Physical geography. By Mary Somerville ...

CHAP. XIX. THE MISSISSIPPI. 247
enters the Frozen Ocean in the Esquimaux country beyond the
Arctic Circle; and the Colville, a very large river, the upper course
of which in the Russian possessions is very little known, enters the
sea near Point Barrow, in 152~ W. longitude. All these rivers are
frozen more than half the year, and the Mackenzie, in consequence
of its length and direction from south to north, is subject to floods
like the Siberian rivers, because its lower course remains frozen for
several hundred miles long after the upper part is thawed, and the
water, finding no outlet, flows over the ice and inundates the
plains.
South of the table-land the valley of the Mississippi extends for
1000 miles, and this greatest of North American rivers has its origin in the junction of the streams from the small lakes Itasca and
Ussawa, on the table-land at no greater height than 1500 feet above
the sea. Before their junction these streams frequently spread out
into sheets of water, and the Mississippi does the same in the upper
part of its course. This river flows from north to south through
more degrees of latitude than any other, and receives so many tributaries of the higher order, that it would be difficult even to name
them. Among those that swell its volume from the Rocky Mountains, the Missouri, the Arkansas, and the Red River are the largest,
each being in itself a mighty stream, receiving tributaries without
number. Before their junction the Missouri is a stream much superior to the Mississippi both in length and volume, and has many
affluents larger than the Rhine. It rises in about 44~ N. lat., and
runs partly in a longitudinal valley of the Rocky Mountains, and
partly at their foot, and drains the whole of the country on the right
bank of the Mississippi between the 49th and 40th parallels of north
latitude. It descends in cataracts through the mountain regions,
and in the plains it sometimes passes through large prairies and
sometimes through dense forests, in all accomplishing 3000 miles in
a very tortuous and generally south-eastern direction till it joins the
Mississippi near the town of St. Louis. Lower down, the Mississippi is joined by the Arkansas, 2000 miles long, with many tributaries, and then by the Red River, the former from the Rocky
Mountains; the latter, which rises in the table-land of New Mexico, is fed by rivers from the Sierra del Sacramento, and enters the
main stream not far from the beginning of the delta, at the head of
which the Mississippi sends off a large branch called the Atchafalaya to the south, and then turning to the east it discharges itself by
five mouths at the extremity of a long tongue of land which stretches
50 miles into the Gulf of Mexico, having formed a delta considerably larger than that of the Nile. The shore is lined with shallow
salt lagoons; the greater part of the delta is covered with water and
unhealthy marshes, the abode of the crocodile, and during the floods
it is a muddy sea. This river is navigable for 2240 miles. Its