"You've hit it just right," he said, "mud is earth or soil that has been washed down by the river. That's what makes the bottom of the river so irregular and why it's always shifting. You can see for yourselves, boys, that if the bottom of the Mississippi is just made of light mud, light enough to be carried down as muddy water for hundreds of miles, any little change in the current of the river will stir up that mud again and scoop out a hole. If it happens to be near a bank, the bank will be eaten away and, naturally, will cave right in." "About how much mud does the Mississippi carry down, Mr. Levin?" Anton asked. "In flood time, as much as a thousand tons a minute will be carried past here." Ross whistled. "A thousand tons a minute!" he exclaimed. "Why, I should think that would fill up the river in no time." "It would," the Weather Man answered, "if the river stood still. In flood time, however, the water is flowing rapidly and takes the mud clear down to the delta. That's why there is always so much new land being made at the mouth of the river. You could buy a piece of land under water now, Ross, if you wanted, and be quite sure that in twenty years' time there would be land there for a farm." "But a thousand tons a minute!" the boy repeated, "that seems huge!" "It is pretty big," the Forecaster agreed, "but I'll show you where it comes from. You know, boys, generally the land slopes down in the direction of the river, doesn't it?" "Yes," assented the two boys, "it's supposed to. But it doesn't here. The lie of the land is away from the river." "That's just exactly the point," the Forecaster declared. "The banks of the Mississippi range in height from about twenty to forty feet above extreme low water. As the river, in times of flood, rises as high as forty to fifty feet above low water, unless there were levees, the river would overflow its banks every spring or flood time." "It does, quite often, even yet," commented Ross, looking on the flooded scene around him. "Well," said the Weather Man, "the present levee system only dates back to the end of the Civil War, although there were levees built during the first settlement of New Orleans, two centuries ago. Remember, though, that the Mississippi has been flowing down its present bed for several hundred thousand years, with a flood every spring, so that the overflow has had its effect. Of course, before the land was broken up by farming, there wasn't as much earth carried down into the river to make mud as there is now. "When the Mississippi River, with its heavy sediment, overflows the banks into the swamps, it's easy to see that the current will be slower in the flooded area than in the main bed of the river." "Of course," agreed Ross, "but what has that got to do with it?"