Neb. Rev. Stat. § 31-227

The city council of any city of the metropolitan class shall have concurrent jurisdiction with the county board of any county having a population of two hundred thousand inhabitants or more, and the power to cause all natural watercourses, as defined in section 31-228 , which are within its corporate limits or within three miles thereof, to be kept clean and free of obstructions in such manner as to permit the natural flow thereof.