Notwithstanding the powers and authority conferred on the district by this chapter, it is not the intent of this chapter to limit, abridge, restrict, or curtail any existing power granted to any municipality by law, but to provide a vehicle through which municipalities may more effectively perform their governmental and corporate functions, and the provisions of this chapter shall be cumulative, additional, and supplemental. Notwithstanding provisions of this chapter to the contrary, a municipality shall not be required to secure the approval by the district of any feasibility study, plans, or projects which the municipality may elect to undertake separately. Wherever any public agencies are now authorized to join with other municipalities, and undertake projects, then such municipalities may elect to proceed under such general laws, or may elect to come within the provisions of this chapter. Notwithstanding any regulation or requirement of any federal agency to the contrary, the governing body of the district shall not act for or on behalf of any public agency of the district in its dealings with such federal agency without the express consent of the public agency involved. It is the intent of this chapter that the governing body of the district shall not be vested with any governmental powers or functions other than those voluntarily conferred upon it by public agencies in the manner provided in this chapter.