Event‐Based Suspended Sediment Budgets Across a Gravel Bed River Network

Abstract This study explores the variability of suspended sediment transport down a river main stem by identifying sediment sources and in‐stream processes. It examines event‐scale suspended sediment budgets across a catchment with diverse sediment sources and assesses the influence of hydrological drivers such as spatial and temporal variability in river flow and rainfall, as well as in in‐channel physical processes, on downstream variations in fine sediment loads during runoff events and flow‐suspended sediment concentration hysteresis. To achieve this, we established a network of four monitoring sites along the gravel‐bed Oreti River in Southland, New Zealand. The monitoring stations were designed to differentiate dominant suspended sediment contributions from hillslope sources, braided reach deposition, and re‐entrainment and alluvial bank erosion. Continuous records of flow and flow‐weighted cross‐section averaged suspended sediment concentration, proxied by turbidity, were measured at each site over 55 flood events of varying size. 700 auto‐samples and 40 depth‐integrated samples were collected from different sizes of flood events to calibrate turbidity to sediment concentration. Event‐based suspended sediment budgets were quantified by calculating net suspended sediment gains or losses between monitoring sites, incorporating uncertainties to ensure a rigorous comparison. Results highlight variability in suspended sediment storage and transport during similarly sized floods, driven by hydrological factors such as rainfall distribution and flood magnitude, as well as by the timing of suspended sediment delivery. Despite this variability, diffuse source erosion and net deposition within the braided reach increased with flood size. These findings provide valuable insights for evaluating and improving suspended sediment routing models.