Said districts, when established, shall be for the purpose of the control and preservation of the purity of the waters of any rivers, creeks, bayous, lakes, canals, streams or other waters of any kind and character situated or flowing, in whole or in part, through the said district, or any part thereof, by the prevention of the inflow of salt water or other deleterious substances, or by the changing of said waters from salt to fresh water, and the impounding of fresh water for such purposes.