Because the sources of fine-grained sediment problems in river systems are diffuse, it is essential to assemble catchment scale information for informing management strategies. Sediment source-tracing procedures have increasingly been adopted in this respect. Accordingly, a recently refined composite tracing procedure was used to investigate contemporary sources of fine-grained channel bed sediment in 11 sub-catchments (364 km2) of the River Wensum Demonstration Test Catchment, in the eastern UK. The procedure incorporated a combination of statistical tests for discriminating source end members, plus numerical mass balance modelling incorporating weightings for within-source tracer variations and tracer-specific discriminatory power, as well as a combination of local and genetic algorithm optimisation coupled with Monte Carlo uncertainty analysis. Replicate Monte Carlo runs demonstrated the convergence of the modelling outputs within ±1% of the average medians. Relative frequency-weighted average median source type contributions were estimated to range between 27%–69% (agricultural topsoils; predicted deviate median inputs 0%–98% and 9%–100%), 0%–38% (damaged road verges; predicted deviate median inputs 0%–58% and 0%–100%) and 21%–48% (channel banks/subsurface sources; predicted deviate median inputs 0%–50% and 4%–50%). The study provides further evidence of the importance of channel banks and damaged road verges as sediment sources and the need to include such areas in catchment sediment management strategies.