Provided further, that any county in the State of Mississippi through which a river or other stream flows, and which stream is classed as navigable and maintained as such by any agency of the federal government, and wherein harbors, wharves, ports, terminals or other facilities exist or are proposed or established hereafter for the handling of inbound or outbound waterborne cargo moving in interstate or foreign commerce, and where there is maintained a channel, harbor or port of such depth necessary for said purposes, then in such event such county may come under the provisions of this article if and when its application to come under the provisions of this article is approved by the Mississippi Board of Economic Development, which board shall adjudicate and determine the feasibility of said project so applied for and whether or not it is economically feasible. Certified copies of the order of said board shall be filed with the auditor of public accounts and the tax collector of such county affected.