A federal judge has dismissed a lawsuit over water management at Round Butte Dam in Central Oregon.

The environmental group Deschutes River Alliance argued in court that Portland General Electric and the Confederated Tribes of the Warm Springs Reservation were violating water quality standards on the lower Deschutes River.

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Round Butte Dam holds back the waters of the Deschutes River to form Lake Billy Chinook near Madras, Ore. Don Ryan/AP

Dam managers had installed a water mixing device to improve temperatures for fish. The device is designed to mix warm water from the top of the dam reservoir with cold water from the bottom, to more closely mimic temperatures of a natural river.

The alliance argued that after the device was installed, the quality of the water flowing through the dam violated the Clean Water Act.

In a ruling Friday, U.S. District Court Judge Michael Simon concluded the group didn't have enough evidence to support a water quality violation.

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