Neb. Rev. Stat. § 22-194

Where any county is bounded by the middle of the channel of any stream or watercourse, and by reason of any change of such channel any island or tract of land shall be thrown to the other side of such bounding channel, after the original organization and establishment of the boundaries of any county, the old channel of the stream or watercourse shall, for all county and state purposes, be deemed the channel thereof.