[Footnote 4: "The bed of the river is so broad that the channel meanders from side to side within the bed, just as the bed itself meanders from bluff to bluff; and, as by erosions and deposits, the river, in long periods of time, traverses the valley, so the channel traverses the bed from bank to bank, justifying the remark often heard, that 'not a square rod of the bed could be pointed out that had not, at some time, been covered by the track of steamboats.'"--J.H. SIMPSON, _Col. Eng., Brevet Brig.-Gen., U.S.A._] PHYSICAL. The lower Mississippi is among the muddiest streams in the world. During the average year it brings down 7,500,000,000 cubic yards of sediment, discharging it along the lower course, or pushing it into the Gulf. As one thinks of the small amount of sediment held in a gallon or two of river water, a comprehension of this vast amount of silt is impossible. It is enough to cover a square mile in area to a depth of 268 feet. In five hundred years it would build above the sea level a State as large and as high as Rhode Island. Thus, by means of this sediment, the river has pushed its mouths fifty miles into the sea, confining its flow within narrow strips of land--natural levees made by the river itself. The Mississippi is notable for its varying length. Within the memory of the oldest pilot the length of the river between St. Louis and New Orleans has varied more than one hundred and fifty miles, being sometimes longer and sometimes shorter, as the year may be one of drought or of excessive rainfall. Occasionally the river will shorten itself a score of miles at a single leap. The shortening invariably takes place at one of its long sinuous curves for which it is so remarkable. At a season when the volume of water begins to increase, the narrow neck of the loop gives way little by little under the continuous impact of the strengthening current. Narrower and narrower it grows as the water ceaselessly cuts away the bank. Finally the barrier is broken; there is a tumultuous meeting of waters; the next steamboat that comes along goes through a new cut; and a moat or ox-bow lake is the only reminder of the former channel.[5]