Section 68 - Bridges spanning canal channels1. When a bridge spanning the Oswego canal or that portion of the Erie canal between the Hudson river and its junction with the Oswego canal, is to be reconstructed, or a new bridge is to be built over such sections of the canal system, such reconstructed or new bridge shall be so built as to provide a clear passageway between the bottom clearance line of the bridge, if of the fixed type, or the bottom clearance line when raised, if of the movable type, and the surface of the water at its highest navigable stage of not less than twenty feet. When a bridge spanning the Champlain canal, the Cayuga and Seneca canals, or that portion of the Erie canal westerly of Three Rivers is to be reconstructed or a new bridge is to be built over such sections of the canal system, such reconstructed or new bridge shall be so built as to provide a clear passageway between the bottom clearance line of the bridge, if of the fixed type, or the bottom clearance line when raised, if of the movable type, and the surface of the water at its highest navigable stage, of not less than fifteen and one-half feet. The commissioner of transportation may, however, if in his judgment the additional cost is not unreasonable, require that such bridges be reconstructed or constructed to provide a clearance of not less than twenty feet or that the substructure of such bridge be so constructed that the superstructure may be raised to provide a clearance of twenty feet without rebuilding the foundation of said substructure.2. The provisions of subdivision one of this section shall not apply to the reconstruction of the State Street bridge, Seneca Street bridge and the Buffalo Street bridge, all over the Cayuga inlet in the city of Ithaca, county of Tompkins.