Water and material circulations and human activities at the river basin were discussed, and countermeasures to conserve and rehabilitate fine and comfortable aquatic environments were proposed. At a river basin, conservation of forest, farm-land and other permeable lands, development of sewage systems and measures against household effluents are considered to be important countermeasures. In the river, it is important to rehabilitate natural-type river, to intensify self-purification capacity. Environmental conservation movements by citizens are also important and cooperation of administration and citizens is necessary for establishment of consultation and agreement.
Precipitation particles are mainly formed by ice-crystal process, warm rain process is only observed in tropical area. Chemical substances are scavenged by precipitation particles under the in-cloud process and below-cloud process. The ratios of Cｌ-/Na+ and Mg2+/Na+ in all the winter precipitation samples collected at Tohkamachi are nearly equal to that found in sea water. These results suggest that sources other than sea water can be considered negligible. It is concluded that sea water is the predominant source of Na+, Cl-, and Mg2+ in the winter precipitation. The origin of NO3- and NH4+ in the winter precipitation samples collected at Tohkamachi is considered to be non sea salt. The origins of SO42- and Ca2+ are both sea salt and non sea salt. NO3- and nssSO42- are the acidic materials and decline pH value of the winter precipitation. NH4+ and nssCa2+ are the neutralization materials of acid precipitation. Synoptic meteorological conditions of winter precipitation at Tohkamachi are classified into three types. These three types consist of the winter monsoon, a cyclone positioned over the Sea of Japan, and a cyclone located over the Pacific Ocean near the main island of Japan. Sea salt particles contained in cloud droplets are collected by snow crystals through the in-cloud scavenging process. The higher the height of the convective mixing layer, the greater the rate of collection of the sea salt particles by the snow crystals. The duration of the convective mixing affects the rate of collection of the sea salt particles by the snow crystals. A correlation can be observed between the height of the convective mixing layer and the concentration of Na+ in winter precipitation at Tohkamachi. And a mutual relationships between the temperatures at the top of convective mixing layer and the δ18O values in winter precipitation samples are observed for each weather condition that produces winter precipitation. The regression equations differ for each weather condition. Winter precipitation samples formed during the winter monsoon have larger δ18O values than these formed under other conditions at the same temperature. The δ18O values of samples precipitated under conditions when the cyclone is positioned over the Pacific Ocean are the smallest. The cyclone system that passes over the Pacific Ocean has a relatively long life time, and precipitates heavy isotopic particles during the synoptic times associated with this period in the south-west area of Japan. As the result of this phenomena, the samples precipitated in Tohkamachi under conditions of the cyclone positioned over the Pacific Ocean are considered to have a relatively small δ18O values.