What Forests Give

FORESTS AND FLOODS

The forests can help us against the floods which are doing such terrible damage along our river valleys, in much the same way that they help us against that greater danger, erosion. Floods are more dramatic than erosion, because a flood, like a charge of cavalry, does all its damage in a short time, while erosion just keeps gnawing away year after year like a malarial fever. Forests cannot prevent the winds from bringing in the rain clouds and pouring the water in torrents upon the land, but they can help the land to receive it with very little damage.

To understand just how forests can do this, it is necessary to reason back from a flood to what caused it.

In the summer of a flood year the Mississippi River down near its mouth, is a menacing gray monster swinging sullenly from side to side against the levees that hold it. It does terrible work that nothing can stop. There are many things in that river besides water. It is heavy with soil and no more transparent than a plowed field. Strange things bob up and go under again—things which once belonged to people up the river—boards from their houses, rails from their fences, tables from their parlors, cows and pigs and chickens from their barnyards. All these are the wreckage from a great battle which the water has won in that long war between it and the land that has been going on since the Laurentian hills—the first rocks of this continent—pushed up out of the sea.

The result of this battle is not only a defeat for the land, but it is also a defeat for the American people, which leaves them poorer, and more unhappy, and far more discouraged than they have been before.

Why has this battle been lost?

This is what happened in one flood year: After a winter with more snow than usual in northern Minnesota—snow that had been blown into gullies and drifted into hollows in the bare lands from which the forests had been cut—there came, very early in March, a thaw. It began to rain steadily and hard, and the ice in the little lakes—Cass Lake and Winnebegoshish and the rest—went soft and slushy. As the ground was still frozen hard, the rain could not sink into it. Where it fell on cultivated fields it ran off in muddy streams between the stubble rows of last year's wheat; where it beat down on cut-over forest lands, it carved out gullies; but where it fell upon thick new forests or upon fine old ones, it found a cover of snow which the trees had kept from blowing away so that the land below was not deeply frozen, and the rain could sink in. The rains continued, and the water that ran off the hard ground made the brooks rise high as the ice that had bound them began to break. So many of these streams had emptied into the Mississippi by the time it reached Minneapolis that it could splash high up at the bridges that span it and roar furiously through the deep channel where it turns the wheels of the great flour mills. It surged on past the turn where Red Wing is set in its circle of hills, which as yet showed no sign of green. But the Mississippi was not yet in flood.

Lower down, the Wisconsin River poured in the water it had gathered from the miles and miles of cut-over, burned-over lands where there were no trees to keep the heavy snow from running off in torrents. From the west, the Des Moines turned in the run-off from the rain-drenched prairie of Iowa, a slow stream from a nearly level land where the bogs had been drained, the ponds emptied, the streams straightened, whatever could hold back the water destroyed in order to put every inch of the rich prairie land under the plow. Farther down the Illinois came in from the east, usually a wide, obedient stream meandering through flat lands, but now so full that it stole over its banks. With all this load of water the Mississippi was high, the little bychannels and swamps along the way were full of water, the low marshy islands had disappeared; but the main channel is wide, and nowhere was the river over its banks. This was not yet a flood.

But far to the west in Wyoming and Montana where the rivers rise that flow into the Missouri—the Yellowstone, the Musselshell, the Powder River and the Big Horn—there had also been heavy rain, and the snow from the mountainsides and the steep rocky slopes, went off in freshets. In most years the mouths of these rivers as well as the upper Missouri, into which they empty, would still be ice-bound, but this year there was a warm spell; the ice was ready to break under the rush of the water, and go down the Missouri with it—through the dry-farming region of North Dakota and South Dakota, through Nebraska and Missouri. Opposite the white cliffs of Alton this wide, slow, muddy river joined the Mississippi. The Mississippi struggled with this great volume of water. It took more room for itself where the land is low. It pushed hard against the levees, which tried to hold it in. It was full and savage, but yet there was no flood.

Slowly the Mississippi circled around one side of the peninsula which holds the city of Cairo, while along the other side rolled in the Ohio. If the water on this river had gone down the valley 10 days earlier, as it usually does, then the Mississippi could have carried all the water which poured into it at the point of the peninsula, and there would have been no flood. But up in New York State and western Pennsylvania there had been a late cold snap so that the heavy snow did not melt till thick clouds came to rake the hills and let down a curtain of rain. Then 2 weeks late the complaining brooks broke their ice and slid down over the frozen hillsides into the creeks that drained the farm lands, and the creeks rushed the water along to the Allegheny River, which carried the run-off to the Ohio, which rose out of its bed. All through Ohio and Pennsylvania and Indiana the spring had been held back, and the streams were now running full. The Ohio River rode high below Cincinnati, and as it neared its mouth, the Cumberland and the Tennessee joined it, straining full. So that instead of being a falling river at Cairo, the Ohio delivered to the Mississippi at the base of the peninsula more than 1,500,000 cubic feet of water every second. This is more than the Great River could carry within its banks. It had to have more room. The flood began!

On both sides of the river from there on levees are braced against the water like a man leaning against a door to hold it shut. The tops of the levees are higher than the land back of them, and against them the river rose inch by inch, foot by foot. There is a point beyond which no wall of earth can withstand the pressure of water. The river grew higher and where the top of a levee had crumbled, it reached in a thin trickle over the top. The trickle widened to a sliding sheet of water, the levee melted under it, and the real flood had come.

The Levee Breaks! (Courtesy, The National Geographic Society)

Usually it does not come—this flood—with a rush and a roar, before people in the lowlands have time to reach a safe place. When the early risers in the river towns find pools of yellow water where there was dry land the night before; when Main Street is not a dry road or even a muddy one, but a quiet shallow lake growing deeper and deeper, rising above the curb, over the sidewalks, seeping under the doors into the houses; then the people look for their old boats, for they know that the river has spilled onto the land and that a flood has come.

Further south there were more and more breaks in the levees. Sometimes they began where a crayfish had burrowed and the water, driving through the tiny hole after it, had washed out a tunnel for itself; sometimes where a heavy rain had turned a levee into mud. Miles of continuous embankment held back the river from millions of acres on which the crops were now beginning to show green, and all along them thousands of men piled up sandbags and shoveled earth to keep them firm. All day they worked, and at night they planted torches along the levees and worked on. For this was a flood!

The water was only 2 feet from the top—18 inches—16. More sandbags! Shovel faster! Twelve inches from the top—10—and then, as with a careless push of a mighty shoulder, a whole section of the levee quivered, bent, was gone! There was no seeping, no gradual coming of water now. A wall of water toppled over upon the land. Houses and barns were lifted up from their foundations and floated away; there were beds and chairs and tables; there were horses and cattle; there were all the things that help to make people comfortable and prosperous, for this was a real flood. A battle which the water had won! A defeat for the people who live beside the river!

Flooded Town. (Official photograph, U.S. Army Air Corps)

And there was still another great body of water slowly pushing its way toward the Mississippi. Almost without a ripple the wide, shallow Arkansas turned in its waters. Fifty inches of rain on its headwaters that year, 52 inches near the mouth; cloudbursts on the bare, deforested heights of the Ozarks. More water than any other river, the Arkansas added to the flood. Still lower down the Red River poured in what it had gathered in Texas and Louisiana.

Straight in the path of the oncoming flood crest sat the city of New Orleans on land lower than the surface of the river. The levees surround it like the edge of a bowl. They had never broken yet, and there were thousands of sandbags ready to build them higher still. But 14 inches of rain had just fallen. Suppose this time the levees should not hold! The city was afraid. The flood came on.

On both sides of the city the land lies low and interlaced with streams and estuaries. It is good land on which to raise sugarcane and rice, and it is full of muskrats whose fur is a valuable crop. Levees hold the river back from this profitable land. Will they hold?

Above New Orleans levees had broken on both sides of the river, but in spite of all the water that had drained away through these gaps, the river continued to rise. A million sandbags had been piled along the rim of the bowl in which the city lies, but this time the river was going to take more land. All that men can do in the face of such a flood is to choose whether they will give it the city or the farm lands. Usually it is decided to offer the fields to the river. The levees are blown up, and the river takes the land of the muskrats, the rice, and the sugarcane instead of the city of New Orleans.

It is a great victory of the water over the land.

These great floods on the Mississippi do not come every year. Usually there is one only every 5 or 6 years when the snow melts and the rain falls at the exact time to bring the high waters on two or three of the main rivers into the Mississippi at once. But when they come they destroy the homes and the crops of many people. Is there no way in which we can say to the river, "Stay where you belong!"?

Working on the Mississippi Levee. (Courtesy, The National Geographic Society)

Shall we say this to the river by building more levees? Stronger ones? Higher ones? We have found that when the Mississippi is in flood it pays very little attention to commands enforced only by levees. Already we have built 1,100 miles of levees on the Mississippi itself, and 180 miles on the Arkansas, and many miles on both the Ohio and the Missouri. We must keep them, and perhaps we must build more, but levees alone are not enough.

Perhaps the river will obey us if we give it more room of its own. We are trying that. We are sending dredges to scoop mud from the bottom and make the channel deeper. We have a machine which noses along the shallow places with a great flexible pipe and sucks out the sand. In some places instead of making the river deeper, we are making it wider. There is a plan to give the river more land opposite the city of Cairo and some just below the mouth of the Arkansas, and more still south of the Red River, where it can pour its water down through the Atchafalaya River into the Gulf by an extra mouth. All these are good plans, for there is no use trying to keep a large river in a small bed.

In some places we are holding the water back in reservoirs of stone and concrete—throwing great dams across the Mississippi and its tributaries. With these dams we not only catch the waste water which would otherwise swell the flood, but we have it where we can put it to work.

But to impound rivers and streams in reservoirs and to hold them back behind dams are remedies merely. Would it not be better to prevent floods? To keep so much water from trying to run down the Mississippi at once? Certainly it would be better! But is not that a very difficult thing to do? Yes, it is difficult, but with the help of the forests we can do it.

Of course there is no possibility that forests, however much land they cover or however thick and tall they grow, can prevent-floods. How could they? Only some overwhelming power which could say to the oceans, "Just so much water you may give up to make rain clouds of", could do this. Water that is lifted up from the ocean is certain to come down to earth again. We can do nothing when the warm air lifts water vapor from the waves, carries it over the land on a wandering erratic wind, and pours it down suddenly, except prepare the land to deal safely with it.

Forests are one means to that end. What exactly can they do?

Behind the Mississippi and her tributaries are 800,000,000 acres of land on which the floods originate in rain and melting snow. One hundred and sixty million of these acres are or can be covered with forests. What help can they give to the reservoirs, and dams, and levees in keeping the rivers in their courses?

To control floods it is necessary to interfere with that persistent force of gravitation which continually pulls water down to the level of the ocean. Nothing can hold water back when it is gathered together at the mouth of the Mississippi; almost anything can deter a raindrop.

Forest Floor

Consider again what a forest is. Underneath the ground is a thick network of roots, strong, long fibers which keep the loose soil made spongy by wood mold, porous enough to hold water. Upon the forest floors lies the duff, a layer of branches, dead grasses, and leaves which may be as much as a foot thick. In through it are mosses and ferns. Now moss can absorb from 200 to 900 times its weight in water. If the dead leaves on the forest floor are from birch, maple, or other hardwoods, they will absorb 150 to 220 times their weight. If they are pine needles they will absorb from 120 to 135 times their weight. Such a porous mat will hold a a good deal of water, especially if this is not hurled down upon it so rapidly that the forest floor has no time to take it in. Rain coming down on the forest in summer is met first by the thick canopy of leaves which so breaks its fall that it drips quietly upon the undergrowth and from there slides gradually to the litter on the forest floor. Some of it stops there in the cups which the dead leaves make of themselves—hard dead oak leaves which will each hold a big tablespoonful of water, maple leaves which hold half as much, star gum leaves brimming like bowls. Each one of these tiny reservoirs is small in itself, but consider how many leaves there are! The water they hold stays there till it dries. In winter the leafless branches of the trees form a windbreak. The snow, instead of being piled up in drifts, sifts down upon the forest floor. In evergreen forests much of it stays on the branches. As spring comes on, melting begins early in the woods, but it takes from four to eight times as long for the same amount of snow to melt in the forest as it does on the open ground, and since the soil in the forest is less likely to be frozen, more of the snow water can soak down into it. The roots of the trees absorb this water and carry it to the leaves, and the leaves gives it back to the air by a process called transpiration. In a year an acre of spruce will transpire the equivalent of 8-1/2 inches of rain over that area; an acre of oak, 5 inches; an acre of beech, 10 inches. These are the equivalent of very heavy rains, and so far as floods are concerned it is as though the rains had not fallen. Only when there is more rain than can be absorbed by the soil or drawn up by the trees is there any to run off in the forest streams; only when there is a great deal more, do the streams overflow. If the streams do not overflow, neither do the rivers.

The place to prevent a flood is not at the mouth of a river, nor along its course, but where it starts—around the little forest streams on the steep hillsides and the remote rivers, and the brooks in the midst of the forests.