They shall also determine by their verdict what amount, if any, to be paid annually to the plaintiff, would be a just and reasonable compensation for the damages that may be thereafter caused by the dam, so long as it is used in conformity with the verdict; and also what amount in gross would be a just and reasonable compensation for all damages thereafter to be caused by such use of the dam and for the right to maintain and use it forever as aforesaid.
Mass. Gen. Laws ch. 253, § 10