Plaintiffs own an island and defendants own the mainland. In such a case, the same riparian rights appertain to the island as to the mainland. Therefore, where two tributaries form both sides of the island meet and form one stream, the center lines of the two streams are extended until they meet and form the center line of the single stream and become the boundary lines. Application of Central Neb. Public Power & Irrigation Dist., 138 Neb. 742, 295 N.W. 386 (1941).