nt of junction three powerful States meet. Illinois, here
bounded on either side by the great river and its tributary, lies on
the north; on the east it is separated by the Ohio from Kentucky, on
the west by the Mississippi from Missouri. Of the three Illinois was
devoted to the cause of the Union, but the allegiance of the two
others, both slave-holding, was very doubtful at the time of the
outbreak of hostilities.

The general course of the Mississippi here being south, while that of
the Ohio is southwest, the southern part of Illinois projects like a
wedge between the two other States. At the extreme point of the wedge,
where the rivers meet, is a low point of land, subject, in its
unprotected state, to frequent overflows by the rising of the waters.
On this point, protected by dikes or levees, is built the town of
Cairo, which from its position became, during the war, the naval
arsenal and depot of the Union flotilla operating in the Mississippi
Valley.

From Cairo to the mouths of the Mississippi is a distance of ten
hundred and ninety-seven miles by the stream. So devious, however, is
the course of the latter that the two points are only four hundred and
eighty miles apart in a due north and south line; for the river,
after having inclined to the westward till it has increased its
longitude by some two degrees and a half, again bends to the east,
reaching the Gulf on the meridian of Cairo. Throughout this long
distance the character of the river-bed is practically unchanged. The
stream flows through an alluvial region, beginning a few miles above
Cairo, which is naturally subject to overflow during floods; but the
surrounding country is protected against such calamities by raised
embankments, or dikes, known throughout that region as levees.

The river and its tributaries are subject to very great variations of
height, which are often sudden and unexpected, but when observed
through a series of years present a certain regularity. They depend
upon the rains and the melting of the snows in their basins. The
greatest average height is attained in the late winter and early
spring months; another rise takes place in the early summer; the
months of August, September, and October give the lowest water, the
rise following them being due to the autumnal rains. It will be seen
at times that these rises and falls, especially when sudden, had their
bearing upon the operations of both army and navy.

At a few points of th