Sediment pulse evolution and the role of network structure

Sediment pulses are triggered through a variety of mechanisms, from landslides to land use change. How do these pulses move through the fluvial system, and how do they evolve? In a system with perfect sediment connectivity, the erosional response to a perturbation and the resulting signal at the river mouth would match, however, this rarely occurs. Many studies have addressed reach-scale dynamics of sediment pulses and how they translate or disperse downstream. At the watershed scale, network structure and storage become more important in modulating the sediment signal. Here, we review the current literature on sediment pulse behavior, and then address the role of network structure on maintaining, dispersing, or transforming sediment pulses in a fluvial system. We use a reduced-complexity network routing model that simulates the movement of bed material through a river basin. This model is run in the Greater Blue Earth River (GBER) basin in Minnesota, USA, first with spatially uniform inputs and then with inputs constrained by a detailed sediment budget. Once the system reaches equilibrium, a sediment pulse is introduced, first at a single location and then throughout the system, and tracked as it evolves downstream. Results indicate that pulses able to translate downstream disperse in place upon arriving at over-capacity reaches as sediment goes into storage. In the GBER basin, these zones occur just upstream of a knickpoint that is propagating upstream through all mainstem channels. As the pulses get caught in these sediment “bottlenecks,” there is a decoupling of the original pulse of sediment and the resulting bed material wave. These results show that the network structure, both in terms of network geometry and the spatial pattern of transport capacity, can play a dominant role in sediment connectivity and should be considered when predicting sediment pulse behavior at the watershed scale.