After the formation of any levee district under the provisions of sections 245.285 to 245.545, the county commission of the county in which such district lies, or when it lies in two or more counties, the county commission of each county in said district, shall cause the county assessor of their respective counties composing said levee district, at the first annual assessment to be made under the general revenue laws of the state, to assess the value of all lands in said levee district subject to overflow or inundation or endangered or liable to be endangered by bank erosion or wash from rivers, and to be benefitted by said work, having reference to the value of said lands without the work contemplated by sections 245.285 to 245.545, and shall assess the value thereof as improved by said work, in an assessment book to be provided for that purpose.