18 Surficial geology and neotectonics in the West: Celebrating and advancing the legacy of Kirk Bryan through field-based geomorphology studies

This session invites and examines recent field-based geomorphology studies in the West. Contributions regarding soils geomorphology, neotectonics, pediments, fluvial terraces, and landscape development are invited.

Tuesday, 19 May 2026
8:00 AM - 12:00 PM
Alvarado A

Biblical Flood in the Rio Grande? Testing Models of Rapid Gorge Incision and River Integration in the Birth of the Rio Grande

Paper number: 18-3
Program: Technical Sessions
Time: 8:50 AM – 9:10 AM

Authors

Abstract

Some of the most enigmatic landscapes shaped by the interplay of tectonics, climate, and volcanics are continental rift settings such as the Río Grande rift. The Río Grande, the axial drainage of the Río Grande rift, is the fourth longest drainage in North America. However, the recent evolution of the geologically young Río Grande is still debated. The river established its modern route with headwaters in the San Juan Mountains following the 440 ka spillover of pluvial Lake Alamosa. The extent to which streams were integrated into an ancestral Río Grande prior to the spillover of Lake Alamosa in its upper-most basin, and the significance of spill over (top-down) and headward (bottom-up) incision are not well constrained. While the timing of events that contributed to the final integration of the Río Grande to its modern headwaters has been studied, the rate at which spillover may have incised the gorge and the celerity of knickpoints through the local river network have not been quantified. 

We present new models for rapid Río Grande gorge incision due to progressive spillover of Pleistocene Lake Alamosa. Using tributary terraces and profile geometries, we reconstruct paleoprofiles of tributary streams along the modern gorge to estimate confluence elevations and pre-spillover topography. From this we test the ability for flood waters to incise into the Servilleta basalts by plucking. Estimated incision rates are used to quantify celerities for major knickpoints that together form the largest knickzone in entire length of the Río Grande. This study helps explain carving and final integration of the upper-most rift basin since the middle-Pleistocene and explores the role of climate cycles in creating hydrological connectivity among major rift basins. Our results will yield new models for the birth of continental scale rivers and rapid incision of volcanic landscapes due to spillover events. 

Geological Society of America Abstracts with Programs. Vol. 58, No. 4

doi: 10.1130/abs/2026RM-410599

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