Zero Datum : A reference "zero" elevation for a stream or river gage. This "zero" can be referenced (usually within ten feet of the bottom of the channel) to mean sea level, or to any other recognized datum
National Weather Service NWS Forecast Office, National Oceanic and Atmospheric Administration NOAA, SH Manual, Glossary of Hydrologic Terms, Washington, DC, USA 2015. http://www.nws.noaa.gov/om/hod/SHManual/SHMan014_g lossary.htm, (HydroI - Amer) {889}