"What I don't see," said Ross, "is why the Government doesn't build a really high levee all the way along the river. I don't mean just a few feet higher, but a regular wall 'way higher than the river ever goes. I mean a regular stone wall, twice as high as any levee that we've got now. I should think that would make the river behave." "It would and it wouldn't," replied the Weather Man. "What are you going to build that wall on? On the ground?" "I suppose so," said Ross. "I hadn't thought much about that." "Indeed you hadn't," his friend replied. "You've got to remember, Ross, that the Mississippi doesn't run in a straight line; it bends and twists like a snake. In the bends the current strikes on the outwardly curving bank, and, as you know, the water is always deep there. This causes a rapid caving and erosion of the bank. At the foot of each bend, the main flow crosses to the other side, where it strikes the bank which has become concave there, and eats into that bank just as, a few hundred yards higher, it has been eating into the opposite one." "I know you've always got to pilot a boat first on one side of the river and then on the other," said Ross thoughtfully. "You have. And, if you remember, you'll see that it is generally on the side nearest to the concave shore that the boats pass." "Yes," agreed Ross thoughtfully, "I guess it is." "Now, you can easily see," the Forecaster continued, "that the river might keep its own channel clean if it flowed straight down with a current of equal strength. But, as the current crosses from side to side, it slackens speed at each of these crossings. Therefore, as the current becomes slower, it drops some of the heavier particles of sand or mud, forming a bar at every bend, sometimes so high as to prevent navigation." "That's what the dredges are for, isn't it?" asked Ross. "Yes. The Government has twelve large dredges at work all the time, keeping the navigation channel open." "I don't see, yet, why the stone wall idea wouldn't work," protested Ross. "I'm just showing you," was the reply. "If you built your heavy wall on the bank, the water would strike the concave bank at one of these crossings, eat away the earth under the wall and your wall would topple in. Then the current would cross the stream, undermine the bank on the other side and your masonry would crumble there, too. So much for the wall." "Suppose you sunk that wall, away down deep, below the level of the bottom of the river?" suggested Ross. "That might work," the expert replied, "but it would cost more money than the United States could afford to spend. Besides, Ross, where would you build this wall? Right on the bank?" "Of course." "But the Mississippi is half a mile wide at some places and three miles wide at others. If the river were absolutely walled in, you'd have swift currents at one place and slow in another. Then your channel would fill up in the wide places and you'd be as badly off as before."