Federal water engineers on Thursday launched the long-planned and controversial Chatfield Reservoir water supply project, closing a deal with Colorado sponsors.
Audubon Society opponents filed a lawsuit in federal court trying to block construction.
A reallocation of the South Platte River water that is captured in the reservoir, created in 1975 for flood control, is expected to add 2.8 billion gallons a year to water supplies.
But the project will inundate 10 percent of the premier state park.