Lowell,

this was a good effort at driving the logic to the "bottom of the
funnel" to get at the logical end of interpretation. But (ampersand):
take three of your paragraphs alone, which I've numbered below:

Number 1 says, in effect, "we (US-MEX) posted a border marker on the
bridge at the middle of the river". In case the river changes, and
one of us doesn't undo the change, then the river remains the border
for all purposes EXCEPT those relating to the bridge itself".

I agree that we have a "differentiation" - a "tongue" or tunnel of
Mexican airspace here that juts out north of the center of the river,
which is filled with a bridge around which we can draw an
international border (which, where drawn at the end of the tunnel can
be moved as the sign is moved along the bridge span.) This tongue
comes down to earth north of the river at whatever spot(s)on earth are
occupied by the foundations over which bridge supports are built.

With respect to your paragraph that I numbered as "2.", I interpret
"rights other than those relating to the bridge itself" in your
paragraph 1, not to mean "for the purposes of the bridge", which you
defined as "carrying traffic". For one thing, these rights with
respect to the "bridge itself" come into play only "in case later
changes occur" (i.e. river movement, for one). Once the river
changes, THEN other rights relating to the bridge itself come into
play. Prior to any change, the Mexican side would have rights to park
under the bridge on its own territory, because prior to any change,
the side of the bridge marked by the marker is right above the middle
of the river - everything south is Mexican. If the river moved south,
the north side under the bridge becomes American, but limited to the
extent of "rights with respect to the bridge", which I interpret to
mean that the rights the mexicans previously had to park under the
bridge for purposes of the bridge (i.e, to drive a truck under it to
put up a scaffold so that Mexican workers could scrape rust off the
underside, etc, etc.) is a right Mexico had "relating to the bridge"
before the river moved, and a right they don't lose under the treaty
just because the river moved, the border on the ground went with it
placing the ground in the US.

This is exactly the same case as we have in the Vennbahn, that we had
in Steinstuecken before German unification, that existed in one spot
where the border between German Eupen and Malmedy was at a bridge, and
which we may have with respect to the bridge over the
German-Luxembourg condominium. It parallels the tunnel of airspace of
occupied West Berlin under allied sovereignty that jutted out above
and across East German airspace that had upper and lower limits of
altitude by treaty, within which East Germany had no sovereign right,
except that it wasn't filled with a bridge.

"Vertically differentiated international borders"... I like the sound
of that! :-) Even the acronym is useful = "V-dibs".

Len

Lowell wrote and quoted:

1. "Any rights OTHER than those relating to the BRIDGE ITSELF shall
be determined, IN CASE LATER CHANGES OCCUR, in accordance with the
provisions of this Treaty," [which is to say, the middle of the main
channel]."

2. "The purpose of bridges is to carry traffic of various sorts
across the river. For those purposes (only), the monument on the
bridge is observed. For everything else, the boundary goes wherever
the river accretes..."

3. "I fear that honesty forces me to admit that what we have here is
either a true vertical differentiation or something functionally
similar. Whichever one calls it is only a matter of semantics. I
have now come to believe that Mr. Rubio of the IBWC was entirely
correct when he enunciated the agency's interpretation to me by
telephone that the accreted land beneath the monumented Mexican
segment of the bridge is sovereign American territory. (The same
could be said for the airspace above it.)"