Results from a zircon U-Pb study of the metamorphic basement of Vietnam reveal that a large part of the continental crust was affected by a short-lived episode of ductile deformation and high-temperature metamorphism between 258 ؎ 6 Ma and 243 ؎ 5 Ma. Although coincident with final stages of North-South China collision (Qinling orogenesis), the thermotectonism in Vietnam was caused by accretion of Sibumasu to Indochina-South China. This accretion event (Indosinian orogeny) has regional significance because it contributed to the final stages of North-South China collision, an aspect not explicitly recognized in Qinling orogenic models.

The initiation and evolution of a kilometre-scale, sand braid-bar was monitored during a 28-month survey period from 1993 to 1996 in one of the world's largest braided rivers, the Jamuna River, Bangladesh. Repeated bathymetric surveys through two monsoon flood seasons, combined with bar-top surveys during exposure of the bar at low flow, provide the most detailed chronology of braid-bar growth yet compiled for a large sand-bed river. During rising and peak flow of the 1994 monsoon flood, a 1.5-km-long, 0.5-km-wide, 12-m-high, symmetrical mid-channel bar was deposited in the centre of a major channel downstream of a zone of flow convergence and significant bank erosion. Initial deposition and growth of the bar core were probably caused by amalgamation of dunes that are present in the Jamuna channels at all flow stages. Bar-top aggradation continued through downstream migration of an `accretionary dune front', a 3-m-high, angle-of-repose slipface that was composed of amalgamated, 0.5- to 1-m-high dunes. At waning and low flow, the mid-channel bar widened by up to 1 km through the lateral accretion of dunes onto the margins of the initial bar core. A low-velocity zone in the sheltered wake region of the bar-tail led to the accumulation of substantial volumes of silts and clays. During the rising and peak flows of the next monsoon flood, the mid-channel bar extended its bar-tail by up to 1.5 km, as one of the anabranches became dominant, and flow was deflected across the bar-tail. Accretion at the bar-tail generated a lobate, transverse bar-front with a 10-m-high, angle-of-repose avalanche face. Emergence of several smaller bars along this depositional front produced an overall reach morphology that more closely resembled an alternate bar rather than several mid-channel bars. The conversion of a mid-channel bar to an alternate bar is contrary to many previous descriptions of the braiding process

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Progress towards a fuller understanding of the dynamics and deposits of braided rivers demands an interdisciplinary approach to a host of unresolved problems. Although many advances have been made within recent years in interpreting the mechanics of flow, transport of sediment and sedimentary architecture of braided rivers many key issues remain to be addressed. In particular, several areas demand attention: the mechanisms of braid bar initiation; confluence-diffluence dynamics, the nature of sedimentary facies over a range of grain sizes and the influence of flow stage and aggradational regime upon the depositional architecture over a range of channel scales. This paper focuses upon these issues and highlights several areas of fruitful future interdisciplinary collaboration.Braided rivers form important topics of study for many scientists and one of the primary aims of this volume is to bring together work from many disciplines in an integrated approach to braided rivers. For the geomorphologist braided fluvial systems are abundant within upland and proglacial settings and are agents of considerable erosion and sediment transport. For engineers the high rates of sediment transport, deposition and erosion combined with frequent channel shifting and rapid bank erosion may pose considerable design problems both to withinchannel structures, such as bridge piers (e.g. Mosley 1982a; Sutherland 1986) and braidplain edge constructions such as roads and railways Finally, for the geologist braided rivers form important agents of deposition that have been responsible for the accumulation of many sedimentary sequences that form valuable aquifers, hydrocarbon reservoirs and sites for heavy mineral accumulation. Because of these abundant and diverse applications, knowledge of the mechanics and deposits of braided rivers is vital within many areas and yet, when compared to the wealth of literature upon meandering systems, they have been comparatively understudied. This may, in part, be due to the difficulty of measuring flow, sediment transport and morphology in the rapidly shifting braided river environment. Future progress in understanding the mechanics and morphology of braided rivers demands interdisciplinary collaboration and calls for a more integrated approach across the sciences than may have been present until comparatively recently. This paper highlights some specific areas upon which our knowledge of braided rivers may be fruitfully extended by adopting such an interdisciplinary scope. Zones of flow convergence and divergenceBraided rivers are characterized by 'having a number of alluvial channels with bars and islands .between meeting and dividing again, and presenting from the air the intertwining effect of a braid' (Lane 1957). The division and joining of channels are essential features of braided rivers and, whilst the bars within these rivers have received attention from both geomorphologists and sedimentologists, the areas of flow convergence and divergence have not been incorporated into braided river deposit...

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