§ 251. County agency. The board of supervisors may appoint or establish an officer, board or body, or may designate an existing officer, board or body, or public authority which possesses the express power to act as such an agency, to act as a county water, water quality treatment, sewer, wastewater disposal, drainage, refuse or public inland lake protection and rehabilitation agency (hereinafter referred to in this article as the "agency") having the powers hereinafter prescribed in sections two hundred fifty-two, two hundred fifty-three and two hundred fifty-four, and such other powers and duties as the board of supervisors may determine necessary to carry into effect the provisions of this article. Except in the case of a public authority, the agency may also be designated as the administrative head or body of any county district which may be established pursuant to the provisions of this article. All matters relating to the membership of such agency, including, but not limited to, numbers, method of selection, tenure, qualifications and compensation, shall be determined by the board of supervisors.