Abstract
Leaf pack distribution was investigated in 270-m interval of a second-order forest stream, the Yanase River. The dominant tree type was Keyaki (Zelkova serrata). Leaf pack sampling was conducted in pools and riffles. At the first, the leaf pack distribution area was estimated, and then sample was taken by hand and fishing net in the pools. In the riffles, 1.0-m-x-1.0-m quadrates were located first, and then the leaf pack was sampled. All leaf pack was sorted into leaves and branches. At the time of leaf pack sampling, physical stream variables were also measured. The variables were water depth and current velocity. In the riffles, stone width, stone height and stone distance were measured. The collected leaf pack was categorized by their accumulation types: LS (Leaf retained by Stone), SLP (Settled Leaf in Pool), SLPi (Settled Leaf in Pool at inner bend), SLPo (Settled Leaf in Pool at outer bend) and DD (Debris Dam). The accumulated leaf pack amounts at each leaf pack type were compared with the measured physical stream variables, and their correlation was tested. From the results, for leaf pack accumulation in riffles, stones projecting above water surface and their arrangements influence to entrap or release leaf litters in transport. In pools, leaf pack accumulation is assumed to occur at high stream flow where there are sufficient pool spaces. At stream meandering points, leaf pack accumulation is influenced by production of a secondary flow, and the leaf pack has a tendency to be pushed over to the inner side of the bend. Finally, debris dams contribute greatly to accumulate leaf packs even when the number of the dam is few. LS was the largest in leaf pack density and the stones in the riffles were the most retentive structures.
