Recent Developments in Fluvial Sedimentology
This volume brings together 36 of the manuscripts that were presented at the Third International Fluvial Sedimentology Conference hosted by Colorado State University in August, 1985. Fluvial systems and sedimentary rock sequences discussed range in age from Holocene to Precambrian and include many diverse areas. The principal objective of the volume was to document the recent developments in the application of facies analysis to the reconstruction of the architecture of fluvial systems. Reconstruction of architecture is an integral part of the overall process of basin analysis.
This study follows the pioneering work of Coleman (1969) and presents the results of a study of a 200-km-long reach of the Brahmaputra River in Bangladesh. Channel pattern and migration has been monitored using LANDSAT imagery and historic maps. Channel pattern in the Brahmaputra is varied. The river is mostly braided but some reaches are anastomosed or meandering. Channel movement is dominated by lateral migration with some minor channel switching and one major avulsion in the last 200 years. There is a hierarchy of channels in the Brahmaputra. First-order channels encompass the whole river and may comprise several second-order channels which, in turn, have third-order channels within them. The channels divide and rejoin around bars which scale with the bankfull width and depth of each channel. Bar types include lateral (point) bars, diagonal bars, medial bars and tributary bars. Bars sometimes coalesce to form semi-permanent bar assemblages. Channel cross sections observed by echo-sounding show a high degree of channel asymmetry and extremely complex cross sections which can be related to bar development. The pattern of deposition is divided into four styles: new mid-channel bars (13 percent), channel abandonment (15 percent), lateral accretion to bank (19 percent) and addition to bars (53 percent). The latter is the most important and is subdivided into three elements of deposition: upstream accretion (14 percent) downstream accretion (29 percent) and flank accretion (57 percent).