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SUMMARY OF REGIONAL CONDITIONS

In the Northeast, in the region of the Great Lakes, and on the Columbia River, the silt burden of streams is on the whole negligible. There are no marked periods of extremely low water, disastrous floods are infrequent, and there is a low ratio of minimum to maximum flowage. The forest cover exercises little influence in determining the life of reservoirs.

The streams of the Middle Atlantic States, in spite of a wide margin between maximum and minimum discharge, have a much lower turbidity than those farther south. The drainage basins of many of them, however, are on sandy, shaley, or very permeable soils, especially at their headwaters, and for this reason erosion of soil proceeds at a comparatively slow rate even on denuded sites. Forests exercise a considerable protective function.

The streams of the Great Plains, and likewise the western Colorado River, flow through vast areas of unconsolidated soils on which, because of irregular and deficient rainfall, it is not possible in many places to maintain vegetative cover of sufficient density to obtain protection against erosion. Forest cover, however, exercises a high influence upon the headwaters of such of these streams as lie within the forested portions of the Rocky Mountains. The streams which have their headwaters in central Texas are in the same general class as those of the Great Plains. The rainfall, however, is more irregular, at times torrential, followed by long periods of scant precipitation. A consequence of this is extremely erratic flow of the rivers. The basins of these streams are largely covered with open parklike stands of trees, and there is a fairly good cover of grass, which in this region forms a more efficient protection than the scattered trees. A few areas of real forest occur, however, chiefly on the low mountains, and they should be carefully protected. On the whole, however, the protection afforded by forest cover is inconsequential.

It is in the Appalachian and adjacent Piedmont regions from Ohio and Maryland to northern Mississippi, and in the similar Ozark region of Missouri and Arkansas, in the mountains of middle and southern California, and on the slopes of the Rocky Mountains south of middle Colorado that forests seem to exercise their maximum influence in the protection of reservoirs. In these regions rainfall is irregular and often concentrated; soil where denuded erodes easily. contributing a high solid burden to streams; snowfall is light; and there is little lake storage. When forests are reestablished on such denuded lands, and reestablishment is readily effected, erosion of soil is largely checked.

Table 1 shows, for typical streams in different hydrographic regions of the United States, the comparative amount of turbidity. relation between maximum and minimum flow, and precipitation on the basin of the stream with maximum rainfall for each region.

TABLE 1.-Flow and turbidity of typical streams of different sections of the United States

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Connecticut, above Orford, N. H. (1904–1923): Maximum, March, 1913; minimum, September 28, 1908. Susquehanna, above Harrisburg, Pa. (1889-1920): Maximum, June 2, 1889; minimum, September 28-29,

1900.

Savannah, above Augusta, Ga. (1884-1891): Maximum, 1888; minimum, October 16, 1904.10
Colorado, above Austin, Tex. (1898-1923): Maximum, April, 1900; minimum, August, 1918.11
Kansas, above Lecompton, Kans. (1899-1906): Maximum, May 31, 1903; minimum, March 6, 1902.12
Although the conditions of the Kansas at Lecompton are so unfavorable, on the headwaters of the Mis-
souri River at Fort Benton, drainage area 24,600 square miles, the conditions are as follows: Maximum
flow, 2.2 second-feet; minimum flow, 0.09 second-foot; relation, 1 to 24.13

Sacramento, above Castella, Calif. (1910-1920): Maximum, January 2, 1914; minimum, August 5 to
September 21, 1920.14 This is the fork of the Sacramento River which heads in the mountains.
Columbia, above The Dalles, Oreg. (1879-1919): Maximum, 1894; minimum, January, 1890.15

IU. S. Geological Survey.

U. S. Weather Bureau.

This low minimum evidently the result of artificial storage on preceding day.

The rainfall given is that at Helen mine, the heaviest in the basin. The average rainfall for the basin is 23 inches.

The material comes largely from the Idaho plains, which are treeless.

For Grand, Green, and Gunnison above junction, since only scant figures are available for any one

stream.

1925.

June 22, 1916. This was chiefly from the Salt River, but no records on that stream.
Dry for one or more months each year.

NEWELL, F. H. RESULTS OF STREAM MEASUREMENTS. U. S. Geol. Survey Ann. Rpt. (1892-93) 14 (2): 148. 1894.

"GROVER, N. C., GRAY, G. A., and ELLSWORTH, C. E. SURFACE WATER SUPPLY OF THE UNITED STATES, WESTERN GULF OF MEXICO BASINS. U. S. Geol. Survey Water-Supply Paper 478: 29.

1918. PART VIII.

1922.

1921.

12 KANSAS WATER COMMISSION. SURFACE WATERS OF KANSAS, 1895-1919. 463 p., illus. Topeka. GROVER, N. C., LAMB, W. A., and FOLLANSBEE, R. SURFACE WATER SUPPLY OF THE UNITED STATES, 1918. PART VI. MISSOURI RIVER BASIN. U. S. Geol. Survey Water-Supply Paper 476: 17. 1921. GROVER, N. C., MCGLASHAN, H. D., and HENSHAW, F. F. SURFACE WATER SUPPLY OF THE UNITED STATES, 1919 AND 1920. PACIFIC SLOPE BASINS IN CALIFORNIA. U. S. Geol. Survey Water-Supply

PART XI.

Paper 511, 456 p., illus. 1923.

PARKER, G. L., and LEE, L. SUMMARY OF HYDROMETRIC DATA IN WASHINGTON, 1878-1919. U. S. Geol. Survey Water-Supply Paper 492, 363 p., illus. 1923.

If the average quantities of suspended matter in the water and the wide ratios between maximum and minimum flow are used as a guide, the streams of the Plains region and those of Texas and the Southwest are more urgently in need of protection than those of any other group. The very conditions, however, which determine the wide fluctuations between maximum and minimum flow-that is, the extremely irregular rainfall and the long periods of drought broken by short periods of most concentrated precipitation (practically 24 inches having fallen in a single day in Texas, which is three-fourths of the precipitation for an average year)-make it

difficult or even impossible to maintain a forest cover over those portions of the basins of these streams from which the turbidity is derived. In many places it is impossible to maintain a sufficient cover of shrubs and grasses to afford even a moderate protection against erosion of the soil.

It is significant that most of the turbidity of the streams of the Plains seems to come from below the mountains. As explained in the note to Table 1, the headwaters of the Missouri River show a most equable flowage as a mountain stream, the maximum flood flow being only 24 times the low-water flow. The regimen of the Missouri River as it emerges from the mountains is changed by the tributaries from the Plains. From these tributaries also comes the large burden of solid material which is borne by the Missouri in its lower reaches and which gives it the unenviable reputation of being one of our muddiest rivers, a stream which can well be compared to the Yellow River of China. The headwaters of streams which pass through the Plains but head within the mountains are materially benefited by protection; and all protective cover, whether of grass, brush, or forest, should be most carefully maintained.

At the other extreme from the rivers of the Plains, including those of Arizona and those of the southern portion of the Great Basin, are those of the Northeast, the Lakes region, including Wisconsin and Minnesota, and the extreme Northwest. The streams of these regions have a remarkably low turbidity and a low ratio of maximum to minimum flow. Reservoirs constructed for storage of storm water consequently have a long life. Not that protection is not desirable, even in the case of these streams, but of all our rivers, the streams of these sections most nearly approach the ideal in the eveness of their flow. The distribution of rainfall in these sections is such that there is little land which can not be protected by means of grass or forest cover.

The streams of the southern Appalachian and Piedmont region and those of California, seem to occupy an intermediate position. Normally they have moderate turbidity and have fairly even flow in comparison with the streams of the Plains and with those of Texas; but their flow is widely fluctuating when compared with the streams of the Northeast. The rainfall is periodically heavy, although sufficiently well distributed to maintain an ample forest cover for thorough protection. In the Appalachians more than 35 inches of rain have fallen within a month and in the mountains of California more than 71 inches within the same period. In both regions extremely concentrated precipitation frequently takes place, more than an inch sometimes falling within much less than an hour. The soils are such that, when they are denuded or exposed, erosion proceeds at a hight rate; but it is always possible to obtain thorough protection by forest or other cover, the forest being as a rule decidedly most efficient and preferable.

SUMMARY

On account of the wide fluctuations in the discharge of streams employed for the generation of power, especially hydroelectric de

velopments, reservoirs are being located on their headwaters for the purpose of storing storm waters and thus equalizing the flow of the streams and obtaining a more thorough utilization of power.

If it were ascertained that the pondage of a reservoir located on a head stream was in danger of being materially reduced through erosion of soil from denuded land, it would be justifiable from an investment point of view to expend in the preservation of the storage capacity or in the extension of its life a sum of money which would not exceed the present value of the prolonged life of this storage capacity.

The acquisition of the denuded land on which erosion is most excessive and the planting of it to trees, not only offer means of greatly reducing the erosion, but promise returns from the investment so made. If such denuded lands can be acquired at a sufficiently low price, the returns from the growth of the timber may be expected to pay for the entire cost, with interest, of the land and its afforestation. Under some conditions the cost of the land and its afforestation would be so high that the growth of the timber would probably not return a current rate of interest on the investment. In such cases it would be economically desirable for the additional cost of the land and afforestation to be charged against the prolonged utility value of the reservoir. There would then be justification for expending in this way such a sum as would equal the present value of the prolonged life of the reservoir.

One of the most important considerations in determining the location of reservoirs for such a purpose is the life of the reservoir and the rate at which the initial pondage is lost through siltage due to erosion of soil.

The streams in the different parts of the United States may be grouped in relation to the quantity and irregularity of their runoff, their turbidity, and the value of forest cover in protecting their headwaters, and thus influencing the quantity of their solid burden. In such a classification it appears that, because of the character of the soil, the rather small amount and even distribution of the precipitation, and the extensive storage of water in lakes and mountain snow fields, forests exercise a generally low influence in the Northeast, in the Lake regions and in the Northwest.

At the other extreme are the rivers of the Great Plains and Texas and the Colorado River. These streams flow through regions which, on account of the extreme irregularity and scantiness of the rainfall, are largely treeless or wooded only with scattered groves, and in consequence the influence of the forest cover is negligible.

There is an intermediate type of stream on whose watershed the forests do exercise a most important function in the protection of reservoirs. This type embraces the streams of the southern Appalachians, those of the southern Piedmont region, most of those that rise in the mountains of southern California, and the headwaters such of those as have their sources in the southern part of the Rocky Mountain system. They are within regions of irregular and usually heavy rainfall. Their headwaters at least are in forest regions. Wherever extensive areas have been denuded and exposed

to concentrated rainfall, most of these streams have during floods high turbidity. If there have been no clearings on the headwaters of streams within these regions, or if the surface has not been denuded or exposed by fires, erosion is slight, there is a small quantity of solid burden, and silting threatens the capacity of storage reservoirs only to a slight degree. If there is excessive erosion due to soil exposure, it is possible to reestablish forest conditions and by doing so reduce erosion and lessen sedimentation.

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