Topological controls on catchment‐scale sediment dynamics

Abstract

The episodic transfer of sediment from source to sink is a fundamental process in fluvial systems that influences river morphology, aquatic and riparian ecosystems, and risk from a variety of associated natural hazards.The hierarchical structure of river networks has been identified as a key control on spatiotemporal patterns of sediment routing at the catchmentscale, but very few studies have systematically explored this relationship.In this paper, we investigate the role that drainage network topology plays in modulating sediment flux and morphodynamic activity. We simulate the geomorphological responses of four topologically distinct catchments from New Zealand's South Island to sequences of flood events using a landscape evolution model. Spatiotemporal variation in different types of geomorphological activity is assessed via a link-based framework, and potential interrelationships between within-network changes and discharge and sediment yield at the catchment outlets are explored to provide insights into relative levels of network connectivity. We also investigate the occurrence of geomorphic 'hotspots' in relation to network topology, and their impact on the downstream transfer of sediment in different network 'types'. Dissected networks were found to exhibit much greater spatiotemporal variability in geomorphological activity compared to narrow, elongated networks where change was concentrated in mainstem reaches. The frequency and significance of geomorphological hotspots are shown to vary between network types, with strong contrasts evident between dissected networks with steep topography and elongated networks with more gentle gradients. Dissected networks exhibited mostly non-linear relationships between within-network geomorphological activity and outlet discharge and sediment yield. However, moderate-strong linear relationships between these variables were observed in mainstemdominated networks, indicating much greater levels of connectivity across a range of flow conditions. We discuss the implications of these findings on the transformation of environmental signals through fluvial systems with different topological structures, and the differential responses of catchments to disturbance events.