As used in this part:

    (a) "Board" means the county board of commissioners.

    (b) "Dams" means dams, embankments, dikes, pumps, weirs, locks, gates, tubes, ditches, or any other devices or construction to impound or release water.

    (c) "Local unit" means any city, village, township, or soil conservation district acting through its governing body.

    (d) "Optimum flow" means that rate and quantity of flow in any stream as determined in accordance with this part.

    (e) "Plan" means a plan adopted by the board or boards and approved by an order of the department for the best development, utilization, and conservation of the surplus water of the state.

    (f) "Surplus water" means water that may be impounded without decreasing the flow of a river or stream below its optimum flow.