http://www.asprs.org/asprs/resources/grids/07-99-honduras.pdf
says the gthn agreement provides for the border to move in 
unison with its defining rivers 
whenever & regardless of how they may change their courses
in a possibly unique exception to standard international practice
the article also maintains that the hnnisv tridominium was 
actually finalized by the 3 parties in 1993 in accordance with the 
icj judgment & advice of 1992