How can a River Stage be Negative?
A river’s stage at a point (a gage reading) is not an absolute measure of the depth of the water in the channel, rather it is a depth with respect to an HISTORICAL DATUM LEVEL. That (DATUM) level is chosen considering many factors, like the USGS references (or benchmarks) that are near to the gage site, or an historical level that may have been used for a hundred years or more. These gage zero levels are not changed very often. Silt may deposit in the river channel over time (filling the channel up), or the channel bottom may be scoured out to a deeper level by strong currents. Still, the gage zero datum levels are not changed to keep continuity! For example, most of the gages in the Susquehanna Basin experienced their record Flooding during Agnes in 1972. If the Gage ZERO level was changed on a whim, all the historical data for the gage site could be rendered useless! So, when the gage reads ZERO or NEGATIVE – it does NOT mean that the river has gone totally dry or is running below gr
