"In that case, then," said Anton thoughtfully, "the Mississippi runs in a groove on the top of a hill."
"That's it exactly," the Forecaster said, "and some of the most fertile fields lie in the lowlands made of the fine mud at the bottom of this hill. It's just like that hollow where your house was, Anton. The flood hasn't done much damage south of here because all the waters poured down into that fine plantation land where your place was located."
"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."