Mark Twain 
Life on the Mississippi [EPUB ebook] 

It is a remarkable river in this: that instead of widening toward its mouth, it grows narrower; grows narrower and deeper. From the junction of the Ohio to a point half way down to the sea, the width averages a mile in high water: thence to the sea the width steadily diminishes, until, at the ‘Passes, ‘ above the mouth, it is but little over half a mile. At the junction of the Ohio the Mississippi’s depth is eighty-seven feet; the depth increases gradually, reaching one hundred and twenty-nine just above the mouth.
The difference in rise and fall is also remarkable—not in the upper, but in the lower river. The rise is tolerably uniform down to Natchez (three hundred and sixty miles above the mouth)—about fifty feet. But at Bayou La Fourche the river rises only twenty-four feet; at New Orleans only fifteen, and just above the mouth only two and one half.
An article in the New Orleans ‘Times-Democrat, ‘ based upon reports of able engineers, states that the river annually empties four hundred and six million tons of mud into the Gulf of Mexico—which brings to mind Captain Marryat’s rude name for the Mississippi—’the Great Sewer.’ This mud, solidified, would make a mass a mile square and two hundred and forty-one feet high.
The mud deposit gradually extends the land—but only gradually; it has extended it not quite a third of a mile in the two hundred years which have elapsed since the river took its place in history. The belief of the scientific people is, that the mouth used to be at Baton Rouge, where the hills cease, and that the two hundred miles of land between there and the Gulf was built by the river. This gives us the age of that piece of country, without any trouble at all—one hundred and twenty thousand years. Yet it is much the youthfullest batch of country that lies around there anywhere.