Geology of Large River Systems

−1 , and annual suspended and dissolved load = 100 mt (Hovius, 1998). Of the world's rivers, large rivers are signifi cant because they are, now, and have been in the past, important links of the sediment transfer system from the continents to the world's ocean basins. Large rivers play a major role in sculpturing the topography of the earth. In a classic paper, Potter (1978) indicated that most large rivers 'have large, long-lived deltas which have played a major role in both deep and shallow waters'. Most large rivers and their valleys have alluvial strati- graphic records that enable an understanding of their development on different timescales - century, millennial, tens of thousands of years, million and tens of millions of years. The subject of large rivers can be approached from different perspectives, from a modern perspective that emphasizes the hydrological, sediment transport and network organization of large rivers, and also from a stratigraphic perspective that emphasizes the reconstruc- tion of ancient large river systems through the methods of sedimentary basin analysis. Many of the great ancient civilizations were established in the vicinity of ancient large rivers. Large rivers are important constituents of the hydrological cycle; they not only possess inorganic constituents, but also support biotic systems. Huge deposits of coal within the fl uvial Gondwana sequences emphasize the signifi cance of fl uvial deposits of ancient rivers. Apart from this, the placer deposits of large rivers are also economically sig- nifi cant as they may contain uranium, gold or diamonds. The deltas of large rivers are very important sites for vegetation growth, e.g. the Sundarbans delta at the mouth of the Ganga-Brahmaputra system. This chapter reviews the origin and evolution of large rivers across the globe in a variety of tectono- climatic settings. We assess geological criteria for the defi nition of large rivers. Maps of the distribution of modern large rivers in different tectonic settings and cli- matic regimes are used to throw light on their genesis and sustenance. Hydrological and sediment dispersal aspects of the large rivers have been discussed in the light of climatic variations and source area characteristics. A direct manifestation of sediment dispersal by large river systems is the sedimentary architecture developing below the alluvial plains, and not surprisingly, the large rivers display a strong variability in terms of their alluvial archi- tecture. We describe some of these settings, including longitudinal trunk systems, radial fans, fan-interfan systems and interfl uve settings, mostly with examples drawn from our own work in the Ganga plains. Many of the big rivers originate in active mountain belts and ter- minate in open oceans after draining through large alluvial plains. These systems are infl uenced by sea level changes, tectonics and climate change in different parts of their