Abstract: The basic features and spatio-temporal distribution of grain size of sediment in Qumalai reach of the source region of Changjiang River are examined through measurements and comparisons. Research results indicate that: sand and gravel is the dominant sediment in the sampling reach, composing saltation load and suspended load with no rolling sediment. The natural frequency distribution curves are mostly of double peaks, implying that the bed sediment in Qumalai reach is of poor sorting resulting from river hydrodynamics; while the frequency distribution curves of terrace sediment are mostly of multiple peaks, also indicating poor sorting, which may be induced by both river dynamics and glacier dynamics. Sediments in the vicinity of river trough in a same section are coarse, which is consistent with flow dynamics. In addition, the grain size of shoal sediments changes alternately between coarse and fine in vertical direction, which could reflect the hydrological rhythm of rivers in different historical periods and the qualitative change of climate.The distribution of sediments in the vertical direction can supply basic data for river process analysis and hydraulic inversion.

Key words: source region of the Changjiang River, Qumalai, sediment, grain size parameters, riverbed