Keywords: 
• straight stream channel /
• grain size /
• heavy mineral

Abstract: The flume study was made at Environment Research Center of Tsukuba University, Japan. The observation of the bedload transportation and the grain size analysis of samples collected along the channel from upper to downstream showed that selective sorting plays a very important part in heavy mineral concentrations. The results were as follows: 1) The sediment from superficial layer was fining downst reamward. As a result, the percentage of heavy mineral fractions increased with increasing distance off the flume inlet. The longer the distance, the higher the percentage was. The maximum reached 48% near the flume outlet, whereas the initial population was lower than 5%. 2) Heavy particles were characterized by good sorting and fine diameter size. Mean particle size ranged from 1.5 to 2.5φ. Major content was fine sand. 3) The percentage of the heavy mineral fractions changed with the variation of bedforms. Stoss side of a bar, trough and pool were traps of heavy particles. 4) According to the texture of the sediment, a bar was divided into three parts. The superficial layer of the tail was observed to contain large amount of fine and medium sand and to be rich in heavy minerals. In contrast, the head of a bar was mainly composed of gravel and coarse sand, which contains few heavy particles. The body of a bar mediated between the tail and head. 5) On transection, three layers with abundant heavy minerals were observed, but only the lag layer was continuous and signified to forming placers.