In the fourth construction, the Nakagawa mineral poison settling basin was build in the mainstream of the flume (completed in 1948), and after that, the damages by the mining pollution in the lower river were reduced considerably.
第4次工事で、用水本流上に中川鉱毒沈砂池（1948年竣工）が設けられ、下流部の鉱毒被害は激減した。