I’ve received a few emails about this recently, so here’s the deal. It depends on who you ask.
According to the National Park Service, every agency has a different estimation. The sign post the beginning of the river in Lake Itasca State Park says 2, 552 miles. The U.S. Geologic Survey gives 2,300 miles. The EPA puts the length at 2,320 miles, and the Mississippi National River and Recreation Area gives its length at 2,350 miles.
So whomever you ask, it’s a long, long way down! We estimated that we paddled about 2.5 million strokes over the 73 days we were on the Mississippi and Atchafalaya Rivers.

Don’t the boundaries change year to year–and hence the length? I know that it can’t change as it day in the time of Twain’s “Life on the Mississippi” because of various dams and barriers, but aren’t there still places where it change courses somewhat?
You don’t get those huge changes in mileage from year to year as much, but again, it all depends on who’s doing the measuring. Is it the middle of the shipping channel or closest from point to point? The upper stretch would depend on how the marshes shape up from year to year too. I usually just call it 2,300 miles and call it a day.
Take care,
-John