That if the board of directors shall determine that it is necessary or advisable to alter, change or relocate the channel or any part of the channel of any natural watercourse within the district, to make or excavate a cutoff or a new channel therefor, it may cause such work to be done, and, to pay for the cost of such work, issue bonds to be paid by general taxation or by levying of special assessments upon the property benefited in the manner herein provided.