No predictions loaded for 1991. This site serves NOAA predictions from its own store, and this period has not been fetched for Newark Slough yet. Nothing is estimated or interpolated in its place. NOAA's own page for the station has the official table: tidesandcurrents.noaa.gov.
tidealmanac.site/us/ca/newark-slough-9414506/1991
Newark Slough, CA tide table 1991 (printable) · Tide Almanac