Adam,
You wrote:
> In other words, the line started at the confluence of the two rivers,
> but the border started where that line crossed the Colorado.
Not exactly. The MXUS boundary 1848-1853 descended the Gila to a point at "the
middle of the Rio Gila where it unites with the Colorado" and from that point
took a bee-line for the Pacific below San Diego, crossing the Colorado several
miles downstream at the current AZCAMX tripoint.
So, the MXUX boundary of 1848 came down the middle of the Gila and just touched
the south bank of the Colorado in the mouth of the Gila, not making tripoint
there with the boundary of California (as admitted in 1850), which was the
middle of the Colorado. Thus, the broad bend in the Colorado that now skirts
the northern end of Yuma was a pene-enclave of the New Mexico Territory
(established 1850), joined to the rest of NM only by half the width of the
Colorado at the confluence of the Gila. The southern boundary of NM was
described as "Beginning at a point in the Colorado River where the boundary line
with the Republic of Mexico crosses the same; thence eastwardly with the said
boundary line..." This would have carried it through the northern end of
current Yuma on the vestigial cadastral line that we see on modern maps and then
up the Gila eastward.
The Gadsden Purchase of 1853 added to the US the land between the MXUS boundary
described above and the current MXUS boundary. This erased the part of MXUS
that is now the ghost line through Yuma, causing MXUS to go down the Colorado
southward from modern AZCAMX to the modern MXUS geodesic segment that you
mention below, thus enlarging the New Mexico Territory.
> I wonder how the western end of that line was chosen. It seems likely
> that it was just chosen as a location that allowed for the area
> around San Diego Bay to be in the USA but not much more. Seems odd
> that they didn't set the border on the Pacific at, say, the mouth of
> the Tijuana River, which would be a couple miles north of where it is.
The Treaty of Guadalupe Hidalgo of 1848 specified "a point on the coast of the
Pacific Ocean, distant one marine league due south of the southernmost point of
the port of San Deigo, according to the plan of said port made in the year 1782
by Don Juan Pantoja, second sailing-master of the Spanish fleet, and published
at Madrid in the year 1802, in the atlas to the voyage of the schooners Sutil
and Mexicana; of which plan a copy is hereunto added, signed, and sealed by the
respective plenipotentiaries." [Shades of Mason and Dixon hunting the
southernmost point in Philadelphia!]
> While we're at it, I wonder what the history of the geodetic line
> that forms the WNW/ESE southern border of Arizona/Gadsden Purchase
> is. How was it chosen? A map of Baja California shows Mexico Hwy. 2
> extending for about 15 miles WNW of the azbcso tripoint, roughly
> along the same alignment as the WNW/ESE line in question. Hmm.
The whole purpose of the Gadsden Purchase was for the US to acquire a desirable
railroad route. James Gadsden was, in fact, a railroad executive who was
appointed Minister to Mexico for the negotiations. The boundary that finally
emerged was rather arbitrary, designed to enclose the needed railroad route.
The geodesic segment has its eastern terminus at 31°20" N. Lat. and 111° W.
Long. It runs "thence in a straight line to a point on the Colorado River
twenty English miles below the junction of the Gila and Colorado Rivers. It has
no vestige west of the Colorado. Mexico highway 2 roughly parallels the
geodesic segment. After crossing the Colorado, it continues in the same
direction, straight across the desert, aimed generally at Mexicali.
Lowell G. McManus
Leesville, Louisiana, USA