The board of supervisors may establish and license toll ferries, and fix the toll; and where the public convenience and travel justifies it, may establish and maintain free ferries, or in its discretion toll ferries at public expense; and the board of supervisors shall fix the salary of the ferryman or ferrymen to run such free or toll ferry. The board of supervisors may make allowance to pay for ferry boats and things to run same, and may establish ferries over rivers or other watercourses where public road is parallel with and on one bank of the river or watercourse, and public or private roads on the opposite bank.