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EROSION CHECKED BY WOODS

FIG. 1.-Deep gully extending up to the edge of yellow pine forest. The forest checked erosion. San Bernardino County, Calif.

ROOTS OF TREES BINDING THE SOIL

FIG. 2.-Roots and rootlets of forest trees in the soil, showing how their network binds the soil and protects it from erosion. When a tree dies the soil and subsoil become filled with hundreds of small aqueducts where the roots were. This makes the earth permeable to a great depth

ests have generalized too much from particular cases, those who rely upon mechanical means of stream control have likewise drawn their conclusions from inadequate data and from a too restricted field. At any rate it will be shown that forests, under certain conditions, have a most significant influence in determining the permanence of the development of a stream-development which enables storm water to be stored for power purposes, which to a limited extent equalizes stream flow for navigation, and which under some conditions may be of material benefit in the control of floods."

Even if it were conceded, as is claimed by Mead, that forests have little influence upon stream flow and the development of water resources in northern Wisconsin, and in regions of similar sandy or glaciated soils, the situation would not be the same in other regions with different soil and climatic conditions. The protective influence of the forest is of great importance in most regions subject to excessive erosion when the naked soil is exposed. In the sandy soils of the Coastal Plain region of the Southern States, where the conditions are very similar to those in the Great Lakes Region, forest influences are prevailingly at a minimum and are often negligible, but these influences increase in other regions, apparently reaching their maxima, as will be shown further on, in the Sierras of California, in the southern Appalachians, and in the southern Piedmont.

The discussion in this bulletin will be restricted to erosion, the sections of the United States where it is most prevalent, the conditions. of rainfall, surface and soil which cause excessive erosion, and the means of reducing it for the protection of water-power resources.

SOURCES OF SOLID BURDEN OF STREAMS

The solid burden carried by streams is derived from several sources, summarized as follows:

Farming lands, especially such as are situated on steep slopes.

Woodland and brush land in which the soil cover of litter does not afford an adequate protection.

Grasslands, especially thinly stocked prairies and the plains of the Western and Southwestern States where the sod, or sod and brush, is not sufficiently dense to protect the soil and prevent erosion.

Earth roads subject to degradation, ditches along roads, and unprotected cuts and fills along railroads and highways.

Washings of ore and clay, mine water, etc., which in place of being retained in settling basins are disposed of in waterways.

Corrasion of banks of streams.

which he supports by figures of run-off, that forests may have a limited influence, at the headwaters of the Mississippi River, in obstructing the run-off, and that brush land efficaciously performs the same function. POTTER, C. L. SOME SUGGESTED WAYS OF CONTROLLING THE MISSISSIPPI FLOODS. Engin. News-Rec., 94: 558. 1925. The repository of arguments for and against the benefits of forests to stream flow is a paper by H. M. Chittenden, with its accompanying discussoin. CHITTENDEN, H. M. FORESTS AND RESERVOIRS IN THEIR RELATION TO STREAM FLOW, WITH PARTICULAR REFERENCE TO NAVIGABLE RIVERS. Amer. Soc. Civ. Engin. Trans., 62: 245-318, illus. 1909. (Discussion, p. 319546) Unfortunately generalizations control most of the discussions. It is seldom that all the essential facts are given or that proper weight is given to different factors, and this is particularly the case in those portions of the discussion which refer to erosion of soil and corrasion of stream banks.

See LEIGHTON, M. O. RELATION OF WATER CONSERVATION TO FLOOD PREVENTION AND NAVIGATION IN OHIO RIVER. In Preliminary Report of the [U. S.] Inland Waterways Commission, pp. 451-490. 1908. (U. S. Congress, 60th, 1st sess., Senate Doc. 325.) See also [PITTSBURGH, FLOOD COMMISSION.] REPORT OF FLOOD COMMISSION OF PITTSBURGH, PENNA. 252 pp., illus. [Pittsburgh. 1912.]

MEAD. D. W.

THE FLOW OF STREAMS AND THE FACTORS THAT MODIFY IT, WITH SPECIAL REFERENCE TO WISCONSIN CONDITIONS. Wis. Univ. Bul. 425: 62. 1911. (Engin. Ser.,

v. 6, no. 5.)

ASHE, W. W. FORESTS AND THE COST OF TEXTILE PRODUCTION. [1908.]

11 pp. [n. p.]

99224-26-2

The factors which determine the erosion of naked soils are the character of the soil and subsoil, the gradient of the surface, and the amount and character of the rainfall or other precipitation. A soil which is thoroughly granulated, even if situated on a moderate slope, is not subject to erosion. Such soils are fine gravels and coarse sands and soils of finer texture which are granulated on the surface and have thorough subsoil drainage. Soils of finer texture having a deficiency in binding material, such as river silts, loess, or other silty soils, and in the East the well-decomposed micaceous schists, are subject to excessive erosion.

The character of the rainfall is also of importance. Slowly falling rain of large total volume may, through the time allowed for percolation into the subsoil, be largely absorbed, leaving only a small amount to find its way over the surface of the soil directly into drainage channels. A much smaller proportion of the same amount of precipitation would, if concentrated, be absorbed within the same time. The greater the amount which is not absorbed, the greater is the run-off over the surface, with accompanying erosion.

According to Kennedy,10 the quantity of silt which can be carried in suspension by a stream varies according to the square root of the fifth power of its velocity-that is, the possible silt burden increases at a much higher rate than the velocity. In case the velocity of a stream is 2 miles an hour and it is increased during flood period to 4 miles an hour, the quantity of silt which it can carry increases six times. Consequently the capacity of a stream for carrying silt increases enormously during flood periods. Corrasion of banks or channel continues to take place until the maximum silt burden for the velocity is reached. When there is a slacking in velocity for any reason, as when there are pools, or when the stream spreads out over a flood plain, a portion of the silt is deposited. But before a particle is in suspension it must be freed from associated particles either upon the surface of the land, upon the stream bank, or at the bottom of the channel. As a rule a particle must become a part of a bed load before it passes into suspension. So erosion as a rule is determined initially by the capacity of a stream for bed load. The weight of bodies such as sand, gravel, or stone, which can be rolled along the bed of a stream, varies with the velocity of the current,1

11

10 KENNEDY, R. G. THE PREVENTION OF SILTING IN IRRIGATION CANALS. Inst. Civ. Eng. [England], Minutes Proc. (1894/95), 119: 281-290, illus. 1895. Kennedy's formula was based upon studies of canals. Studies upon Texas streams made by R. G. Hemphill, associate irrigation engineer of the Department of Agriculture, seem to show approximately the same percentage of silt, 1.3 by weight, being carried on the Brazos River at Waco by velocities which ranged from 0.69 up to 7.50 per second, and from the surface nearly to the bottom. The fineness and character of the silt is a factor. His investigations do not seem to bear out for Texas conditions Kennedy's statement that corrasion continues to take place until the maximum silt burden for the velocity is secured." Undoubtedly considerable additional study of this problem is necessary. 11 MERRIMAN, M., and MERRIMAN, T. TREATISE ON HYDRAULICS. Ed. 10, rev., pp. 294. 339. New York. 1916. According to Merriman the weight of bodies which can be rolled along the bed of a stream varies as the sixth power of the velocity of the current.

GILBERT, G. K. THE TRANSPORTATION OF DEBRIS BY RUNNING WATER. U. S. Geol. Survey Prof. Paper 86: 10-11. 1914. Gilbert has given the subject thorough study in the laboratory, but states that the results of his investigations are to be regarded only as a contribution to the subject. In discussing the laws which control the movement of the bed load he makes the following general statement: The capacity of the stream to transport débris along its bed is affected by the width of the stream, a broad channel carrying more than a narrow one, by the velocity of the current, the quantity varying greatly for small changes in the velocity along the bed. Bed velocity is affected by slope and depth, increasing with each factor, and depth is affected by discharge and also by slope. The size of the transported particles is a factor of great importance, a greater weight of fine débris being carried than of coarse. But capacity of a stream to transport is less sensi

and for each combination of width, slope, and discharge there is a limiting coarseness of débris above which no transportation takes place. The rate of flow increases with the volume or the depth of the water in the channel. These laws of flow and of the carrying capacity of water at different velocities of current explain the great amount of erosion of soil which takes place in regions of heavy or concentrated rainfall, such as the mountairs of California, the Southwestern States, and the southern Appalachian and adjacent Piedmont regions, regions not only of concentrated rainfall, but of broken surface where it is possible for surface run-off to gather into rapid torrents having great volume and enormous cutting and carrying capacity.

The more thorough the cultivation of farming lands, the deeper the plowing, the more humus incorporated, and the better the granulation and the absorptive capacity of the soil, the lower will be the proportion of rainfall which runs off and the less will be the extent of the erosion. This is a matter of prime importance to the farmer as well as in connection with reducing the turbidity of streams, since, in addition to the physical destruction of land through the formation of gullies by erosion, the muddy effluent bears away the lighter and more fertile parts of the soil, resulting in its constant depletion. At the same time the loss of water which should soak in and be stored in the subsoil for future dry-season needs is often equally serious. Although agricultural lands contribute but a relatively inconsequential part of the total solid burden of most of our great rivers, except in the Southeast, on the whole the wastings from the agricultural lands represent the element of greatest fertility, the humus, the organic components, containing particularly the nitrogen or ammonia compounds. It has been computed 12 that an average of more than 850 pounds of soil are yearly washed from every acre of land on the Yadkin River above Salisbury, N. C. Of this more than 125 pounds are organic matter, the balance being mineral soil. The organic matter is humus, chiefly from farming soils, and where this is the case it must be replaced.

In woodland, erosion is due chiefly to the exposure of the soil through the destruction of the humus and leaf litter by fire or other agency. A forest soil as a rule has excellent subsoil drainage due to root penetration, and the top soil is usually well granulated. Moreover, there is a prevailingly good binder of rootlets. (Pl. 4, figs. 1 and 2.) For these reasons, even when the leaf litter is annually destroyed, erosion is seldom so disastrous as it is in the case of a soil used for farming, or which has been farmed and abandoned. In open parklike woodland and brush land, such as exist in sections where the rainfall is not sufficient to support a dense stand of trees, as is particularly exemplified on the lower mountains of the West and Southwest, the litter and leaves which accumulate are not sufficient to cover the soil and afford it adequate protection. Under

tive to changes in fineness of débris than to changes in discharge or slope. If slope remains the same velocity changing with discharge, capacity for transportation varies with the 3.2 power of the velocity. If discharge remains the same, in which case velocity changes with slope, capacity varies with the 4.0 power of the velocity. If depth remains the same, the velocity changing with simultaneous changes of slope and discharge, capacity varies with the 3.7 power of the velocity.

12 ASHE, W. W. TERRACING OF FARM LANDS. N. C. Geol. and Econ. Survey Bul. 17: 22. 1908.

such conditions and with a concentrated rainfall, or with the rapid melting of snowfall, erosion is active. Since it is not possible to maintain in such sections forest or brush growth of sufficient density to produce adequate litter, it is not possible to reduce erosion of soil below a certain amount, the amount which is fixed by the heaviest litter which can be maintained. (Pl. 5, figs. 1 and 2.)

On the western plains there are extensive areas of unconsolidated soils protected only by a scant stand of grass, with scattered bushes intermixed. Notwithstanding that these lands are nearly level, erosion from them is often excessive. Where the grass cover has deteriorated through excessive grazing, a regulation of range practice may have the double beneficial influence of checking further range depletion, and at the same time, by inducing the more thorough covering of the soil, of lessening erosion. There are undoubtedly extensive areas in Texas and Oklahoma where this condition prevails.

Ditches along roads and cuts and fills along highways and railroads, especially in the Southeastern States where the rainfall is heavy and where grass does not readily set, contribute a large quantity of solid matter to the streams. By the use of Bermuda and carpet grasses in ditches, and these grasses and Japanese honeysuckle on embankments and slopes, much of the erosion from these sources could be prevented or at least greatly reduced. The rapidity with which erosion takes place in roads in the Piedmont and mountain sections of the Southeastern States is shown by the hundreds of miles of hollow roads in this section. A good example of road degradation through erosion is shown in Plate 6, Figure 2, and the deeply washed ditches on either side show the process by which it takes place. The brush in the ditch is the futile means employed to check it.

Friction between the current and the bank is a factor in the corrasion of banks of streams. Corrasion continually takes place, especially during periods of high water, whether resulting from rainfall or from melting snow. It is most active, however, in the streams flowing through the regions of the Great Plains. This results in shiftings of stream channels, which is particularly characteristic of the Missouri, the Arkansas, the Kansas, the Canadian, the Red River, and many of their tributaries. Corrasion is also not uncommon along many eastern streams. It is aggravated in instances by the clearing away from stream banks not only of all trees but of shrubs as well. Upon the decay of their roots the natural soil binder is destroyed. (Pl. 4, fig. 2.) The small roots and rootlets of willows, birches, alders, cane, and similar trees and shrubs grow to the very water's edge. They are often exposed along the banks of the stream and form a loose basketry often quite as effective in bank protection as ripraps.

In the aggregate the solid burden which is added to streams through the washings of ore, clay, etc., is not large in comparison with the turbidity which comes from other sources. Locally, it is of importance. This contamination of streams is sometimes permitted as a result of a sentiment opposed to interfering with the conduct of local industries. Waste of this kind which is deposited in streams must frequently be removed, often at public expense, from navigable channels or from reservoirs within which it has been deposited. In some States, as in California, there are restrictions against such pollution.