Accumulation of allochthonous sediment and litter on a sediment bar subject to sand release from an upstream dam

Release of trapped sediment from a dam is currently considered to be an effective mitigation method in dammed rivers. However, it has a high impact on the ecosystems downstream. In order to mitigate the impact of the sediment trapped by these dams, sediment gates were constructed in the two most downstream dams—Dashidaira Dam and Unazuki Dam—to release trapped sediment into the downstream. Since synchronous sediment release from Dashidaira Dam and Unazuki Dam was initiated, however, the formerly completely stony beds of the sediment bars and the river channel downstream have become covered with sand, and in the last several years they have become remarkably vegetated. Accumulation of sediment and organic matter in densely and lightly treed areas was investigated on a sediment bar subject to sediment release from upstream. Sediment accumulated in a half cone shape behind trees, and the amount of accumulated sediment was larger at sites with greater tree density. The amounts of accumulated litter were also larger at sites with greater tree density. A higher ratio of autochthonous than allochthonous litter was observed. The ratio of TN to TP was 20–25 in this litter, as opposed to 1.0 in the soil; thus plant growth seems to be limited more by levels of nitrogen than phosphorus. Because sediment and organic matter accumulate in reservoirs over time, it was concluded that the present method of releasing trapped sediment from dams is not suitable, and further modification is required.