Water Quality of the Monobe River with Dams (I) : Seasonal Variations and Changes in Surface Water Quality from Upper to Lower Reaches

抄録

Water quality of the Monobe River, which has three dams and two weirs, were monitored 24 times from 2 July to 30 December, 2002 using HORIBA Water Quality Meter. The investigated water quality parameters were water temperature, EC, DO, pH and turbidity. Eight points were selected for this investigation. At three dams, namely Nagase Dam, Yoshino Dam and Suita Dam, vertical changes in water quality were also investigated. In this paper, seasonal variations and changes of surface water quality from the upper reaches to the down reaches of the Monobe River are mainly reported. The analysis revealed the following facts. Firstly, water temperature increases from upper reaches to lower reaches during the research period. Secondly, turbidity rapidly increases after successive heavy rains. Thirdly, water temperature and EC are lowered due to a large amount of rainfall. Fourthly, DO of the midstream rapidly decreases, and turbidity increases abruptly after successive heavy rains. Fifthly, usually EC (Electric Conductivity) is highly affected by coastal population and agricultural lands, but in this study, this point is not clear at all from the upper reaches to the midstream down to Suita Dam. Sixthly, DO and pH values are greater at the stream water surface, but they were relatively low at the dam water surface. Seventhly, Nagase Dam, the largest among three dams, shows the maximum flood peak cut. Eighthly, the inflow into the downstream dam is highly affected by the discharge from the upstream dams both in quality and quantity.