The incident occurred on Saturday at the Punt dal Gall dam in the canton of Graubünden in eastern Switzerland, ATS quoted the power firm Engadiner Kraftwerk and the Swiss National Park as saying.
Earlier Saturday, park rangers raised the alarm after noticing that the water level in the River Spoel had dropped dramatically.
As a result, officials at the dam opted to open a duct to resolve the problem.
But that decision proved to be a major error.
It sent a wave of sediment spiralling downstream, killing the trout and other aquatic wildlife along a six-kilometre (four-mile) stretch of river.
The Punt dal Gall dam is located in the Livigno valley at the border between the Swiss canton of Graubenden and the Italian region of Lombardy.