Large rivers in sedimentary basins: Morphology and form observed from satellite imagery

Preservation of the deposits of big rivers, like any other river, can only occur where the river crosses an area of net aggradation in a sedimentary basin. Many of the world’s big rivers are systems that transfer sediment load from erosional realms to the sea, depositing fluvial successions only where there is accommodation on the coastal plain. However, many of the big rivers (e.g., Parana, Paraguay, Brahmaputra, Ganges, Indus, and Yukon Rivers) also cross continental sedimentary basins (e.g., sedimentary basins with minimal marine influence that lie inside continents) on their way to the oceans. We use satellite imagery to observe the large-scale morphology of big rivers in these continental sedimentary basins. As with other rivers, big rivers lose confinement of their valleys and form distributive fluvial systems (DFS) as they enter the continental sedimentary basins. Commonly, channel size decreases down-DFS, either through infiltration, bifurcation, or evaporation. Several active and/or old channels radiate outward from a DFS apex, and where the river is incised into its DFS, several paleochannel deposits are visible radiating outward from the DFS apex. Between and adjacent to channels, a significant amount of fine-grained sediment is deposited across the DFS surface, leaving high potential for preservation of floodplain deposits, even on large river DFS dominated by braided river systems. Commonly, the big rivers become the axial river in the sedimentary basin, continuing along strike of the basin. In this position, the river becomes confined between opposing DFS or between transverse DFS and the basin edge. In several examples, the river morphology changes upon reaching the sedimentary basin and across the DFS and this morphology may change once again at the toe of the DFS where the river takes the axial position in the basin. For example, the Brahamaputra River upstream from the sedimentary basin is a relatively narrow, single thread channel that is confined in its valley. Upon entering the sedimentary basin, the Brahmaputra River develops a DFS and becomes broadly braided in form. Distally on the DFS, the braided system bifurcates, leaving relatively large areas where floodplain deposits may be preserved. At the toe of the DFS, the Brahmaputra River becomes the axial system for this portion of the foreland basin. In this axial position, it is held between opposing DFS, thus the channel system migrates back and forth between these DFS and fills this portion of the basin with coarse-grained material. Other large rivers show similar change as they enter a continental sedimentary basin. In areal extent, DFS from smaller rivers occupy more of the modern continental sedimentary basins than the big rivers (either in axial or DFS position), therefore deposits of all rivers in sedimentary basins must be considered in order to fully interpret the rock record.