Commissioners are empowered to make contracts in a form and under conditions deemed proper with the persons or corporations owning or proposing to construct any dam across any river in the state, providing for the use of the dam either in whole or in part, or in a general or limited way as may be agreed upon for a public highway. If the dam and proposed highway is in more than one (1) county, the agreement shall be executed by the commissioners of each county. Contracts for the use as a public highway of any dam to be constructed may be executed prior to construction, and shall, subject to the terms of the contract, be public highways.