Mid-Dakota Rural Water System Act of 1991

Mid-Dakota Rural Water System Act of 1991 - Authorizes appropriations to the Secretary of the Interior for: (1) the planning and construction of a rural water treatment and distribution system in portions of specified counties in South Dakota (to be known as the Mid-Dakota Rural Water System); (2) the Untreated Water, Treated Water, and Constructed Wetlands Components of the Wetland Development and Enhancement Component of the System; (3) the South Dakota Wetland Trust; and (4) the Wetland Development and Enhancement Component of the System.

Prohibits the Secretary from obligating any Federal funds for the construction of such System until the Secretary finds that appropriate non-Federal interests have implemented water conservation programs throughout the System's service area.

Provides that mitigation for fish and wildlife losses incurred as a result of the construction and operation of the System shall be on an acre-for-acre basis, based on ecological equivalency, concurrent with project construction.

Directs the Administrator of the Western Area Power Administration to make available, from power designated for future irrigation and drainage pumping for the Pick-Sloan Missouri River Basin Program and subject to certain conditions, the capacity and energy required to meet the pumping requirements of such System.

Authorizes the use of and connection to Government facilities at the Oahe power house and pumping plant for purposes of a water supply for the System.

Provides for the establishment of a Wetlands Trust, to be administered by the South Dakota Game, Fish and Parks Foundation. States that the Trust shall be operated to preserve, enhance, restore, and manage wetlands in South Dakota. Sets forth authorized uses of Trust income.