In the case of a dam ... "all properties" located within a scientifically defined "watershed" and all properties lying within the floodplain of the dam are connected by how water drains from an entire watershed into a lake and how water within the lake may flood specific areas downstream. Since the area of a watershed and the area of a floodplain are often very, very large when compared to the area of a lake, it is possible for some portions of the watershed and floodplain to be physically located in some government unit other than the lake. It is also possible that the government unit authorizing a special assessment levy does not have jurisdiction to include all land within the watershed and floodplain. In this example, the service district would be large enough to include all properties connected to the lake by how water flows. The Special Assessment District would be a smaller area within which the government unit proposing the special assessment has the power to levy a special assessment tax.