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 the banks high land is encountered. On the right, or western, bank there is but one such, at Helena, in the State of Arkansas, between three and four hundred miles below Cairo. On the left bank such points are more numerous. The first is at Columbus, twenty-one miles down the stream; then follow the bluffs at Hickman, in Kentucky; a low ridge (which also extends to the right bank) below New Madrid, rising from one to fifteen feet above overflow; the four Chickasaw bluffs in Tennessee, on the southernmost of which is the city of Memphis; and finally a rapid succession of similar bluffs extending for two hundred and fifty miles, at short intervals, from Vicksburg, in Mississippi, about six hundred miles below Cairo, to Baton Rouge, in Louisiana. Of these last Vicksburg, Grand Gulf, and Port Hudson became the scenes of important events of the war.