When two or more districts have their outlet or discharge into the same natural or man-made watercourse or stream and it becomes necessary to maintain, deepen, enlarge or otherwise improve the natural or man-made 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 all districts emptying into the natural or man-made watercourse or stream. No district shall be liable to contribute for any improvement or costs and expenses incurred in improving the natural or man-made watercourse or stream above the point of discharge of the waters of such district into such natural or man-made watercourse.