“When men began confining the river within its banks at all but the highest stages of water, and in many places at all stages, it couldn’t leave its old channels for new ones, no matter how much it had built up the bottom, and so the bed of the river steadily rose from year to year. That made the surface of the flood water higher, and so men had to build higher and higher levees to keep the floods from burying their plantations. As they have nothing better to make their embankments out of than the soft sandy loam of the bottom lands, the levees are not very strong at best, and the higher they are raised, the greater is the water pressure against them when the river is up. So they often give way, and when they do that the river rushes through the gap, or crevasse, as it is called, rapidly widening and deepening it, and pouring a torrent over all the country within reach. In such a flood as this men are kept watching the levees day and night to stop every little leak, lest it become a crevasse, and it is often necessary to forbid steamboats to pass near the shore, because the swells they make would wash over the tops of the levees and start crevasses in that way. Sometimes a strong wind pushes the water up enough to break a levee and destroy hundreds of lives and millions of dollars’ worth of property, for when a levee breaks, the region behind it is flooded too rapidly to permit much more than escape alive, and often it doesn’t permit even that.” “What a destructive old demon this river is!” said Irv. “Yes, at times,” replied the elder boy. “But it does a lot of good work as well as bad. It created all the lands that it overflows, and if man tries to rob it of its own, I don’t see why it is to be blamed for defending its possessions.” “How do you mean that it created all the lands that it overflows?” asked Constant, who always wanted to learn all he could. “Why, the geologists say that the Gulf of Mexico used to extend to Cairo, covering all the flat region in the Mississippi Valley south, except here and there a high spot like that on which Memphis stands. The high spots were islands in the Gulf.” “But where did the land come from then?” “Why, the Mississippi built it with its mud. It carries enough mud at all times to make half a state, if it were all brought together. When the river’s mouth was at Cairo, the river kept pouring mud into the Gulf. The mud sank, and in that way the shore-line was extended farther and farther south, spreading to the right and left as it went. The river is still doing this down at its mouth below New Orleans, and it has been doing it for millions of years. It has simply filled in all that part of the Gulf that once covered eastern Arkansas, Louisiana, Mississippi, and the lower parts of Kentucky, Tennessee, and Missouri.” “But why don’t other rivers do the same thing?” asked Constant. “They do, in a degree,” said Ed. “You know there is always a bar in the sea just off the mouth of a river.” “Yes, but--”