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be presented au tie appiup of the comprehensive program of flood control in which engineering, forestry, and agriculture should all have a share.
FORESTS AND FLOODS
The Mississippi disaster of 1927 has thrown into bold relief the need for national foresight in dealing with the perpetual menace of floods. Levees, reservoirs, and other structures for holding or diverting water always have been and always will be primary means of flood control; but indispensable as engineering works are to the harnessing of flood crests, they should be backed up by measures to reduce floods at their source, through the restoration of forests and grasslands as soil and water holders and the protection of farm lands from soil washing and gullying.
Excessive rainfall and sudden thaws will always cause floods in rivers unprotected by large natural reservoirs like the Great Lakes. The best of forests can not prevent the discharge of water into stream beds. It can slow down that discharge, however, and absorb a portion of it. Forest cover can not prevent the geological process of erosion. But it can prevent excessive man-made erosion that follows denudation or neglect of the soil.
The excessive silt load carried by so many of our rivers comes from the washing away of topsoils under unskillful methods of farming and grazing, from the gullying of worn-out or abandoned hillside farms, and from the erosion of forest soils after destructive logging or fire. Injury or destruction of the vegetative cover, whether grass or forest, and of its spongy leaf litter and vegetable mold (fig. 1), accelerate the run-off of rain and snow water. In large portions of the Mississippi Valley, once clothed with dense forests, a slow, inconspicuous, but immensely significant process has been going on for decades—the deterioration of the vegetative cover, the loss of leaf litter and humus, and the laying bare of mineral soil. From an increasing area of relatively barren and impervious soil, rain flows
nore swiftly and carries a greater load of silt that chokes river hannels and raises flood crests. Water runs off a tin roof faster han off a straw stack. A bucket half full of water will overflow if illed with mud. These illustrations, exaggerated as they are, picture what happens on denuded and eroded slopes and in silt-laden streams. The effect of the destruction of vegetation and humus seems insiglificant on this or that acre, this or that farm, this or that stretch of woodland. But when we sum up the aggregate volume of erosion and run-off in an immense catchment basin, like that of the Mississippi, and compare this volume with the size of the stream channel that must carry the load of water and soil, we gain a new conception of the importance of surface conditions. Floods in large rivers begin as little drops of water and little grains of sand on millions of acres. Every tree, every bush, every fallen leaf, every blade of grass, every
FIG. 1.-In a well-kept forest the thick carpet of needles, leaves, twigs, and decay
ing wood protects the soil from erosion and acts as a sponge that carries rain
contour furrow, every ounce of porous topsoil that impedes the progress of these drops of water and grains of sand toward the main channel relieves the lower river of some of its load.
The attack on floods should be a unified attack from the sources of the river to its mouth. There must be structures for controlling the water en masse, such as levees, reservoirs, and spillways; but the full measure of protection demands well-kept forests, grasslands, and fields to aid in controlling water before it is massed and con
centrated. Proper use of the land surface of a great watershed will 2
not only reduce the violence of floods but will protect the engineering works themselves from injury by heavy deposits of silt. It is even more important as a means of safeguarding life-giving soil from wastage and sterility. The best of all soil holders is the forest; it is the most absorptive form of vegetative cover; and it yields a crop adapted to the steeper and rougher lands where erosion and
run-off are most rapid and a protective cover of the soil is most needed.
Forests are simply one of the influences affecting the behavior of streams, an influence which may be greater or less in the composite of all the conditions which have to do with the volume and rate of run-off. The precise effect that a given area of forest within a large watershed has upon the discharge into the river can not be determined because so many other factors influence the behavior of streams. One of these factors is the presence or absence of natural reservoirs in the form of lakes and swamps. The St. Lawrence, for instance, is regarded as one of the best-regulated rivers in the world because of the storage system afforded by the Great Lakes. Other factors are the steepness of the slopes, the porosity of the rock and soil, and the nature of the climate. The severity of the Mississippi flood in 1927, for example, was due to a climatic factor, heavy and long-continued rains in the lower portions of the valley. It would be fruitless and shortsighted to exaggerate the effect of forests upon river systems. They are simply one of the beneficial influences that can be brought into play.
The prevention of staggering losses of life and property from flood crests is the primary purpose of river control. But more than that is at stake. Holding soil in place and preserving its fertility, protecting bottom lands back on the small tributaries, and maintaining more regular flow in the feeder streams for the many resulting economic benefits are all objects that should be sought. For some of these protective purposes, forests and other forms of vegetative cover are the only possible means, and through their aggregate effect on all the feeder streams they have an ameliorating influence on the large floods in main rivers. Along with this protecting and ameliorating influence on water flow, forests more than pay their own way by producing marketable crops on land not fit for fields or pastures.
The following discussion of the influence of forests on run-off and erosion is drawn from experience and observations at many widely scattered points, and is designed to sketch a general picture of the relation of forests and other forms of surface protection to run-off and erosion in helping to provide flood protection."
FOREST, SOIL, AND WATER In building up a true conception of the influence of forests on a great river system one can best begin by drawing a simple picture of running water at work on a limited plot of ground, then enlarg. ing this picture to cover minor valleys, and finally multiplying it a thousand or a million fold to attempt some measure of the immense cumulative effect of the destructive forces that are unleashed by abuse of the soil and its natural cover.
1 The following publications have been drawn upon for examples and data, and the reader is referred to them for more detailed discussions of forests, run-off, and erosion : ASHE, W. W. FINANCIAL LIMITATION IN THE EMPLOYMENT OF FOREST COVER
IN PROTI (TING RESERVOIRS. U. S. Dept, Agr. Bul. 1430. 3) p. illus. 1926.
MINNS, E. N. EROSION AND FLOOD PROBLEMS IN CALIFORNIA. Calif. State Bd. of For. Rpt. to legislature of 1921 on Senate Concurrent Res. No. 27.
ZO, RAPHAEL. FORESIS ANI) WATER IN TID LIGHT SCIENTIFIC INVESTIGATION, Final Rpt. Nat. Waterways (om., App. V. Sen. Doc. 469, 62d Cong., 2d sess.
1912, A reprint of tbis report may be obtained, as long as the supply lasts, by applying to the Forest Service, Washington, D. (.
THE FOREST AS A SOIL HOLDER
In watering a lawn one can see how the water trickles along the ground, collides with grass blades, splits up into tiny rivulets, spreads out slowly, and remains clear until it sinks into the soil. The same water falling on bare soil (unless it is absolutely level) gathers headway, collects rapidly in little rills and streams, and with its growing speed and concentration begins to tear loose the finer particles of soil, then sand, and, finally, gravel. Where its speed is checked, the turbid rivulet deposits its load; if it still gathers speed and volume it begins to cut a gully, its load of sand and gravel acting as an abrasive to cut faster and deeper.
This example shows in miniature how vegetation protects the soil and how the absence of vegetation permits soil to be easily carried away.? Vegetation hinders the flow of water, and by spreading it out takes away its force. Thus checked, it can not dislodge the soil particles, which, moreover, are bound together by the tangle of roots. Without vegetation there is nothing but the friction of the soil to keep water from following its inherent habit of going downhill as fast as it can. The cutting power of running water and its ability to transport soil increase greatly with increased speed. After the vegetative cover is removed, rain and sun compact the soil, each succeeding rain makes new gullies or deepens old ones, and the destructive combination of speed and concentration gains headway.
A well-kept forest is the best of natural soil holders. It has a dense ground cover of trees, shrubs, and lesser plants that impedes the flow of water. Tree roots spread far and wide to a depth of 2 feet or more, usually much deeper than the roots of grass or farm crops; and the roots of the lesser plants help to bind the topsoil together. On top of the ground is a mat of leaves or needles, twigs, branches, and dead grass and herbs, varying from a few inches to a foot or more in depth; and beneath this mat is the spongy woods mold or humus derived from the decay of generations of leaf fall. (Fig. 2.)
The canopy of leaves and branches breaks the force of rain and evaporates considerable quantities of it. The rain that penetrates the canopy has next to contend with the leaf litter, which retards the water in its flow over the surface, soaks up part of it, and passes part of it down to the humus and thence into the mineral soil. Also in their life processes, trees absorb large quantities of water from the ground and pass it off as vapor from their leaves. By these means the forest intercepts and evaporates a part of the rainfall, absorbs another part, and slows down the surface flow of the rest, retarding its speed and erosive action on the soil. Even in heavy rains the soil under a dense well-kept forest is little disturbed, and the streams that derive their water from such a forest are likely to remain clear.
*A test made by the Missouri Agricultural Experiment Station showed that during six years erosion had removed soil at the rate of 208 tons per acre front bare uncultivated land, as compared with less than 2 tons per acre from similar land covered with bluegrass sod; in other words, one hundred and twenty-two times as much. See DULEY, F, L., and MILLER, M. F. EROSION AND SURFACE RUX-OFF UNDER DIFFERENT SOIL CONDITIONS.
Mo. Agr. Expt. Sta. Res. Bul. 63. 1923.
THE FOREST AS A WATER HOLDER
The things that make the forest a good soil holder also make it a good water holder. During a rain the flow of surface water is retarded. More of it has a chance to sink down into the absorptive leaf litter, thence into the decayed leaf mold, and so down into the mineral soil and the fissures of the rocks. There it joins the great reservoir of underground water that feeds springs, brooks, and rivers. The deep, tangled root system of a well-kept forest also greatly aids this process of seepage, for water flows along living roots or through the tubes formed by decayed roots, far down into the soil.
Under ordinary conditions forest soils absorb from 20 to 50 per cent of light or moderate rains. Even in a prolonged rain the leaf mulch keeps feeding the water underground and thus empties and
F-48606 FIG. 2.-A forest is the best vegetative cover for holding soil and water. The
canopy of leaves and branches catches and evaporates a great deal of rain. What drips to the ground is impeded by the dense cover of shrubs, ferns, fallen leaves, and decaying wood, which absorbs much water underground, slows down the run-off, and keeps the soil from being washed into the streams
refills itself. In a violent or long-continued rain so much water may fall as to overtax this absorptive capacity, but even so, the forest has already reduced the run-off and continues to impede the remaining surface water in its progress to the stream beds. Such torrents as rush from bare, gullied land are rare in a forest, where during even a heavy rain one always has a sense of the dripping of water rather than of a rapid rush.
U'nderground storage of water by forests is an important natural means of regulating and equalizing stream flow. The immense quantity of water carried into the earth by the absorbent ground cover of a forest may percolate for months before it emerges.
Humus and leaf litter, while thus acting directly as an absorbent, also keep the soil itself mellow, porous, and more permeable. Forest