Coarse Particulate Organic Matter Distribution in the Pools and Riffles of a Second-order Stream

Coarse particulate organic matter distribution was investigated in a 270-m interval of a 2nd-order forest stream, the Yanase River. The dominant tree type was Keyaki (Zelkova serrata). CPOM sampling was conducted, and the sampled CPOM was sorted into leaves and branches, and water depth and current velocity were measured along with stone width, stone height and distance between the stones in the riffles. The collected CPOM was categorized by their accumulation type: LSS, SLP, SLPi, SLPo and DD. LSS was leaf packs at the leading edge of the stones, SLP was the sinking leaf packs in the pools, SLPi was the sinking leaf packs at the inner side of the stream bend in the pools, while SLPo was the sinking leaf packs at the outer side of the stream bend, and finally, DD was the leaf packs in the debris dams. The accumulated CPOM amounts at each leaf pack type were correlated with the measured physical stream variables. From the results, CPOM accumulation in riffles is controlled by stones projecting above the water surface and by their arrangements. In pools, CPOM accumulation occurs at high stream flow where the pools provide low velocities and a thick boundary layer of relatively quiescent flow. At stream meandering points, CPOM accumulation occurs by production of a secondary flow. Finally, CPOM accumulation in debris dams is important where they occur frequently. LSS was the largest in CPOM amount, and the stones in the riffles were the most retentive structures. On the other hand, SLP and SLPo were the least CPOM amounts, and the pools in the mainstream and at the outer side of the stream bend were the least retentive stream morphologies.