Na chiin Huun-Dakota Project Act of 1991

Na chiin Huun-Dakota Project Act of 1991 - Directs the Secretary of the Interior to expend funds for the planning, constructing, operating, and replacing of a municipal, rural, and industrial water system, to be known as the Na chiin Huun-Dakota Project.

Requires the Project to consist of three components: (1) the Three Affiliated Tribes Rural Water Supply System to serve only the Fort Berthold Indian Reservation; (2) the Integrated Rural Water Supply System to serve the Reservation and Bottineau, Burke, Divide, McHenry, McLean, Mountrail, Renville, Ward, and Williams counties, North Dakota, and any other communities to be served by or to benefit from the Project, subject to board approval; and (3) the Rural Water Supply System to serve such counties. Sets forth provisions concerning cooperative agreements, funding, and titles with respect to such Systems. Provides for technical assistance to the Three Affiliated Tribes System. Limits Federal funding for the Rural Water Supply System. Grants credit to the State of North Dakota for moneys expended for the Project prior to enactment of this Act.

Sets forth terms of cooperative agreements. Prohibits the obligation of funds for Project construction until: (1) the requirements of the National Environmental Policy Act of 1969 has been met (concerning environmental impact); and (2) a final feasibility report has been submitted to the Congress.

Requires the Secretary, prior to obligating construction funds and if water conservation measures are economically and financially feasible, to: (1) determine whether plans for the Project include prudent water conservation measures; and (2) issue a public notice finding that such measures are included, when appropriate.

Directs the Three Affiliated Tribes of the Fort Berthold Indian Reservation and the State of North Dakota (represented by a board) to develop water conservation plans and schedules for meeting water conservation objectives.

Requires mitigation for fish and wildlife losses incurred as a result of the Project to be on an acre-for-acre basis, based on ecological equivalency, concurrent with Project construction.

Prohibits funds for the Project from being used for irrigation purposes.

Directs the Administrator of the Western Area Power Administration to make available annually, during May through October, capacity and energy required to meet the pumping and operational uses of the Project. Conditions the availability of such energy such that the Project is required to: (1) be operated on a not-for-profit basis to contract to purchase its entire electric service from suppliers that purchase power from the Administration; (2) adhere to a specified rate schedule; and (3) pay its supplier for electric service in accordance with the supplier's rate schedule.

Authorizes appropriations.