The project, 15 years in the making, will almost double the size of the reservoir that sits on the border of Moffat and Routt counties, and allow officials to maintain water levels in the Yampa river to sustain endangered fish species.

A spokesperson for the Colorado River Water Conservation District said the project should be complete and the reservoir should be open to the public in spring 2007.

The 24.4m high Elkhead dam was built in 1974 to impound a 16.9Mm3 reservoir. When the new project is complete, the reservoir will hold about 30.8Mm3 of water, and the dam will stand 32m high. In addition to a new spillway, the reservoir also will have new outlet works under the dam. The outlet works will allow officials to control flows into Elkhead Creek, which flows into the Yampa river.

The Elkhead reservoir expansion will allow the Upper Colorado River Endangered Fish Recovery Programme to keep the river at the level endangered fish need to survive.

The recovery programme has a lease on 8.6Mm3 of the reservoir for the next 20 years. It can use the water to raise the level of the Yampa whenever it is necessary. The conservation district also can use 8.3Mm3 of water for human use.

The enlargement project is a partnership of the conservation district, the city of Craig, the Craig Station power plant, the recovery programme and Colorado State Park.