128-5 Evaluating Basin-Scale Controls on River Avulsion Using a Global Database

Abstract:

River avulsion occurs when a river relocates from its channel onto an adjacent floodplain. Avulsions can cause catastrophic loss of life, infrastructure damage, and landscape alteration. Beyond their surface hazard potential, avulsion deposits are a major component of the alluvial stratigraphic record. While the local conditions behind avulsion setup (superelevation and gradient advantage) are linked to high channel-belt sedimentation rates, these data are not widely available and do not appear to explain all observed basin-to-basin variation in modern avulsion occurrence. Prior work explains avulsion from the perspective of local topography and cross-sectional morphology. However, these properties are fundamentally related to the characteristics of their upstream catchments. Whether basins that contain avulsing rivers are distinguishable from similar basins that do not is unknown. To test this, we compare climatic, hydrologic, physiographic, and geomorphic parameters from 93 basins with observed avulsions to matched control basins with no documented avulsions. Control basins were selected by minimizing the combined differences in upstream drainage area and mean annual precipitation, allowing other basin characteristics to be compared while accounting for two major controls on river size and water availability. This approach seeks to identify the basin-scale characteristics associated with downstream avulsion susceptibility, providing a framework for recognizing avulsion-prone catchments rather than only individual at-risk river reaches. Quantifying these indicators provides a method for evaluating broad avulsion risk, which can in turn inform hazard, infrastructure, and land-use management decisions in vulnerable regions where avulsions preferentially occur.

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