Method, substrate type and sample replication affect measurements of fine sediment content in river beds

Abstract

Excessive fine sediment (particles <2 mm) in riverbeds negatively impacts river ecology. Riverbed fine sediment measurements are therefore critical in research, monitoring and management seeking to protect, maintain or restore riverine ecosystems. However, there is no single, widely adopted method and limited evidence-based guidance about how to choose between numerous available field techniques to achieve accurate, repeatable and consistent results. We therefore compared the intercorrelations of six commonly employed fine sediment methods across 29 sites (constituting 667 independent observations): the original resuspension method and two alternative turbidity derivatives (turbidity tube and turbidimeter), Wolman pebble counts, McNeil sampling and visual estimations at reach and patch scales. Performance evaluation focused on issues of practical significance, including comparisons of fines content between surface and subsurface measures, local substrate composition, spatial scale of application (reach and patch) and sample replication. Most methods yielded estimates of fines that were strongly correlated with each other, but these differed depending on local substrate composition, suggesting that different methods are better applied to certain substrate types. Differences between reach fine sediment estimates were typically larger as the proportion of fine sediment increased, whilst the converse was true for patch-scale measures. On average, surface measures do not provide reliable information about subsurface fines content. We also found that an inexpensive, rapid version of the resuspension method utilising a turbidity tube performs as well as costlier alternatives, providing a valuable means of estimating fines in most riverine environments. The spatial scale of sampling (reach or patch) and the number of replications made a significant difference to the estimates obtained using visual observations. We make pragmatic recommendations, providing a significant step forward in standardising fine sediment measurement in riverbeds. Practitioners and researchers should select methods that suit local substrate conditions, while recognising that their choices will influence the results obtained.