What does this number measure?
The adopted convention is the Mississippi between Lake Itasca and its confluence with the Ohio at Cairo. The federal USACE Clarks Ferry fact sheet gives 1 250 miles for this course, approximately 2 010 km after conversion and rounding. This is a defined section of the Mississippi, not the Mississippi–Missouri system. The simplified mapping data are presented separately from the length ruler.
This length follows the course adopted by the catalogue. Maps show simplified, sometimes partial courses; an independent bar represents the documented length, distinct from straight-line distance.
Compare the stated dimensions
The comparison uses the number in each fact sheet: altitude, height, diameter, length or distance, depending on the reference. Two values in metres do not necessarily describe the same property.
Source and limitations
Itasca–Ohio/Cairo course: about 1 250 miles according to USACE, or about 2 010 km. Other federal documents round this to about 1 300 miles. Precision to the nearest kilometre is not claimed.
Top view of a simplified geographic outline of the Upper Mississippi, based on the available mapped segments. The locator drawing may be partial, and river systems may combine several branches. The channel width and the textures of the water and banks are interpreted; this is not a satellite photograph. Only the lower bar represents the reference length documented by the source; the width of the map above does not measure that length. Outline: Natural Earth, public domain. Licence: Natural Earth public domain.