Abstract
In 1985, a dam with a discharge gate was built at Dashidaira at Kurobe River, Toyama Prefecture for the first time in Japan. The dam sediments were first flushed out in December, 1991. The sediments with bad smell, such as sludge, spread all over the downstream of Kurobe River. After the first discharge of the dam sediments, benthic fishes decreased year by year in Toyama Bay. In this study, the sediments in Dashidaira Dam Reservoir and Toyama Bay were collected in 1997, 2000 and 2001. The Toyama Bay sediments which were considered to have been affected by discharged dam sediments were collected on July 20, 2000, November 19, 2000, and March 3, 2001. The sediments in the dam reservoir and the bay were studied chemically, physically and mineralogically for comparative study. The quantity of clay minerals, mainly smectite, increase toward the discharge gate at Dashidaira Dam. The distribution of clay minerals approximately corresponded to distribution of the N, C, and S concentration in Toyama Bay. XRD data show similar clay mineral components and patterns between dam sediments and suspended particles from the seabed at the offing of Kurobe River mouth. The sediments in Dashidaira Dam Reservoir and Toyama Bay contain relatively high content of kaolin minerals associated with chlorite, vermiculite, smectite, and mica clay minerals compared with sediments of the other bays. Rainbow trouts in water with smectite suspension result clearly indicated the damage to the fish. In the highest concentration of smectite (lOg/l), all 7 rainbow trouts died in five hours. The fish's gills were deformed and dehydrated, suggesting influence of smectite particles adhered to the surface of the gills. Furthermore, SEM observation and EDX analysis of the flat fish's gill in Toyama Bay clearly showed the presence of particle-like thin films with organic materials sticking on the surface. The volume of discharged sediments from Dashidaira Dam Reservoir is clearly related to the decrease of annual haul of benthic fishes in Toyama Bay within the past nine years (1991-1999).
