Bank strength and erodibility in the Amazon River and their control on large-scale river morphodynamics

Large anabranching sand-bed rivers are characterised by dynamic lateral channel migration, bar aggradation and floodplain accretion. On the Amazon River, variations in bank and floodplain sediments exert a primary control on channel migration rates. In a reach near Tefé, Brazil, the Solimões River shows different migration dynamics along its north and south banks, suggesting that bank strength plays a role in the large-scale and long-term channel and floodplain evolution. Here we present measurements of lower bank strength and sediment resuspension in 29 locations along and across the Solimões River to investigate their spatial variability in sediments of different age and origin by means of a cohesion strength meter and a Pilcon Shear Vane. Results show that the north bank consists mainly of late Holocene sandy and silty deposits, whereas the south bank is characterised by frequent Pleistocene outcrops of cohesive muds and diagenetic iron cements and concretions. The south-bank Pleistocene deposits have on average three times higher bank strength than the younger floodplain deposits along the north bank. When comparing the locations of the Pleistocene deposits with lateral migration rates along both banks for two 40 km reaches, we observe that these sediments occur mainly where the south bank has been eroded, suggesting that they are revealed when the river migrates. Our results suggest: (1) that lateral migration uncovers the less erodible layers that can then deflect the flow towards the north bank; (2) that outcrops of resistant Pleistocene deposits might be abundant underneath the northern alluvial floodplain; and (3) spatial variations in bank erodibility exert a first-order local control on the planform morphology and lateral dynamics of the river. We suggest that such variations in erodibility are equally important for the morphodynamics of other large sand bed rivers that show evidence for the presence of resistant Pleistocene sediments, such as the Nile, Mekong and Mississippi Rivers.