Rules of river avulsion change downstream.

Area of Science:

• Earth and Environmental Sciences
• Geomorphology
• Hydrology

Background:

• River avulsions, where rivers form new paths, significantly impact floodplains and societies.
• Traditional theories attribute avulsions to rivers being perched above floodplains or steeper flanking slopes.
• These mechanisms have been historically considered separately.

Purpose of the Study:

• To test classical theories of river avulsion by quantifying floodplain topography.
• To determine the interplay between channel slope and perched water levels in triggering avulsions.
• To develop a unified theoretical framework for predicting river avulsion vulnerability and path.

Main Methods:

• Quantification of topography surrounding actively avulsing rivers.
• Comparative analysis of topographic conditions near coastal regions versus mountain fronts.
• Development of a new theoretical model integrating slope and perched water level dynamics.

Main Results:

• River avulsions result from a combination of steeper flanking slopes and perched water levels, not solely one factor.
• Near coasts, steeper slopes away from the channel are primary drivers, not perched water.
• Near mountain fronts, rivers avulse when perched above the landscape due to similar path slopes.

Conclusions:

• A new theoretical framework reconciles differing avulsion mechanisms based on geographic context.
• Avulsion risks, particularly in coastal environments, may be underestimated.
• Probabilistic avulsion path prediction offers efficient hazard mapping, benefiting regions like the Global South.