The standard law over rivers as boundaries, that i BELIEVE holds as default,
unless replaced by specific provisions to thecontrary in a treaty, is that
the boundary moves with the river as it meanders naturally. This is the
process of accretion.
However, if the river suddenty changes course, due to flood or a man-made
event (attempts to train the river to meander into the neighbouring
teritory), then the boundary holds to the original course. This is known as
avulsion.

Thus the Mississippi boundaries which follow old river courses that are now
dry, because the river has changed course, cutting off a large loop, due to
floods or man-made cuttings in the necks of loops, etc.

However, some rivers, especially those in Bengal, meander so much, and flood
every year, that to let the boundary move with the river would mean large
differences year to year in territorial control. So when the boundaries of
India and Bangladesh follow the course of the river at demarcation, and are
then fixed on that meandering line, even if it becomes dry a year or two
later. Only in a couple of places do the boundaries follow the rules of
accretion/avulsion.

Thus along the NSW/Vic border, the left bank of the Murray is the state
boundary. Some loops have been cut througgh for navigaiton/floow control
purposes. Even if these have since become the main channel, the boundary
still follows the natural left bank, because the cuts are man made. This
does leave Vic land on the NSW side of the river's current course.

BW

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