As cobble gravel travels downstream, its radius reduces in size. In most cases, this is thought to be a consequence of the way it is carried along by the water flow. To date, the crush and abrasion of cobble gravel has received little attention in the field of Sabo and river studies. However, monitoring indicates that this phenomenon, which occurs with the downstream movement of gravel, is prominent in mountain rivers. The authors therefore carried out a study into the crush and abrasion of gravel using the Los Angeles machine, focusing on cobble gravel from a representative sample of rocks distributed across Japan's mountain rivers. This was done with the aim of identifying the characteristics of the phenomenon and obtaining basic data for further dynamics and hydrology research on gravel crush and abrasion.
@The results of the study reveal new information about the characteristics of crush and abrasion of cobble gravel:
1) The ratio of the weight reduction that occurs (=gravel weight after rotation/gravel weight before rotation) indicates the curved shape that gradually degrades along with the increase in number of rotations, and the characteristics of the curved shape vary according to the type of stone. 2) The rate of increase in the production(=sediment weight produced by crush and abrasion after rotation/cobble gravel weight before rotation)of sediment particles produced by crush and abrasion of cobble gravel as it rotates varies, depending on whether the particles are smaller than2mm or larger than2mm. 3) The production rate of suspended load particles smaller than0.1mm rises in a linear fashion as rotations increase. The same tendency for the production rate to rise can be seen for particles between0.1mm and2mm in size. 4) The production rate of gravel particles larger than2mm is high during the initial period of rotation, and subsequently decreases. 5) Rock types with a high rate of cobble gravel weight reduction (shale, mudstone) produce sediment particles of a roughly equivalent size and quantity, which become finer as they rotate. 6) Types of rock with a low rate of cobble gravel weight reduction (andesite, sandstone, granite)mainly produce sediment, and silt and clay particles that are smaller than2mm. 7) Types of stone with a medium rate of cobble gravel weight reduction(chert, limestone, greenstone)produce forms of sediment that fall between the other two categories.

Key wordsFreduction in grain size, crush, abrasion, grain size distribution, suspended load