Journal Article

How Sediment Supply, Sea‐Level, and Glacial Isostatic Oscillations Drive Alluvial River Long‐Profile Evolution and Terrace Formation

Ruby, A., McNab, F., Wickert, A. D., Fernandes, V. M., Schildgen, T. F.

2026

Overview

For over a century, alluvial river terraces have been used as archives of tectonic deformation or changes in water discharge, sediment supply, and sea level. Despite this long history, such efforts remain challenging: using terraces as deformation markers requires knowledge of their initial geometry, and most attempts to attribute terrace formation to paleoclimate forcing rely on qualitative comparisons between paleoclimate archives and terrace ages. We illustrate how simulating alluvial valley profiles and terrace formation can substantially improve such analyses. We apply a physically‐derived model of alluvial river long profiles to the Río Santa Cruz, a glacially‐fed river in Patagonia with extensive terraces. To explore how different geomorphic drivers affect terrace formation, we impose (a) sinusoidal changes in the input sediment‐to‐water discharge ratio, (b) sediment pulses, (c) sinusoidal surface uplift and subsidence simulating glacial isostatic adjustment (GIA), and (d) sea‐level variations. Each forcing mechanism generates distinct terrace geometries and lag‐time distributions, with the river response time relative to the forcing timescale influencing both. We test which terrace‐formation drivers are most likely to have generated the terraces along the Río Santa Cruz, whose response time is considerably longer than timescales of glacial–interglacial cycles. Our results reveal complex patterns of incision and aggradation, including destructive signal interference leading to terrace‐formation gaps. Although terrace profiles may remain non‐unique, when combined with a quantitative understanding of alluvial river processes, they represent a powerful archive of diverse Earth‐system processes, including variations in water and sediment supply, sea‐level change, GIA, tectonic deformation and mantle dynamics. River terraces—flat, elevated, surfaces alongside rivers that represent ancient floodplains—have been used for over a century as evidence of surface deformation or climatic change. We used a numerical model to improve our understanding of how terraces form, with reference to the Río Santa Cruz, a large river in southern Argentina with extensive terraces. We explored how different driving processes effect terrace formation, specifically: (a) cyclical changes in the amount of sediment and water entering the river, (b) sudden sediment pulses, (c) cyclical upward and downward movements of the surface from advancing and retreating glaciers, and (d) sea‐level variations. We find that different driving processes generate terraces with different, potentially diagnostic, shapes, and that terrace formation may be delayed substantially behind the responsible driving processes. We then test systematically which processes are most likely to have generated the terrace sequence observed along the Río Santa Cruz. In this way, we show how, in combination with models of river evolution, river terraces could be used as archives of past tectonic and climatic change. Modeling alluvial valley profiles and terrace formation can improve their interpretations as paleoenvironmental archives Individual drivers and their combinations lead to distinct terrace profile shapes and extents with predictable lag times River response time relative to the forcing frequency plays an important role in terrace profile shape and spatial extent

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Alluvial rivers / Archives & records / Deformation / fluvial terraces / glacial isostatic adjustment (GIA) / numerical modeling / Paleoclimate / Rivers / Sea level changes / sea‐level change / sediment supply / Sediments / Terraces / Water shortages