A federal judge has overturned water quality rules that were meant to protect southeastern Montana cropland from natural gas drilling but were assailed by Wyoming as a threat to energy production.

The rules covered the Tongue and Powder rivers, which flow north from the rich gas fields of northeastern Wyoming into primarily agricultural land in Montana.

Drafted by Montana and approved by the Environmental Protection Agency, the rules limited how much salty water - a byproduct of drilling - could enter the rivers. State officials said the EPA had not yet begun to enforce the rules, in part because of a pending lawsuit.

In a judgment in that case issued Tuesday, U.S. District Judge Clarence Brimmer in Cheyenne, Wyo., annulled the rules and sent them back to the EPA to reconsider. Brimmer wrote that the EPA had failed to give the water quality standards a full review when it approved them in 2003 and 2008.

The lawsuit has pitted natural gas companies backed by the state of Wyoming against the EPA and Montana.

The case represents one of several running skirmishes between Montana and Wyoming over the rivers that flow north across their shared border.