Rules of river avulsion change downstream

Creators: Gearon, James H.¹; Martin, Harrison K.^{1, 2}; DeLisle, Clarke¹; Barefoot, Eric A.^{1, 3}; Mohrig, David⁴; Paola, Chris⁵; Edmonds, Douglas A.¹

1. Indiana University Bloomington
2. California Institute of Technology
3. University of California, Riverside
4. The University of Texas at Austin
5. University of Minnesota

Abstract

Avulsing rivers create new pathways on the floodplain and the associated flooding can profoundly affect society^1,2,3,4. River avulsions are thought to occur when the water column becomes perched above the floodplain⁵ or when the slope down the flanks of the channel provides a steeper descent than the existing river channel^6,7. We test these classical ideas by quantifying the topography around avulsing rivers and show that these mechanisms, historically invoked separately, work together. Near coasts, rivers avulse when the slope away from the channel is steeper, not because they are perched. The opposite is true near mountain fronts; on fans, the alternative paths are similarly steep to the downstream path, so rivers avulse when they are perched above the surrounding landscape. We reconcile these findings and present a new theoretical framework that identifies which rivers are vulnerable to avulsion and predicts the path of an avulsing river. These first-order rules of avulsion suggest that avulsion risks are underestimated in many coastal environments⁸ and that probabilistic predictions of avulsion pathfinding can efficiently map hazards with minimal information. Applying these principles for risk assessment could particularly benefit the Global South, which is disproportionately affected by avulsions.

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Acknowledgement

D.A.E., J.H.G. and H.K.M. were supported by U.S. National Science Foundation grant EAR-1911321. H.K.M. was also supported by National Aeronautics and Space Administration (NASA) Future Investigators in NASA Earth and Space Science and Technology (FINESST) grant 80NSSC21K1598. E.A.B was supported by US National Science Foundation grant EAR 2052844.

Contributions

J.H.G. and D.A.E. conceived the presented ideas, with help from H.K.M. J.H.G. and D.A.E. wrote the manuscript, with contributions and revisions from E.A.B., H.K.M., C.D., C.P. and D.M. H.K.M. and C.D. contributed to the conception of equation (5). C.P. and D.M. contributed to and reviewed equations (1)–(4). E.A.B., D.M. and C.P. contributed to the design of analyses and interpretation of results. D.A.E. supervised the project.

Data Availability

The authors declare that all data supporting the findings of this study are available at https://doi.org/10.5281/zenodo.10338685 (ref. ⁴⁸).

Code Availability

Code for reproduction, including data cleaning, analysis and plotting, is available at https://zenodo.org/records/13693548 (ref. ⁴⁹).

Supplemental Material

Supplementary Information

This file contains two items: an extended derivation of the softmax random walk algorithm (Supplementary Method) and a data dictionary describing the column names and meanings for the separate supplementary tables.

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