The photo was taken about midway between Leadville and Buena Vista, Colorado. The river is the Arkansas. From 1819 until 1848, the southern bank of the Arkansas River was the boundary between the United States and Mexico from near present Dodge City, Kansas (100th Meridian), to the river’s source near Leadville. The photo was taken from the Mexican side looking across to the United States.
From the source of the Arkansas (in the mountains near Leadville) the boundary ran directly north to the 42nd Parallel (just north of today’s Interstate 80 in Wyoming); then west to the Pacific Ocean. Prior to the Adams-Onis Treaty of 1819, the United States claimed the Rio Grande as the southern border of the Louisiana Purchase. This was consistent with what the French believed they sold in 1803, but not practicable as the 1819 compromise confirmed.
At 1,460 miles, the Arkansas is the fourth longest river in the United States after the Missouri, Mississippi and Rio Grande. It passes through Tulsa and Little Rock and empties into the Mississippi between Memphis and Vicksburg.