The gage datum for this site was lowered by 2 feet on January 13, 2009 because of continued degradation of the channel bed; the change was applied retroactively back to October 16, 2008. Gage datum is an arbitrary elevation, above mean sea level, from which stage or gage-height readings are measured. Adding the gage datum elevation to a gage-height reading will give the water-surface elevation referenced to mean sea level. Gage datums are used locally because readings referenced to mean sea level, generally, have little meaning unless the site is near the sea coast. Ideally, gage datum would be set to the elevation at which water would stop flowing in a channel—zero flow would then occur at zero gage height. Because natural channels commonly degrade (scour) and aggrade (fill), the datum is set to some elevation below the bottom of the bed to avoid negative gage height readings. Continued degradation at this site resulted in a situation where negative gage heights were becoming a possibility. The decrease in the gage datum elevation results in an equal increase in gage height readings, while readings referenced to mean sea level remain unchanged. A new stage-discharge relation (rating), or adjustments to the old rating, will ensure that converting gage-height values to corresponding discharge values will be unaffected by the datum change.