Impact of drainage integration on basin geomorphology and landform evolution: A case study along the Salt and Verde rivers, Sonoran Desert, USA

Abstract

The Salt and Verde rivers, Sonoran Desert, USA, integrated multiple endorheic extensional basins near the start of the Quaternary. Integration began via spillover on both rivers. Spillover resulted in headward erosion in upstream basins, leading to basin-wide incision and excavation of large volumes of sediment transported into downstream basins. As a result, the Verde River aggraded to what is now the highest terrace in the lower Verde River valley (LVRV), the Lousley Hills terrace. Evidence suggests deposition of the Lousley Hills terrace fill led to aggradation and backfilling of adjacent pediments. Ancestral Salt River Deposits (ASRD) of the combined Salt and Verde rivers deposited an extensive alluvial fan in the Higley Basin, raising local base level for over 2 million years. Eventually, this led to aggradational piracy of the Salt River (~0.46 Ma) that integrated the Higley and Luke basins. Consequently, this shortened and steepened the Salt River, resulting in ~30 m of aggradation downstream and headward incision upstream. Headward incision abandoned the ASRD and created the Sawik stream terrace. Piedmont pediment and alluvial fan systems that were formerly adjusted to the ASRD incised in response. Since abandonment, the ASRD floodplain has accumulated a sheet of silt and fine sand from aeolian and local fluvial processes. Sedimentary, metamorphic, and granitic pediments that developed in slowly aggrading endorheic basins display evidence of response to base-level adjustments resulting from drainage integration processes. Classic ballena landforms (eroding alluvial fans) began to form in the LVRV only after drainage integration — providing the first known maximum age for the ballena form. Drainage integration of the Salt and Verde rivers clearly demonstrates the impact of base-level fluctuations on basin-scale geomorphology. However, integration led to very different geomorphic responses in different extensional basins, revealing the difficulty of a one-size-fits-all conceptual model of geomorphic response drainage integration.