The Impact of Warm Water in Upstream on Water Quality of Headwaters Reservoir

As remarkable geographic reasons, most of rivers in Japan form short distance and steep difference of altitude. In additions, the mountainous area is covered by headwater conservation forest to obtain fine quality water as well as suitable place for dam reservoirs. However, for these past 20 years, a water-bloom as the typical water pollution phenomena came to occur in the dam reservoir established to around 500m above sea level of the headwater conservation forest. We investigated the Nutrient salts and water temperatures of the rivers inletting to dam reservoir in Kanto mountain region. The temperatures of the sampling rivers water remarkably increased. In the present study, the effects of inletting river water temperature on the water quality of a reservoir located in Kanto mountain region are investigated by multivariate analysis. As the results, (1) In three decades from 1982 to 2011, feeder rivers water temperature increased. The highest affecting matter is Chlorophyll a which is an index of water-blooms. The Chlorophyll a gradually increased and the water quality deteriorated. The increased water temperature is a primary factor for the water pollution of the reservoir. (2) The selective intake system to control the water temperature of the downstream river started in 1992 is an additional factor to deteriorate the reservoir water quality. It became a trigger to decrease the mid layer water temperature in the reservoir.