LEASH By WARD SHEPARD Forest Inspector, United States Forest Service HE MISSISSIPPI bears seaward in her floods a mighty derelict traffic gathered from far-flung mountains, plains, and farms. It is the traffic of wasted water and wasted soil. From millions of rivulets and creeks and small rivers it comes, each bearing its seemingly insignificant burden, each taken by itself a harmless thing. Yet these burdens, augmented by heavy rains and sudden thaws, converging from a thousand scattered sources, multiply their strength until the Mississippi becomes an unchained monster. Great floods will always come, for man cannot hold back the storm-clouds nor keep their waters from rolling to the sea. But, within certain limits, he can restrain their violence. Levees, if high enough and strong enough, will hold floods to a fixed channel. Storage reservoirs will hold back part of the flood waters. Such works form the main attack on floods. But there is another attack-the natural storage of water-which is of great Floods, importance but which has never been adequately developed in Forests, this country, because for its greatest effectiveness it requires that and vegetative cover, especially forests, be kept at the highest state Levees of efficiency. Soil waste and water waste are at the heart of the flood problem. Yet their very immensity has obscured their importance. It is easy to see the swelling tide of a great flood, the desolation it leaves in its wake; but it is difficult to trace, even on a small scale, the widely scattered causes that contributed each in its way to the catastrophe. It is even more difficult to grasp the cumulative total effect of these causes at work over immense areas of land. That vegetation-particularly forest cover-holds back and absorbs a great deal of water has been conclusively shown by careful scientific experiments both in Europe and in America. In Switzerland two small neighboring valleys-one forested, the other deforested-were studied for many years. Even after very heavy rainfall, the deforested stream carried from two to three times as much water as the forested stream. In California three forested valleys discharged into their streams only 37 per cent of the rainfall during the rainy season of January, February and March. They had absorbed the rest and were still paying it out slowly and in well-sustained streamflow after three months of dry weather. A neighboring deforested valley, on the other hand, quickly poured 95 per cent of its rainfall into the stream and went dry two months later. Other similar examples could be given. Excessive erosion after deforestation is attested by innumerable striking examples from many parts of the world. Many navigable rivers have been so clogged with soil that navigation is no longer possible. Orange groves in California have been partially buried under tons of gravel and bowlders washed from fire-stripped mountain slopes. In one instance soil washed from a burned hillside of 200 acres completely buried a 12-acre alfalfa field. Such instances could be multiplied indefinitely. Accurate scientific observations of this kind are much more easily made in small than in large valleys, where there is such a variation in soils, slopes, vegetation, and many other factors that it is difficult to weigh their relative effects on total streamflow and erosion. Vegetation In spite of the large amount of scientific experiment and observation, especially in small valleys, on the value of vegetation for preventing rapid run-off and erosion, there is still doubt and debate over the meaning of the evidence. The mind finds it difficult to bridge the gap between the individual raindrop and the great flood in the valleys. Yet the greatest flood is merely the final sum total of the discharge of numberless tiny rills and rivulets, which in turn collect rain-water from the ground where it falls. This difficulty comes from failure to take into account the vast extent of land surface on which flood-causing rains fall and the equally vast surface over which vegetative cover may exert its influence. Only an insignificant fraction of the water in a great flood falls directly on the face of the river or flows from its immediate banks. The great bulk of it falls on thousands of square miles of land surface. Part of it seeps into the ground, to emerge months later as springs; some of it flows or trickles over the surface into little rivulets and so makes its way down toward the creeks and rivers. But whatever course it takes, most of it falls in places that afford a greater or less opportunity to cope with water while it is still in small quantities and moving at slow speed. There are only two ways in which man can radically alter the condition of the earth's surface. One way is through tilling the soil. Poor tillage leads to rapid run-off and soil erosion; good tillage leads to absorption and stability of soil. The other great means of changing surface conditions is hrough the use or abuse of vegetative cover. Leaving out of account the influence of growing farm crops, the imporance of vegetation in controlling run-off and erosion in the Mississippi basin can be gauged by the enormous extent of grassland and forest within its confines. Grassland and forest can play their full part in water control only if they are kept vigorous and dense. But both our grasslands and forests have been depleted and their efficiency in water storage impaired over vast areas. The forest is the best of all natural water-holders and soil-holders, but all forms of vegetation, including grass, have similar effects in varying degree. Forest cover baffles water in its efforts to reach the sea. The crowns of the trees intercept and evaporate a good deal of rain and snow, and forests use up large quantities of water in the process of growth. Much of the normal rainfall is absorbed by the thick, soft mat of fallen leaves, needles, twigs, cones, seeds, and decaying wood that covers the forest floor, sometimes to a depth of 3 feet. This mat, acting like a sponge or a wick, carries water down into the porous humus or vegetable mold and thence into the mineral soil. Here it joins the great body of underground water that may seep slowly for months before it emerges in some distant spring or rivulet. Forests During a heavy storm, the absorptive capacity of the forest may be passed. Even so, the forest has already had a helpful effect in absorbing and retarding part of the water. By slowing down the run-off and by taking large quantities of water underground and delaying its passage to the streams, the forest tends to reduce flood heights and to prolong the flow of streams. Forest cover means, then, less fluctuation in the high and low water stages, slower run-off, and better navigation. The forest exerts other powerful restraints on rapid run-off. Snow in the forest melts later and much more slowly than snow in the open, so that its burden is added to the streams after snow water from the open has already passed off. This may prolong high water but makes it less severe. Moreover, the forest is warmer in winter than open country, so that the soil may remain unfrozen or may freeze only slightly, and is thus able to absorb the water from melting snow. In the open, the soil is frozen hard and deep; and the snow, melted by the first warm rains, pours torrents into the streams. The same things that make the forest a good water-holder make it a good soil-holder. And this soil-holding is extremely important not only in flood control but in the conservation of our soil wealth. Floods contain not only water but mud. The mud raises the flood crest, fills river channels, and cuts down their flood capacity, compelling the building of higher levees. It is a constant menace to such engineering works as storage reservoirs, which it fills up with silt. A good, dense forest keeps the soil in place instead of loosing it into the rivers. Its fibrous roots bind the soil together. The thick mat of leaves and humus keeps the water from cutting into the soil. In a well-kept forest the streams usually run clear even after a heavy rain. The destruction of the forest or other vegetation soon exposes the soil, which permits rain-water to run off quickly. The cutting power of water increases even faster than its speed; and the bare soil, no longer protected by leaf-litter, is rapidly washed into the streams. Millions of tons of our richest topsoils are yearly washed into the streams. They are irrecoverably lost to our farm and forest wealth and are adding enormously to the problems of navigation and flood control. Forests vary considerably in their effects on run-off and erosion. They have most influence on steep slopes with impermeable soils; least in level country with porous soils. The processes here described apply to average conditions. The forest, it is true, is only a shallow reservoir, so to speak, but it is vast in extent. This or that acre, this or that stretch of woodland, may seem to have only an insignificant effect on streamflow. But the cumulative effect of thousands of square miles of forest is an immense power for good that must be made use of. Forests It is not generally realized that we have immense areas of denuded and deteriorated forest land not needed or suited for farming. Something like 275 million acres of once forested lands are now in farms, but about 225 million additional acres of forest land have been logged off and not taken for farming. Many of these forests have been denuded or depleted by destructive lumbering and fire. Many of them have been turned into worthless barrens. Most of them are swept by recurrent fires, the vegetation is thinned out, the leaf litter and humus destroyed, and the bare soil exposed to gullying and washing. In the denuded forest, neglected and repeatedly burned, the vegetation finds a scant hold on the sterile soil and is often reduced to a scraggly, open growth of brush and weeds. The soil, gouged and exposed by logging and fire, is no longer protected by a thick mat of litter. Such forests cannot compare as water-holders or soil-holders with a well-kept dense forest with a thick litter and tangled undergrowth. Most of our forests are only partly fulfilling their functions of soilholding and water-holding. They must be built up and restored to their full density; the destroyed humus must be replaced by a new accumulation; the forest must everywhere be kept green, thrifty, and well managed. Such forests can be built up only by universal protection against fire and destructive lumbering. The most critically important forests of the Mississippi basin are those of the western slopes and foothills of the Appalachians. The Ohio and Ten nessee Rivers, which have their sources in these mountains from southern New York to Georgia, collect and pour into the Mississippi something like one-fourth of its total waterflow. Their headwaters drain rugged slopes subject to violent downpours and destructive erosion. Under the attrition of the ages, these ancient mountains had reached a state of equilibrium in which natural erosion was largely checked by the dense cover of virgin forest. Man has disturbed this balance. With fire and ax he had made enormous inroads into the original forest. Destructive logging and fire have long held sway in the Appalachian Mountains. Precious soil that has been tens of thousands of years in building up from weathered rock and decayed leaffall has taken its way toward the sea. The Ozark Mountains, occupying a large stretch of country between the Arkansas and Missouri Rivers, likewise occupy a strategic position. Their forests have been badly depleted by lumbering, fire, and the clearing of slopes for agriculture. The forests of the eastern slopes of the Rocky Mountains, whence flow Systems of Flood Control such important tributaries as the Missouri and the Arkansas, fortunately have far better prospects than the Appalachian forests; for many of them are embraced within the National Forests, which are managed not only for timber production but for stream protection. In the Lake States, forests have a less decisive effect on streamflow and erosion than other forests of the Mississippi basin because of the gentle topography and because of the porous sand and gravel soils, which readily absorb water. The swamp forests, however, are extremely important in their ability to store flood waters. Nowhere in the country have forest depletion and denudation been so extensive as in the Lake States. Throughout the central farming regions, stretching from southern Wisconsin and Minnesota down to the southern pineries, there is in the aggregate an immense acreage of woodland, much of it in farms. Many of these woodlands are injured by fire, overgrazing, and overcutting. Large areas of broken, sloping land have been cleared and are subject to heavy run-off and erosion. Though less important than the mountain forests, they are by no means a negligible factor in erosion and run-off. No system of flood control that leaves out of account these vast and strategically situated forests can ever wholly succeed. Levees will always be the primary means of controlling the lower Mississippi. But green, vigorous, timber-producing forests must become the primary means of holding the headwaters in leash. The benefits of forest cover in controlling streamflow and reducing erosion have been abundantly demonstrated in many countries. |