Controls on the Evolution of River Channel Morphology on Volcanic Islands

Abstract:

River channel morphology is thought to record the interaction of climatic and tectonic drivers of erosion, together with material properties of bedrock and the temporal changes in each of these parameters. However, unraveling the geophysical story told by river networks is complicated by the complexity of the interactions involved and the impracticality of making observations of river network evolution in situ over geological time scales. In this work, we exploit spatial gradients in an effort to understand fundamental controls on river channel morphology in volcanic terrains. We focus primarily on the Banda Arc, a complex tectonic domain with tectonic, volcanic, and climatic gradients that produce islands with river network geometries in a variety of stages and styles of evolution. We compute the Χ statistic – an integration of upstream drainage area over the length of a river – for rivers draining the Banda Arc islands, focusing on the currently and formerly active volcanic islands of the Arc. We compare Χ plots from across the Banda Arc with those from the Hawaiian Islands, which offer a time series of evolving river networks on volcanic islands of similar composition and a more stable tectonic domain to gain improved understanding of the role of tectonics and time in river network evolution. We find major disequilibria across main drainage divides in extinct volcanic terrains with little tectonic activity, as networks are forced away from their initial radial patterns by variations in lithology and/or climate. Tectonically active islands in the Banda Arc have generally smaller disequilibria across divides and produce more regular drainage patterns, indicating that, at the scale of individual volcanic islands, the tectonic signal may dominate in channel morphology.