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Flood Variability Determines the Location of Lobe‐Scale Avulsions on Deltas: Madagascar

Brooke, Sam A. S. and Ganti, Vamsi and Chadwick, Austin J. and Lamb, Michael P. (2020) Flood Variability Determines the Location of Lobe‐Scale Avulsions on Deltas: Madagascar. Geophysical Research Letters, 47 (20). Art. No. e2020GL088797. ISSN 0094-8276. doi:10.1029/2020GL088797.

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River deltas grow through repeated lobe‐scale avulsions, which often occur at a location that correlates with the backwater lengthscale. Competing hypotheses attribute the avulsion node origin to either the morphodynamic feedbacks caused by natural flood discharge variability (backwater hypothesis) or to the prograding delta lobe geometry (geometric hypothesis). Here, using theory, historical flood records, and remotely sensed elevation data, we analyzed five lobe‐scale delta avulsions in Madagascar, captured by Landsat imagery. Avulsion lengths were 5–55 km, distances significantly longer than the backwater lengthscale and inconsistent with the geometric hypothesis. We show that the steep, silt‐bedded rivers of Madagascar have flood‐induced bed scour, driven by backwater hydrodynamics, that propagates farther upstream than the backwater lengthscale. The avulsion lengths are 3.1 ± 1.5 times the predicted flood scour lengths, similar to low‐gradient deltas, and consistent with backwater hypothesis. Results demonstrate that erosion initiated by nonuniform flows in the backwater zone is a primary control on delta avulsion locations.

Item Type:Article
Related URLs:
URLURL TypeDescription
https://earthexplorer.usgs.govRelated ItemUSGS Earth Explorer
https://scihub.copernicus.euRelated ItemCopernicus Open Access Hub discharge data ItemRiver discharge data ItemJRC Global Surface Water 1984-2018 datasets scour-paper/-/releases/v1.2Related ItemData/Code
Brooke, Sam A. S.0000-0002-9063-0609
Ganti, Vamsi0000-0003-2165-6052
Chadwick, Austin J.0000-0002-2552-0083
Lamb, Michael P.0000-0002-5701-0504
Additional Information:© 2020 American Geophysical Union. Issue Online: 20 October 2020; Version of Record online: 20 October 2020; Accepted manuscript online: 21 September 2020; Manuscript accepted: 16 September 2020; Manuscript revised: 12 September 2020; Manuscript received: 14 May 2020. We thank R. C. Mahon for constructive comments on the manuscript. This work was supported by the National Science Foundation EAR 1935669 grant to Ganti and EAR 1427262 to Lamb. Data Availability Statement: Data sets used in this research are publicly available, with USGS/NASA Landsat satellite imagery and SRTM 1 arcsec elevation data downloaded from the USGS Earth Explorer ( and Sentinel 2 imagery downloaded from the Copernicus Open Access Hub (, river discharge data from the RivDIS and GRDC global discharge data sets (GRDC, 2015; Vorosmarty et al., 1998) were downloaded from or requested from and, respectively, with surface water masks provided by the JRC Global Surface Water 1984–2018 data sets (Pekel et al., 2016) downloaded from https://global‐surface‐ Representative global compilation of low gradient avulsions were compiled from previously published studies (Chatanantavet et al., 2012; Ganti et al., 2014, 2019; Jerolmack & Mohrig, 2007; Jerolmack & Swenson, 2007; Moodie et al., 2019). Tabular data for this study have been provided in the supporting information. All code and data used in this study can be downloaded from these sites ( and‐flood‐scour‐paper/‐/releases/v1.2).
Group:Division of Geological and Planetary Sciences
Funding AgencyGrant Number
Subject Keywords:river avulsion; steep deltas; flood scour; backwater; alluvial fans
Issue or Number:20
Record Number:CaltechAUTHORS:20200922-154744961
Persistent URL:
Official Citation:Brooke, S. A. S., Ganti, V., Chadwick, A. J., & Lamb, M. P. (2020). Flood variability determines the location of lobe‐scale avulsions on deltas: Madagascar. Geophysical Research Letters, 47, e2020GL088797.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:105482
Deposited By: Tony Diaz
Deposited On:22 Sep 2020 23:00
Last Modified:01 Jun 2023 22:47

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