The level of extreme low water at the confluence
of the Penobscot River and Kenduskeag Stream, as fixed by the United
States of America Coast and Geodetic Survey, shall be the plan of
reference from which all levels taken by the City Engineer shall be
measured and to which all grades of streets, sewers and drains shall
be referred, and benches shall be established by said Engineer in
different parts of the City, and a description of all such benches
shall be recorded by said Engineer in the City plan books and be indicated
and marked on the plans of the several streets on which benches are
or shall be established, such record and marking to be done from time
to time as such benches are established.