Mason Dam is a water conversation project of the United States Bureau of Reclamation, an earthen and rockfill dam originally constructed from 1965 through 1968. Owned by the Bureau, it is operated by the local Baker Valley Irrigation District. The nearby Thief Valley Reservoir from 1932 is part of the same Baker Project.
The dam is 167 feet high, and the reservoir has a capacity of 114,000 acre-feet. It impounds the water of the Powder River to create Phillips Lake, which covers 2,235 acres and offers recreational fishing, boating, camping, etc. The lake is surrounded by the Wallowa–Whitman National Forest.