The Boy with the U. S. Weather Men Part 7

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," a.s.sented 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?"

"A great deal," the Forecaster replied succinctly. "The faster a river flows, the more sediment it can carry without allowing it to drop to the bottom; the slower it flows, the more readily is the sediment dropped.

If you put some mud in a gla.s.s of water and keep stirring it with a spoon, the mud will never sink to the bottom. Even if you let it stand perfectly still, it will take several days before the finest particles sink to the bottom of the gla.s.s and the water becomes clear."

"Yes," agreed Anton, "I've often wondered why."

"Well," the Weather Man continued, "if you look closely at the mud in the bottom of the gla.s.s, you'll see that the bigger particles are at the bottom and then those a little smaller and so on up, until your top layer is made of a mud composed of particles so fine that you'd have to get a microscope to see them."

"I don't quite see why," said Ross. "I know bigger things are heavier, but why should a big bit of earth sink more quickly than a small bit, when they're both made of just exactly the same stuff?"

The Weather Man looked at him.

"Some of these days," he said, "remind me to talk to you about sunlight and dust, and I'll tell you a heap of things you don't know. Right now, get this idea in your head. The larger a piece of matter is, the smaller is the surface in proportion to the bulk. A feather of swan's down will float in a high wind, but if you roll that feather into a ball, it will fall. Why? You haven't made it any heavier. You've only reduced the amount of surface which was borne up by the air. It's the same way with mud, the bigger pieces sink first because they have less surface in proportion to their weight."

"Yes," answered Ross, "I can see that now."

"Very good, then," the Forecaster continued, "when the Mississippi overflowed its banks and the water got out of the current of the main stream, so that it flowed more gently, the sediment began to fall, the larger pieces first and those that were finer until it was only at the most distant point from the river that the finest mud settled. This has gone on, year after year, for thousands of years.

"Therefore, you see, the lands nearest the river are higher than those farther away. In two big basins, the St. Francis and the Yazoo basins, the slope and the drainage is away from the river, instead of towards it."

"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 sh.o.r.e that the boats pa.s.s."

"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."

"Make it all the same width, then," said Ross.

"Build two-thirds of the whole two thousand miles by some underwater system, constructing the wall under water? If you had ever read of the difficulty of building one lighthouse foundation, my boy, you wouldn't talk so glibly about building huge retaining ma.s.ses of masonry under water."

"Suppose it were done, that way, Mr. Levin," put in Anton, "would that settle it all?"

"You mean--suppose there was a high masonry wall, making a ca.n.a.l equal in width and height from St. Louis to the Gulf, would that turn the Mississippi into a permanent s.h.i.+p channel? Is that what you mean?"

"Yes."

"No, it wouldn't," the expert replied. "What are you going to do with all the little streams that flow into the Mississippi? Think for a minute, boys. You can see that wherever you narrow the banks, the river channel has got to be made deeper to accommodate the water, hasn't it?"

"Yes," both boys agreed, "it has."

"In other words, suppose that before you put up this huge masonry wall, the flood crest was fifty feet at Memphis, then, after the wall was built, the flood crest would be seventy-five or a hundred."

"Suppose it were," said Ross, "the wall would hold it in."

"So you think. There are the tributaries to consider. Take the Yazoo, for example. It flows into the main river until the Mississippi reaches the fifty-foot flood level. If you raise the flood level of the Mississippi to seventy-five feet, the water in the main river will be twenty-five feet higher than the water which used to run into it at the fifty-foot level, won't it?"

Ross whistled.