

River systems of part of the Himalayan foreland, northern Bihar plains, India, are described in terms of their channel morphology, hydrology and suspended sediment characteristics. A simple classification of the river systems based mainly on the source area characteristics is proposed: (i) mountain‐fed, (ii) foothills‐fed, (iii) plains‐fed and (iv) mixed‐fed rivers. Distinct differences are noted between these classes of river systems. Most rivers show evidence of channel movement, mainly by avulsion, but cut‐offs also occur locally. The mountain‐fed rivers are characterized by very high discharge and low suspended sediment concentration and the plains‐fed rivers have relatively low discharge and high suspended sediment concentration. The foothill‐fed rivers have moderate values of discharge and suspended sediment concentration. The mountain‐fed rivers have built megafans of large extent, whereas the foothills‐fed and plains‐fed rivers have formed muddy interfan areas. Semi‐quantitative estimates of water and sediment flux suggest that about 99·9% of the mass transfer into the plains is water, with the remaining 0·1% being sediment, and that 10% of the latter (0·01% of the total) is retained in the basin, the remainder being transferred to the Bengal Fan.

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Modifications to standard definitions of braiding and meandering are proposed to indicate the morphology of every river channel reach quantitatively, whether it has single channel or multiple channels. Sinuosity (P) is defined as, P = Lcmax/LR, where Lcma• is the length of the midline of the channel (in single-channel rivers), or the widest channel (in multi-channel rivers), and LR is the overall length of the reach. Braiding is a measure of channel multiplicity and a new term 'braid-channel ratio' (B) has been defined as,

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