Prediction of anthropogenic debris and its association with geomorphology in US urban streams

Anthropogenic debris in urban streams is a persistent environmental problem, yet previous studies have focused largely on how land use influences debris concentrations, while neglecting the potential role of fluvial geomorphology in mediating storage. To examine relationships between in-stream debris concentrations and different geomorphologic characteristics, catchment characteristics, and catchment and riparian land cover in US urban streams, we collected data on debris (>5 cm), large wood, cross-section and longitudinal profiles, and sediment sizes in 24 stream reaches in two metropolitan areas (Cleveland, Ohio; Charlotte, North Carolina). Debris concentrations ranged from 0.18 to 4.7 pieces/m bankfull width, with an average of 1.55 pieces/m. Plastic comprised 71.8 % of the collected debris, and in two reaches with repeated measurements, debris re-accumulated quickly following removal. In city-specific multiple linear regression models, debris concentrations across stream reaches was explained as well or better by geomorphologic variables than GIS variables, but when data from the two cities were combined, the opposite was true. Cross-section characteristics were among the strongest predictors of debris concentration in both cities. Our analysis suggests that roughness associated with stream banks plays an important role in debris storage, through trapping debris on riparian vegetation and by creating width constrictions that lead to low velocity zones and debris settling on the bed. Future work on interactions between bank and vegetative roughness and anthropogenic debris may reveal generalizable predictors of debris storage in urban streams.