When two or more districts shall have their outlet or discharge into the same natural watercourse or stream, each district shall be assessed for the cost of such work in the same ratio to such total cost as the discharge of waters of such district bears to the combined discharge of waters of the several districts emptying into such natural watercourse or stream; but no district shall be liable to contribute for any improvement or costs and expenses incurred in improving such natural watercourse or stream above the point of discharge of the waters of such district into the same.