A federal district court judge ruled last week that the U.S. Army Corps of Engineers violated the National Environmental Protection Act and the Clean Water Act when it approved expanding a Colorado reservoir. But a footnote to that decision is even more significant, experts and environmentalists say, with potentially far-reaching impacts on water management in the West and current negotiations to cut back use of the declining Colorado River.  Since 2002, Denver Water, which supplies 1.5 million people in the Denver metropolitan area, has been seeking to expand the Gross Reservoir. … The diversion of more water from the already over-appropriated Colorado River would threaten the wildlife that depend on the waterway and put the state at risk of violating the guidelines that regulate the river’s water supply, environmentalists have argued. Senior federal judge Christine Arguello agreed, noting that diverting more water from the Colorado River could result in forced reductions for the state.