Mich. Comp. Laws § 120.6

Sec. 6. A port district may be comprised of more than 1 whole county if the electors in such counties so elect, and the same procedure shall be followed as is prescribed in this act for the formation of a port district coextensive with a county, except that the board of county supervisors of the respective counties composing the proposed district shall each act in the submission of the proposition and have charge of the elections in their respective counties. No lesser port district shall ever be created within the limits in whole or in part of any port district. No port district shall consist of more than 5 whole contiguous counties.