Mechanics of river avulsions on deltas and their deposits
River deltas are highly dynamic, often fan-shaped depositional systems that form when rivers drain into a standing body of water (e.g., ocean or lake). They host over a half billion people worldwide and are currently under threat of drowning and destruction by relative sea-level rise, subsidence, and anthropogenic interference. Many river deltas develop planform fan shapes through avulsions, whereby major river channel shifts occur periodically via “channel jumping” about a persistent spatial node, thus determining their fundamental length scale. Emerging theories suggest that the size of deltas is set by backwater hydrodynamics; however, these ideas are difficult to test on natural deltas, which evolve on centennial to millennial timescales. Using physical experiments coupled with observations of the dynamics of modern and ancient deltaic evolution, we showed that deltas grow through successive deposition of lobes that maintain a constant size that scales with backwater hydrodynamics. The characteristic size of deltas emerges because of a preferential avulsion node that remains fixed spatially relative to the prograding shoreline, and is a consequence of multiple river floods that produce persistent morphodynamic river-bed adjustment within the backwater zone. Moreover, river floods cause erosion in the lowermost reaches of the alluvial rivers near their coastline, which may leave erosional boundaries in the sedimentary record that appear similar to those previously interpreted to be a result of relative sea-level fall. Our work has implications for the sustainable management of deltas, decoding their stratigraphic record, and identifying ancient standing bodies of water on other planets such as Mars.
Selected Articles on this Topic
- Ganti, V., A. J. Chadwick, H. J. Hassenruck-Gudipati, and M. P. Lamb (2016), Avulsion cycles and their stratigraphic signature on an experimental backwater-controlled delta, J. Geophys. Res. – Earth Surf., 112, doi:10.1002/2016JF003915.
- Ganti, V., A. J. Chadwick, H. J. Hassenruck-Gudipati, B. M. Fuller, and M. P. Lamb (2016), Experimental river delta size set by multiple floods and backwater hydrodynamics, Science Advances, 2, e1501768, doi: 10.1126/sciadv.1501768.
- Ganti, V., Z. Chu, M. P. Lamb, J. A. Nittrouer, and G. Parker (2014), Testing morphodynamic controls on the location and frequency of river avulsions on fans versus deltas: Huanghe (Yellow River), China, Geophys. Res. Lett., 41, 7882–7890, doi:10.1002/2014GL061918.
- Geleynse, N., V. R. Voller, C. Paola, and V. Ganti (2012), Characterization of river delta shorelines, Geophys. Res. Lett., 39, L17402, doi:10.1029/2012GL052845.