Abstract: In line with demarcation theory and threshhold size, 10 groups of demarcating grain sizes of alluvial river sediment with threshhold sizes from maximum 2.0 mm to minimum 0.001 mm are summarized for the first time, and the features and pedigrees of the 10-group demarcating grain sizes are given. The 10 groups involve sediment classification, settling velocity of spheric particle, mono-mineral sediment, clay and suspensoid sol, sediment flocculation, sediment thixotropy, initial dry density, initial sand-wave, bed-load and suspended-load, bed-material-load and wash-load. The essential core of the 10-group demarcating grain-sizes lies in the demarcation between coarse sand and fine sand. By means of attribute-based classification method, the 10-group demarcating grain-sizes can be divided into 3 types: typeⅠ, mainly dominated by sediment features; typeⅡ, relying on features of sediment and flow; and type Ⅲ, depending on features of sediment, flow and fluvial processes. Meanwhile, according to ascertainty-based classification method, the 10 groups can also be divided into 3 classes: class A with definite demarcating meaning and explicit threshhold size; class C with less definite demarcating meaning or less explicit threshhold size; and class B, which is between class A and class C. Finally, issues and further researches associated with demarcating grain sizes of river sediment, such as division between coarse sand and fine sand, relationship between demarcating grain size and critical velocity, and relationship between settling velocity of sediment and incipient velocity of flow, are introduced.The research has helped to improve the systematicness and importance of demarcating grain-size of river sediment and to establish intertink between sediment micro-feature and river macro-feature.

Key words: river sediment, demarcating grain size, threshold size, critical velocity, fluvial processes