Boundary problems on the southern border of the United States are markedly different from those along the Canadian line. Topographic, climatic, and local conditions contribute to make the southern boundary an unstable one over a large part of its course.
This boundary has a total length of approximately 2,013.4 miles. Its eastern portion, 1,321 miles, follows the Rio Grande; then come $532.9 miles in four straight courses along New Mexico and Arizona; then 19.1 miles of water boundary along the Colorado River, and 140.4 miles in a straight line across California to the Pacific Ocean.1
The land boundary with Mexico presents few problems. While in many places it traverses a rugged terrane, the arid climate does not produce the prolific vegetation which so quickly obscures the monuments on the northern border. There is consequently no need for vista cutting or the trimming of second growth.
The treaty of December 30, 1853 with Mexico (10 Stat. L. 1031) defining the southern boundary specifies that along the Rio Grande and the Colorado River the middle of the stream forms the boundary line. As these rivers, particularly the Rio Grande, are continually changing their courses, this means a fluctuating boundary, and the transfer of territory from one country to the other.2____________________