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The terms "datum" and "elevation", such as used in the "Location" or "Gage" paragraphs in USGS publication and on Web pages, refers to the base, or 0.0 foot gage-height (stage), for a gage. The term datum is used when the base gage-height has been surveyed in from known benchmarks or with precision GPS. The term elevation is used when the base gage-height has been determined from less accurate means, such as maps, barometer, etc. The datum or elevation will be reported to either NGVD of 1929 or NAVD of 1988, or to a different datum which is noted in the description of the gage.
The USGS in Washington State has used the National Geodetic Vertical Datum (NGVD) of 1929 as the datum for gages to determine elevation of gage-height data. Since the early 1990's the emphasis has been changing to use the new North American Vertical Datum (NAVD) of 1988, see a description of the differences between the two datums. You can compute the difference with the National Geodetic Survey Web page. Addition information is available from the NGS and their FAQ Web page.
Currently only a few USGS stream and reservoir gages in Washington have been converted to reporting the gage datum and elevation data in NAVD of 1988. The rest still use the NGVD of 1929. This will change over the next few years. The change will be indicated on the individual station-data Web pages and in the manuscript for the individual stations in the Annual Data Report. You can follow the links on the Web pages to the information for the station(s).