The rights of landowners, municipalities, and persons to the waters in a flood control district, or in waters affected by any State or Federal flood control works or improvements for domestic use, water supply, industrial purposes, water power or for any other lawful purposes shall extend only to such rights as are owned or possessed by them prior to the adoption of the official plans of such flood control works and improvements, and to such use as could be made of such waters if the public improvements and works herein authorized had not been made. Whenever such public improvements and works make possible a greater, better or more convenient use of, or benefit from the waters in a flood control district, the right of such greater, better or more convenient use of or benefit from such waters shall be the property of the Commonwealth, saving the rights of the Federal Government as to navigation. Such rights may be leased, sold or assigned only as the Legislature may hereafter provide, but at all times the primary purpose of flood control shall be preserved. All lands or property, or both, through or over which or bounding which are streams, the beds of which streams are the property of the owners of said lands, shall not be deprived of such reasonable flow of water, as shall be determined and fixed by the board as necessary for the maintenance of fish and aquatic life, and for domestic and industrial use and consumption.