From the mouth of the Ohio to the Gulf, the Mississippi flood plain covers 30,000 square miles. Over this area, sediment to an average depth of fifty feet has been laid down. In earlier times the river flooded this whole area, when freshets swelled its tributaries in the spring. The flood plain then became a sea, in the middle of which the river's current flowed swiftly. The slow-flowing water on each side of the main current let go of its burden of sediment and formed a double ridge. Between these two natural walls the main river flowed. When its level fell, two side streams, running parallel with the main river drained the flood plains on each side into the main tributaries to right and left. These natural walls deposited when the river was in flood are called _levees_. Each heavy flood builds them higher, and the bed of the stream rises by deposits of sediment. So it happens that the level of the river bed is higher than the level of its flood plain. This is an interesting fact in geology. But the people who have taken possession of the rich flood plain of the Mississippi River, who have built their homes there, drained and cultivated the land, and built cities and towns on the areas reclaimed from swamps, recognize the elevation of the river bed as the greatest danger that threatens them. Suppose a flood should come. Even if it does not overflow the levees, it may break through the natural banks and thus overflow the cities and the farm lands to left and right. Instead of living in constant fear of such a calamity, the people of the Mississippi flood plain have sought safety by making artificial levees, to make floods impossible. These are built upon the natural levees. As the river bed rises by the deposit of mud, the levees are built higher to contain the rising waters. No longer does the rich soil of the Mississippi flood plain receive layers of sediment from the river's overflow. The river very rarely breaks through a levee. The United States Government has spent great sums in walling in the river, and each state along its banks does its share toward paying for this self-protection.