The dams have a mechanism set to remove the sediments. In the case of the seven folks, there is an outlet which releases water as flush so as to clean the dams. Kengen wont tell you this but its what actually happen. During rainy season they flush out water through bottom outlets found at the bottom of the dam. As a result people down stream experience unexpected and uncontrolled floods. Also the water during those time are actually brown. These flash floods happen twice every year during rainy seasons.