The boards of supervisors of all drainage districts lying west of the Cascade Mountains, whether or not organized under the Drainage District Act, shall have supervision and control of all drainage ditches, laterals, drains, canals, sloughs, waterways or conduits within the boundaries of their districts and may prescribe the width and grade thereof. They may construct and maintain ditches, laterals, drains, canals, sloughs, waterways or conduits within the boundaries of their districts.