Idaho Code § 70-1206
At the same election at which a proposition for annexation of land to an existing district is submitted to vote, if the area to be annexed includes land in a county or counties other than the county in which the original port district exists, one (1) commissioner shall be elected by the voters in such area within the adjacent county or counties to represent such area in case such annexation shall be accomplished as a result of such election. Such commissioner shall hold office for a term of six (6) years and until his successor is elected and qualified. Such commissioner and his successor shall be elected by vote only of the residents of that portion of such county or counties lying within such port district. Such commissioner shall have the same qualifications as herein provided for other commissioners of the district, and shall be a resident of such area. If the annexation shall be accomplished, the port commission shall thereafter consist of the three (3) commissioners of the original port district and the commissioner for such adjacent county or counties. In like manner, in the event of any subsequent annexations, a commissioner having the qualifications herein set forth shall be elected to serve for a six (6) year term as commissioner for such adjacent county or counties, and the port commission shall be expanded to include the commissioner from each such annexed area; provided, that a port commission shall never exceed five (5) commissioners and no commissioner shall be elected to represent any area annexed to any port commission already having, or being authorized by law to have, five (5) commissioners.
Idaho Code § 70-1206