Abstract
Water-retention at a dam reservoir affects water quality in the lower reaches of the river. Those effects were assessed by comparison with a nearby river of similar size. Reservoir drainage operation ware conducted to repair dam equipment during this investigation period. For both rivers, it was confirmed that the NO3-N concentration tended to rise and that the EC and SiO32- tended to decline during the snow-melt period. Nitric acid from the atmosphere attached to the snow, which then flowed out as surface water without dissolving soil minerals into the melting snow water. The SS value rose when the river was swollen by rains, during the snow-melt period, and during the drainage operation period. The SS mainly comprised inorganic matter, with low and small changes in VSS. The Chl-a value tended to rise by the dam reservoir, but it was around 5μg/L even when it rose. Little influence was noted by the increase of phytoplankton. The BOD value was higher than that of a control river, but its mean value was less than 1 mg/L, which might be attributable to the small dam capacity and the low phosphorus content of inflowing rivers. The water temperature rose around 5°C in the dam downstream. During drainage operation, the values of SS and NH4-N rose. However, those values were not sufficiently high to affect aquatic organisms of the lower reaches.
