The survey, conducted in 2000 and covering 782 dams with a capacity of 1 million cubic meters or more, showed that 124 had reservoirs where mud accounted for 20 percent or more of the volume. Power generation companies own most of the dams.
Since sediment affects not only flood control, but also the river environment, as well as being expensive to dredge, the problem is a headache for dam operators and fuels opposition to the nation's dam construction policy.
``Japan's rivers are steeper than their European or American counterparts, but such factors were not properly factored in when planning dams,'' said Tetsuo Ueno of the Japan Institute of Land and Environmental Studies, a private group.
``Priority was apparently put on economic development and the harmful effects were underestimated. Demolition must be considered for utility dams whose water rights are to expire,'' said Ueno, a disaster prevention researcher at Kyoto University.
The survey by the Ministry of Land, Infrastructure and Transport covered dams run by central and local governments, electric companies and the Water Resources Development Public Corp.
The worst was Chubu Electric Power Co.'s Senzu Dam in Shizuoka Prefecture, which was 97.7 percent filled with mud.
Of the 124 dams that are filled 20 percent or more, 83 are owned by the nation's nine power utilities and the Electric Power Development Co., and 25 are managed by prefectural governments. By region, many are concentrated in central Japan. Thirteen are in the Kiso River and its tributaries, nine each in the Oi River and Tenryu River and seven each in the Tone River and the Sho River.
History plays a part. Many of the dams were built either in the 1920s and '30s or the postwar reconstruction period of the '50s and '60s. After 1957, dam designs were supposed to limit sediment buildup to levels that would not hamper operations during the 100 years of a dam's expected working life.
Of the 45 dams designed that way, 18 have already exceeded the maximum level anticipated, with the ministry's Shinaki Dam in Gunma Prefecture the worst at 75.8 percent.
Sediment adds to the danger of flooding. The Yasuoka Dam on the Tenryu River inundated its surrounding area in Nagano Prefecture in 1961 and 1983, labeling the community with the unwanted nickname ``flood village.''
Power dams tend to have higher sedimentation rates, as utilities make little effort to dredge them. As long as a dam can still hold water, sediment does not affect power generation.
Dredging can cost central and local government millions of yen per dam annually, but no clear answers to the muddy problem are in sight.
One attempt to drain mud via a sluice on the Kurobe River's Dashidaira Dam damaged fishing resources in Toyama Bay because of sludge containing rotten leaves.
Source: The Asahi Shimbun
