ダム堆積物の連携排砂が黒部川の下流に与える影響, その2 : 連携排砂時の水質変動と懸濁物質の特性

In 1985, Dashidaira dam with discharge gate had been built for the first time in Japan, at Kurobe River in Toyama Prefecture. Six years later, in 1991, the dam sediments were flushed out for the first time and impacted on the downstream of Kurobe River. The dam sediments and the suspension contained a large amount of organic matter (with high C, N and S contents) with sludge-smell. The annual catch of bentic fishes (exp. flatfish) and the annual haul of wakame seaweed have been decreased year by year since the first discharge of the dam sediments in 1991. Until now, it has been reported that the sludge has deposited in the Toyama Bay which is apprehensive of influence of discharged dam sediments, because suspended particles affect the fish\u27s gill respiration. In this study, suspension of river water were collected at three bridges during discharge from Dashidaira and Unazuki Dam in June 19th-22nd and in June 30th-July 2nd, 2001. River water quality and suspended solid were analyzed chemically, physically and mineralogically. Extremely low DO and Eh values were observed at the periods during first discharge in June 19th-22nd at two bridges. Moreover, during second discharge in June 30th-July 2nd, DO and Eh values were constant. Therefore it was shown that drastic decreased of DO and Eh values were a peculiar phenomenon during first discharge in June 19th-22th, 2001. The results of NCS elemental analyses were suggested that high organic contents were related with drastic decreased of DO and Eh values. Clay mineralogy of both Unazuki Dam sediments and suspended solid on the seabed at the offing of Kurobe River mouth, show abundant semctite with chlorite, mica clay minerals and kaolin minerals, suggesting those are the almost same origin. The dam sediments associated with organic matter impacted on downstream and the seabed, and was the cause of affecting bentic fishes and wakame seaweed