The commissioner may adjourn the hearing from time to time in his discretion. He shall hear the parties and may make such independent investigation as he may deem desirable, and shall either grant or refuse the prayer of the petition. If he shall grant the same, he shall designate the kind of bridge to be built, and shall cause to be prepared the plans and specifications therefor: Provided, That if the same be on a navigable stream the approval of the board of supervisors of the proper county shall be secured before any construction work shall be done.