‘An owner of land on the shore of an un-navigable river, in the absence of restrictions in his grant, owns to the thread of the stream, and his riparian rights extend to existing and subsequently formed islands.’ Mingus v. Bell, 148 Neb. 735, 29 N.W.2d 332 (1947) (quoting Haney v. Hewitt, 105 Neb. 746, 181 N.W. 861 (1921)). See also In re Central Nebraska Public Power & Irrigation District, 138 Neb. 742, 295 N.W. 386 (1941); Hardt v. Orr, 142 Neb. 460, 6 N.W.2d 589 (1942); State v. Ecklund, 147 Neb. 508, 23 N.W.2d 782 (1946).