Suspended sediment source and propagation during monsoon events across nested sub-catchments with contrasted land uses in Laos

•Monsoon storm flow events were monitored along a tributary of the Mekong in Laos.•Fallout radionuclide activities discriminated surface soil and sub-surface sources of sediments.•The two sources of sediments were cultivated lands and stream banks − channel deposits.•Radionuclides and total organic matter composition measurements provided consistent particle fingerprinting.•In addition construction sites connected to the river also contributed to fine suspended sediment loads.

Study regionHouay Xon catchment in northern Laos.Study focusBecause agricultural headwater catchments of SE Asia are prone to erosion and deliver a significant proportion of the total suspended sediment supply to major rivers and floodplains, the potential sources of sediments and their dynamics were studied for two successive storm flow events in June 2013. Characterization of suspended sediment loads was carried out along a continuum of 7 monitoring stations, combining analyses of fallout radionuclides, particle borne organic matter and stream water properties.New hydrological insightsRadionuclide activities showed that remobilization of soil particles deposited during the previous rainy season or supplied by riverbank erosion is the dominant process, although pulses of surface-soil derived sediments also propagate downstream. This interpretation is supported by suspended organic matter data that also fingerprints the mixing of surface soil vs. subsurface particles. The study moreover highlights the advantages and the drawbacks of combining fallout radionuclides, particle borne organic matter composition and stream water characteristics to discriminate and quantify sediment sources and dynamics in rural areas undergoing urban sprawl.