And river meanders are doing just that, meandering to and fro, changing (albeit slowly) continuously. Modern borders need to be tied geodetic datums, which has been also mentioned are also constantly changing (although probably more slowly than most rivers) and need to be updated continuously. The new datums coming out of the GRAV-D project will address the need for continuous updating (https://www.ngs.noaa.gov/datums/newdatums/index.shtml).