Idaho Code § 42-2929
In case there are any lands included in any drainage district which cannot be assessed because of the title being in the United States government, or because of it being land not yet proved up on [upon], or being Indian land, or for any other reason, then in that event, the drainage commissioners are, at any future date when said lands come under the jurisdiction of the drainage district, authorized to assess said lands for all benefits bestowed, the same as though said lands were originally assessable, and notice of the intention to so assess together with the amounts of assessment filed by the drainage commissioners with the recorder of the county in which said district is located, shall be notice to all subsequent purchasers and encumbrancers.
Idaho Code § 42-2929