I was totally wrong earlier tonight when I wrote that the first break between
the Americas, given sufficient rise in sea level, would be at the approximate
location of the canal in Panamá.

Before the excavation of the Culebra or Gaillard Cut across the lowest point in
the Panamanian cordillera, the elevation there was 333.5 feet. A few hundred
miles to the northwest, however, one can go from the Caribbean Sea, up the Río
San Juan, and into Lake Nicaragua, which has a natural surface elevation of 110
feet and a bottom well below sea level. This lake is separated from the Pacific
Ocean by the narrow Isthmus of Rivas, the lowest point on which is only 154 feet
above sea level. This, then, is where the Central American Land Bridge would
first breech if sea level were to rise.

Since I am hardly prepared to accept Costa Rica as a South American nation, I
will revert to the narrowest point on the Isthmus of Panamá, a dozen or so miles
east of the canal, as the appropriate division.

To accept the juncture of the tectonic Caribbean and South American plates as
the limit between North and South America would put the northern fringes of
Colombia and Venezuela in the former. I don't think that we want to do that
either. Furthermore, this plate juncture is invisible on the surface.

Lowell G. McManus
Leesville, Louisiana, USA