The aim of this research was to evaluate the role of riparian vegetation of a headwater stream of northern Lao in trapping hillslope runoff and sediments. In the rainy season 2005, six sites on three vegetation types (natural grass, bamboo and banana) were equipped with Gerlach troughs. Three troughs were placed at the upper rim of the riparian area and three at the lower rim to measure event inflow and outflow runoff volumes and sediment loads. Site vegetation cover, grass biomass, and undergrowth height were monitored monthly. Runoff (TER) and sediment load (TETS) trapping efficiencies were highest in natural grass sites (TER = 0.25; TETS = 0.13), whereas they were negative in cultivated sites (TER = -0.28; TETS = -0.94 under bamboo; TER = -1.36, TETS = -2.02 under banana). TER and TETS were correlated to undergrowth density and height, which was highest in natural grass, but was also important where banana was cultivated without destroying undergrowth vegetation. Trapping efficiencies were also correlated to the ratio between riparian area and upslope contributing area. To reduce the amounts of hillslope sediment delivery to streams and preserve the water quality of water bodies, it is important to maintain a buffer area between cultivated fields and streams. The minimum width of the riparian zone should be adapted to the size of the upslope cultivated areas.

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