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

Abstract 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.