Mike,

You wrote:

> you have confirmed we need to begin from the rio source
> whether
> the natural sawanabori headspring source
> or
> the meta sawanabori thalweg point on the physical crest line
> somewhere still unknown above the headspring source

The Republic of Texas statute (which by typographical error yesterday I dated as
1936 rather than 1836) has the boundary ascending the Rio Grande from its mouth
to its SOURCE, thence due north. Therefore, I would choose the ultimate
headspring as the departure point for the northward line. This is because the
word "source" comes to us from the French, in which one of its meanings is a
spring of water, being an Old French conjugation of the verb "sourdre" (to
rise).

Where a boundary is specified to follow a river headward, then to run along the
ridge of a divide, we would be justified in ascending above the spring; but
where the source is specified as the beginning of a geogesic line, I believe
that we must use the spring itself.

The question remains: "Which spring?" I will endeavor to gather a few examples
of maps that label different branches as the "Rio Grande," but I won't have
those for you before your library there closes for the weekend.

Lowell G. McManus
Leesville, Louisiana, USA