If you are accustomed to planning paddling trips based on readings from the Lake Charlotte gauge, you are going to have to reinterpret them due to a recent change in the way they are reported.
The very short answer: Add 6.64 feet to the reported number, and that will tell you what it would have been in the old system. For example, if the gauge reads 2.98 feet, that would have been the same as 9.62 feet in the old system.
The reason for the change: Contrary to popular opinion, sea level is not the same everywhere. It is locally affected by ocean currents, the shape of the shoreline, land subsidence, and numerous other factors. Because these all change over time, it is occasionally necessary to adjust the method for figuring out local sea level. Historically, this has been based on data from many local gauging stations, some of which are no longer in operation. The older system used for reporting Lake Charlotte gauge readings was based upon a large data set from 1929. Understandably, quite a bit of adjustment has been required since then. To address this type of problem, a newer system using more recent data was created. You may have wondered how the readings at Lake Charlotte could be so high when the elevation difference between Lake Charlotte and the mouth of the Trinity River is not really very great. Well, that is the reason. The new system gives a much more realistic measure of how high the water level in Lake Charlotte actually is relative to local sea level.
The details: In the previous system, the “zero point” or datum for the Lake Charlotte gauge was -5.81 feet relative to the National Geodetic Vertical Datum of 1929 (NGVD29). The new datum for the Lake Charlotte gauge is based on the North American Vertical Datum of 1988 (NAVD88). You can learn more about all of this at the website for the National Geodetic Survey at https://geodesy.noaa.gov/datums/vertical/index.shtml. Of course, the story isn’t over because the earth continues to change and new technology becomes available. As described at https://geodesy.noaa.gov/INFO/OnePagers/NewDatumsOnePager.pdf, updates to the NAVD88 system are already in the pipeline.
The change-over from NGVD29 to NAVD88 at Lake Charlotte occurred between 11:15 AM and 11:30AM on November 9, 2022. So, if you were paddling out there, you wouldn’t have experienced anything catastrophic.
