Chapter III: The Work of Running Water (2)

The cycle begins with the beginning of valley development, and at that stage drainage is in its _infancy_. The type of the infant valley is the gully or ravine (Figs. 67 and 68). It has steep slopes and a narrow bottom. Fig. 1 of Plate IV represents similar, or rather older, ravines in contour (shore of Lake Michigan, just north of Chicago). With age, the valley widens, lengthens, and deepens, and passes from infancy to _youth_. In this stage also the valleys are relatively narrow, and the divides between them broad. They may be deep or shallow, according to the height of the land in which they are cut, and the fall of the water flowing through them; but in any case the streams flowing through them have done but a small part of the work they are to do before the country they drain is base-leveled. Figs. 69 and 70, respectively, represent youthful valleys in regions of moderate and great relief. Fig. 2, Plate IV, shows a youthful valley in a region of slight relief (near Casselton, N. D., lat. 46° 40′, long. 97° 25′). The uppermost line in Fig. 64 likewise represents topographic youth, as shown in cross-profile.

Not only are narrow valleys said to be young, but the territory affected by them is said to be in its topographic youth, since but a small part of the time necessary to reduce it to base-level has elapsed. An area is in its topographic youth when considerable portions of it are still unaffected by valleys. Thus the areas (as a whole), as well as the valleys, represented on Plate IV, are in their topographic youth. It is often convenient to recognize various sub-stages, such as early, middle, and late, within the youthful stage of valleys or topographies. The different parts of the areas shown on Plate IV, for example, represent different stages of youth.

Youthful streams, as well as youthful topographies, have their distinctive characteristics. They are usually swift; their cutting is mainly at the bottom rather than at the sides, and their courses are often marked by rapids and falls.

As valleys approach base-level they develop flats. As the valleys and their flats widen, and as their tributaries increase in numbers and size, a stage of erosion is presently reached where but little of the original upland surface remains. The country is largely reduced to slopes. In this condition the drainage and the topography which it has determined are said to be _mature_. Mature topography is shown in contours in the figures of Plate V, and in the northern part of Plate VI, where slopes, rather than upland or valley flats, predominate. Fig. 1 of Plate V represents an area in southeastern Kentucky (lat. 37° 12′, long. 83° 10′); Fig. 2, an area in western Virginia. Plate VI represents an area in southern California, somewhat west of San Bernardino. The three areas are alike in representing mature drainage, though not of equal stages of advancement. The striking differences of topography of the three areas are the result of differences in rock structure and altitude, and will be considered later. Mature topography is also shown in Fig. 71, where the relief is moderate, and in Figs. 72 and 73, where it is great. Figs. 72 and 73 illustrate clearly the universal tendency of rivers in regions of notable relief to develop new flats well below the old surface of the region. At the same time that these low-lying flats are developing, tributary drainage is dissecting and roughening the upper surfaces. This process is well shown in Fig. 73. In both Figs. 72 and 73 the summits of the mountains on either side of the valleys appear to have had about the same elevation. The new flat is therefore developed at the expense of the old flat. As will be seen in the sequel, the first flat which a stream develops along its course is usually somewhat above base-level. It is a _graded_ flat.

Fig. 1. KENTUCKY.
U. S. Geol. Surv.

Fig. 2. VIRGINIA.
U. S. Geol. Surv.]

PARTS OF LOS ANGELES AND SAN BERNARDINO COUNTIES, CALIFORNIA.
U. S. Geol. Surv.]

The same processes which have made young valleys mature will in time work further changes. When the gradients of the valleys have become low and their bottoms wide, and when the intervening ridges and hills have become narrow and small, the drainage and the drainage topography have reached _old age_, and the streams are in a condition of senility. This is illustrated by Fig. 1, Plate VII (central Kansas), and in section by the third and lower lines in Fig. 64. Topographic old age sometimes has a different expression; this is shown in Fig. 74, where most of the surface has been brought low. The elevations which rise above the general plain are small in area, but have abrupt slopes. This phase of old-age topography is usually the result of the unequal resistance of the rock degraded. The effects of unequal rock-resistance will be considered later.

The marks of old streams are as characteristic as those of young ones. They have low gradients and are sluggish. Instead of lowering their channels steadily they cut them down in flood, and fill them up when their currents are not swollen. They meander widely in their flat-bottomed valleys (Fig. 1, Pl. VII, Central Kansas) and their erosion, except in time of flood, is largely lateral.

If the processes of degradation were to continue until the land surface was brought to sea-level, and this might be done by solution though not by mechanical erosion of running water, the rivers would no longer flow, and the drainage system would have reached the end of its history—_death_.

Not only do valleys normally pass from birth to youth, from youth to maturity, and from maturity to old age, but a single river system may show these various stages of development in its various parts. Thus in the area shown in Fig. 2, Plate VII (north central Kansas), there is a tract (extreme southwest) where the erosion history is scarcely begun. The zone of land a little farther northeast, and just reached by the heads of the valleys (same figure), is in its youth. The well-drained and uneven tract southwest of the flat of the Solomon River is in maturity, while the flat of the main valley has the general characteristics of old age.

The age of valleys in terms of erosion is also expressed more or less perfectly by their cross-sections. The line 1–1 (and 1′-1′) of Fig. 64 represents in cross-section a narrow V-shaped valley. Such a section is always indicative of youth. The stream which developed it cut chiefly at its bottom, not at its sides. It was therefore rapid, and rapid streams are young. The line 2–2, (2′-2′) (Fig. 64) shows the same valley at a later and maturer stage when downward cutting has nearly ceased. The widening of the valley by slope wash has become relatively more important than before, and the stream has so far lost velocity as the result of diminished gradient as to be unable to carry away all the detritus washed down from the sides. As a result of deposition at the bases of the side slopes, a concave curve has been developed. Up the valley from the point where such a section as is represented by 2–2 occurs, the valley may still have a section similar to that represented by 1–1.

Fig. 1. KANSAS.
U. S. Geol. Surv.

Fig. 2. KANSAS.
U. S. Geol. Surv.]

ABOUT 15 MILES SOUTHWEST OF ST. LOUIS, MISSOURI.
U. S. Geol. Surv.]

Still later stages of development are represented by the cross-sections 3–3 and 4–4. Not only has the valley become larger, but the stream has deposited detritus (not shown in the figure) in the bottom of its valley, developing an alluvial flat. On this flat the stream meanders, and the valley may be widened by the undercutting of the bluffs wherever the stream in its wanderings reaches them (Pl. VIII, near St. Louis). A valley might possess the characteristics shown by the cross-sections 3–3, 2–2, and 1–1, Fig. 64, in its lower, middle, and upper courses, respectively.

The preceding discussion, and the illustrations which accompany it, give some idea of the topography which characterizes an area in various stages of its erosion history. Whether the valleys are deep or shallow, and the intervening ridges high or low, depends on the original height of the land and its distance from the sea. The higher the land, and the nearer it is to the sea, the greater the relief developed by erosion. A plateau near the sea may become mountainous in the mature stage of its erosion history, while a plain in the same situation would only become hilly. A plateau in the heart of a continent would have less relief in its maturity than one of equal elevation near the sea, since the grade-plain in the former position is higher than in the latter. Plates IV and IX show youthful topography where the relief is relatively slight, and Plate X shows youthful topography where the relief is great. Similarly, Plates V and VI show mature topography where the relief is great, and Fig. 1, Plate III, shows mature topography where the relief is relatively slight.

Topographic youth, topographic maturity, and topographic old age are also indicated in other ways, and especially by the presence of features which rivers tend to destroy. If, for example, the surface of the land, well above the valley bottoms, is marked by numerous ponds and marshes, it is clear that drainage has not yet progressed beyond its early stages, for, unless the lakes be very deep, valleys working back into the land will find and drain them before topographic maturity has been reached. Their presence is evidence that the region where they occur has not yet been thoroughly dissected by erosion lines, and therefore has not reached maturity. Still other marks of topographic youth, such as rapids, falls, etc., as well as marks of topographic maturity and old age, will be mentioned in the following pages.

GENERAL CHARACTERISTICS OF TOPOGRAPHIES DEVELOPED BY RIVER EROSION.

With the characteristics of river valleys and the methods by which they grow clearly in mind it is easy to say whether rivers have been the chief agents in the development of a given topography. River valleys are distinguished from other depressions on land surfaces by their linear form and, leaving out of consideration the relatively insignificant inequalities in a stream’s channel, by the fact that any point in the bottom of a river valley is lower than any other point farther up the stream in the same valley, and higher than any point farther down the stream. The second point might be otherwise stated by saying that every valley excavated by erosion leads to a lower valley, or to the sea, or an inland basin. Streams which dry up, or otherwise disappear as they flow, constitute partial exceptions. If, therefore, the depressions on a land surface are linear, lead to other and deeper valleys, and finally to an inland basin, or the sea, and if the elevations between these valleys are such as might have been left by the excavation of the valleys, it is generally clear that rain and rivers have been the chief factors in the development of the topography. If, on the other hand, a surface is characterized by topographic features which streams cannot develop, such as enclosed depressions, or hills and ridges whose arrangement is independent of drainage lines, other agents besides rain and surface streams have been concerned in its development.

SPECIAL FEATURES RESULTING FROM SPECIAL CONDITIONS OF EROSION.

Many striking topographic and scenic features result from rain and river erosion. Some of them depend primarily on the conditions of erosion, such as climate, altitude, etc., while others depend largely on the structure and resistance of the rock. Between these two classes there is no sharp line of demarkation. Illustrations of two types, dependent largely but by no means wholly on conditions independent of the rock, are cited at this point. Others will be mentioned in other connections.

=Bad-land topography.=—To a type of topography developed in early maturity in certain high regions where the rock is but slightly, though unequally, resistant, a special name is sometimes given. Such regions are termed _bad lands_. Some idea of bad-land topography is gained from Figs. 75 to 78. Bad-land topography is found in various localities in the West, but especially in western Nebraska and Wyoming, and the western parts of the Dakotas. The formations here are often beds of sandstone or shale, alternating with unindurated beds of clay. Climatic factors are also concerned in the development of bad-land topography. A semi-arid climate, where the precipitation is much concentrated, seems to be most favorable for its development. The bad-land topography is most striking in early maturity.

=Special forms of valleys; canyons.=—Various conditions influence the size and shape of valleys, especially in the early stage of their development. If the altitude of the land be great, the gradient of the streams at this stage will be high. A high gradient means a swift stream, and a swift stream erodes chiefly at its bottom. High altitudes therefore favor the development of deep valleys. Such valleys will be narrow if the conditions which determine widening are absent or unfavorable. Since slope wash is one of the main factors in the widening of valleys, an arid climate favors the development of narrow valleys, if there be sufficient water to maintain a vigorous stream. Narrowness and steepness of slopes will also be favored if the valley is cut in rock which is capable of standing with steep faces. Thus a stream may develop a narrow valley in indurated rock where it would not do so in loose gravel, and, other things being equal, it will develop a narrower valley in rock which is horizontally bedded than in rock the beds of which are inclined. Aridity, high altitude, and the proper sort of rock structure therefore favor the development of canyons, and many of the young valleys in the western part of the United States where these conditions prevail, belong to this class.

While all canyons are valleys, most valleys are not canyons. The distinction between a canyon and a valley which is not a canyon is not sharp. The canyon depends for its distinctive character on the relation of depth, width, and angle of slope to one another; but any definition of the depth, width, and angle of slope necessary to constitute a valley a canyon is arbitrary.[31] In popular usage the rule seems to be that if a valley is sufficiently deep, narrow, and steep-sided to be distinctly striking, it is called a canyon in regions where that term is in use. Whether a valley is deep, narrow, and steep-sided enough to be striking clearly depends on the observer. The Colorado Canyon (Figs. 79 and 80) is the greatest canyon known, but it is rarely more than a mile deep, and where its depth approaches this figure it is often eight, ten, or even twelve miles wide from rim to rim. Its width at bottom is little more than the width of the stream; that is, a few hundred feet. Its cross-profile throughout much of its course is therefore not in keeping with the conventional idea of a canyon. With a depth of one mile and a width of eight, the slope, if uniform, would have an angle of less than 15°. Such a valley is represented in Fig. 81. As a matter of fact the slopes of a canyon are not commonly uniform. The slopes represented in Fig. 82 correspond more nearly than those of Fig. 81, to the actual slopes of the Colorado Canyon. The inequalities of slope are occasioned by the inequalities of hardness. It is perhaps needless to say that to an observer on the rim of the canyon the slopes seem several times as steep as those shown in the diagrams.

Like all valleys which are narrow relative to their depth, the Colorado Canyon, great as it is, is a young valley; for it represents but a small part of the work which the stream must do to bring its drainage basin to base-level.

While aridity and altitude are conditions which favor the development of canyons, as shown by the fact that most canyons are high and dry regions, they are not indispensable. Niagara River has a canyon below its falls (Pl. IX), and the surrounding region is neither high nor arid. The narrow part of the valley has been developed by the recession of the falls, and is so young that side erosion has not yet widened the valley or lowered its angle of slope to such an extent as to destroy its canyon character. This canyon is often called a _gorge_, a term frequently applied to small valleys of the canyon type.

Plate X shows portions of the canyons of the Yellowstone and the Colorado rivers respectively. In the first the contour interval is 100 feet, and in the second, 250 feet. The horizontal scale is ¹⁄₁₂₅₀₀₀ (about 2 miles to the inch) in the first, and ¹⁄₂₅₀₀₀₀ in the second. These scales should be borne in mind in interpreting the map.

Falls, rapids, narrows, and other peculiar features, due primarily to inequalities in the hardness of the rock affected by erosion, will be considered later.

THE STRUGGLE FOR EXISTENCE AMONG VALLEYS AND STREAMS.

It is not to be inferred that every gully becomes a valley, nor that every small valley becomes a large one. Among valleys, as among living things, there is a struggle for existence, and fitness determines growth and survival. At an early stage of its erosion history the number of small valleys in a given area is often great, while at a later stage the number is less and the size of the survivors greater.

NIAGARA FALLS.
U. S. Geol. Surv.]

Fig. 1. YELLOWSTONE PARK.
U. S. Geol. Surv.

Fig. 2. ARIZONA.
U. S. Geol. Surv.]

One phase of the struggle for existence is often well illustrated on a freshly exposed slope of clay. The number of miniature gullies which develop on such a slope, even in a single shower, may be very large (Fig. 84); but the history of many of them is ephemeral. If two adjacent ones are of unequal depth the widening of the deeper narrows and finally eliminates the divide between them, and the two become one (Fig. 85).

Another phase of the struggle for existence is shown in other situations. Examination of a good map of the north shore of Lake Superior or the west shore of Lake Michigan shows a large number of small streams and gullies (Fig. 1, Pl. IV). The valleys are short and narrow, and between and beyond them are considerable areas untouched by erosion. The drainage near the lake is therefore young, and each of the small valleys is growing. This condition of things is perhaps typical of that which has been, is, or will be along the average coast at a certain stage in its erosion history. No equal stretch of coast-line where erosion is far advanced can boast of a number of large rivers comparable to that of the many small ones along the coasts mentioned. It therefore seems evident that of these many small streams a few only will attain considerable size.

Some of the methods by which the growth of the many is arrested are easily understood. Some of the young valleys on a given coast will work their heads back into the land faster than others because of inequalities of slope and material. This will be true of the tributaries no less than of their mains. If valleys develop in ways other than by head erosion (see p. 73) the chances are also against their equality of growth. If two streams, such as _a_ and _c_, Fig. 86, develop faster than the intermediate stream _b_, it is clear that their tributaries may work back into the territory which at the outset drained into _b_, so as to cut off the supply of water from the latter stream (compare _a′b′c′_, Fig. 87). As a result, the growth of _b_ will be checked, and ultimately stopped. Similarly other valleys, such as _f_, will get the better of their neighbors, and many of the competitors, as _b_, _d_, _e_, and _g_ will soon drop out of the race. Between the stronger streams competition still goes on. If _a′_ and _f′_ develop faster than _c′_ its prospective drainage territory will be preëmpted by its rivals (compare Figs. 87 and 88). Thus as the result of the unequal rate at which valleys are lengthened, the larger number of those which come into existence are arrested in their development. As a result of growth in the manner indicated, the basins of even the large streams remain narrow at their lower ends while they expand above. This is the usual form of a drainage basin the development of which has been normal.

Did valleys grow in length only, competition would not destroy the small ones; it would simply limit them. But valleys widen as well as lengthen, and by widening, adjacent valleys may eliminate the divide between them and become one. The elimination of the intervening ridge may be by lateral planation (p. 82), or, if the valleys be of unequal depth, by slope wash (see Fig. 85). By these and other processes many young valleys are dwarfed, and many others are destroyed.

=Piracy.=—Streams do not always hold the courses which they establish for themselves at the outset. If the valley occupied by the stream _a_, Fig. 89, is deepened more rapidly than the valley occupied by _b_, a tributary from the former, _c_, may work back across the inter-stream area to e and steal the head waters of that stream (Fig. 90). The tributary which does the stealing is known as a _pirate_. Stream _f_ (Fig. 90) is said to be _beheaded_, and its upper portion, _de_, diverted. The beheaded stream is diminished in volume; or if its total supply of water came in above the point of tapping it would disappear altogether.

The process may not end even here. If after the diversion of _de_ the point in the channel to the left is lowered faster than the channel of the beheaded stream _f_, the divide between _dg_ and the head of _f_ (Fig. 90) will be shifted down the valley of the latter, as shown in Fig. 91. The shifting will go on until the divide reaches a position of stability, that is, until erosion on its opposite sides is equal.

The foregoing case may be called _foreign piracy_ because the valleys of different systems are concerned. _Domestic piracy_ may also take place, as illustrated in the accompanying diagrams (Figs. 92 and 93). Here a tributary to a crooked river may develop, working back until it taps the main at a higher point, thus straightening the course of the stream. The change takes place only when the highest point in the tributary valley is brought below the surface of the water in the main stream at the point where the tapping takes place. This would be likely to occur only after the main stream had attained a low gradient, for so long as it is deepening its channel notably, the small amount of water flowing through the tributary valley would not be likely to bring it down to the level of the main. In any case the flow of water from the main stream through the new valley would be likely to be started during flood, and at such time the erosion in the new channel would be great. The complete and final diversion of the stream through the new channel might be a slow process.

Piracy may occur where the material in which the valleys are cut is homogeneous; but, as will be seen later, heterogeneity of material, by determining unequal rates of erosion, stimulates the piratical proclivities of streams.

An actual case of piracy is shown on Plate XI. North and South Lakes formerly drained westward to the Schoharie Creek, the present head of which is in the extreme northwest corner of the map. The head of Kaaterskill Creek, which had a much higher gradient, worked back and captured the head of the westward-flowing stream, diverting the drainage from North and South Lakes to itself. Schoharie Creek was thus beheaded.

Plaatekill Creek, near the south limit of the map, appears to have beheaded the creek flowing west and northwest, similarly diverting its head waters. The Dells, Wis., quadrangle (U. S. Geol. Surv.) affords an illustration of domestic piracy.

RATE OF DEGRADATION.

The amount of mechanical sediment which the Mississippi River carries to the Gulf of Mexico is estimated to represent a rate of degradation for the Mississippi basin of about one foot in 5000 years. But the mechanical sediment carried to the Gulf does not really represent the total degradation of the basin, for the water which sinks beneath the surface is dissolving more or less rock substance, especially lime carbonate. This material is carried to the sea in solution, and does not appear in the sediment on which the above estimate is based. Taking into account the matter dissolved by the water and carried to the sea in solution, the average rate of degradation for the Mississippi basin is estimated at one foot in 3000 to 4000 years.

It is not to be inferred that this rate is uniform, or even that erosion at any rate whatsoever is taking place in all parts of the basin. Such is not the fact. On the whole the rate of erosion is doubtless greatest toward the margins of the basins where the land is in its topographic youth or early maturity. It is notably less in the middle courses of the valleys, and erosion is locally exceeded by deposition along the lower courses of the Mississippi and some of its main tributaries.

The average elevation of North America is not accurately known, but it is probably not far from 2000 feet. If the present rate of degradation, say one foot in 3500 years, were to continue, it would take something like 7,000,000 years to bring the continent to sea-level. But this rate of degradation could not continue to the end, for as the continent became lower streams would become sluggish and erosion less rapid. Long before the continent reached base-level the rate of degradation, so far as dependent on mechanical erosion, would become so slow that the time necessary to bring the continent to sea-level would be almost inconceivably prolonged. Furthermore, it is quite possible that the land is suffering, or is liable to suffer, uplift, relative or absolute. If the rate of rise were equal to the rate of degradation the average height of the continent would of course not be affected.

The amount of sediment carried by streams in suspension varies
notably according to the stage of the water. During a year when
the stream was under careful study the Mississippi at Carrollton
(Miss.) was found to carry ¹⁄₆₈₁ of its weight of sediment during
the high-water stage of June, and ¹⁄₆₃₈₃ during the low-water of
October, the average for the year being ¹⁄₁₈₀₈. The average of a
greater number of records gives about ¹⁄₁₅₀₀ as the average ratio
between the weight of the sediment and the weight of the water.
This corresponds to about ¹⁄₂₉₀₀ by volume, the average specific
gravity being about 1.9. The amount of material carried in the
upper part of the water was notably less than that carried at
greater depths, but that carried midway between top and bottom was
about the same as that carried at the bottom.[32]

The discharge of the Mississippi River is about 19,500,000,000,000
cubic feet of water per year, and the sediment it carries in
suspension is estimated to weigh about 812,500,000,000 pounds. This
is equivalent to about 6,714,694,400 cubic feet. It is estimated
that about 750,000,000 cubic feet of sediment is rolled along the
bottom, giving a total of 7,468,694,400 cubic feet as the aggregate
annual load carried to the Gulf by the river. This would be
adequate to cover an area one square mile in extent to the depth of
268 feet per year.

PART OF THE CATSKILLS, NEW YORK.
U. S. Geol. Surv.]

Fig. 1. NEW MEXICO.
U. S. Geol. Surv.

Fig. 2. VIRGINIA, WEST VIRGINIA AND MARYLAND.
U. S. Geol. Surv.]

ANALYSES OF AMERICAN RIVER-WATERS.[33]

[+Reduced to Parts per 1000 by Dr. H. J. Van Hoesen.+]

+----------------------------------+------------------+----------------+
|Name of river | Bear | Croton |
| | | |
|Collected at | Evanston, Wy. | Reservoir, New |
| | | York City |
| | | |
|Date | Dec., 1873 | 1881 |
| | | |
|Analyst | F. W. Clarke | E. Waller |
| | | |
|Reference |Bulletin No. 9, U.| Water supply of|
| | S. Geol. Surv., | New York City,|
| | p. 30 | 1881 |
+----------------------------------+------------------+----------------+
|Sodium, Na | .0082 | [2].00298 |
| | | |
|Potassium, K | ...... | .00154 |
| | | |
|Calcium, Ca | .0432 | .00905 |
| | | |
|Magnesium, Mg | .0125 | .00336 |
| | | |
|Chlorine, Cl | .0049 | .00213 |
| | | |
|Carbonic acid, CO₂, | [3].0982 |[34].02248 |
| | | |
|Sulphuric acid, SO₃ | .0105 | .00441 |
| | | |
|Phosphoric acid, H₃PO | | |
| | | |
|Nitric acid, HNO₃ | ...... | ...... |
| | | |
|Silica, SiO₂ | .0070 | .03360 |
| | | |
|Alumina, Al₂O₃ | | |
| | | |
|Sesquioxide of iron, Fe₂O₃ | | ...... |
| | | |
|Sesquioxides of iron and alumina, | | |
| Fe₂O₃ and Al₂O₃ | | .00078 |
| | | |
| „ „ iron and manganese,| | |
| Fe₂O₃ and Mn₂O₃ | | |
| | | |
|Carbonates of iron and manganese, | | |
| FeCO₃ and MnCO₃ | | |
| | | |
|Oxide of iron, FeO | | |
| | | |
| „ „ manganese, MnO | | |
| | | |
|Hydrogen in bicarbonates, H | | |
| | | |
|Chloride and sulphate of sodium, | | |
| NaCl, and Na₂SO₄ | | |
| | | |
|Ammonia, NH₄ | | ...... |
| | | |
|Organic matter | | .00400 |
| | | |
|Carbonates and sulphates of | | |
| Na, K, and Mg | ...... | ...... |
| |------------------+----------------+
| | .1845 | .08433 |
+----------------------------------+------------------+----------------+

--------------+--------------+---------------+----------------+-------------+--------------+
Cumberland | Delaware | Hudson, N. Y. | James | Los Angeles | Maumee, O. |
| | | | | |
Reservoir at | Reservoir at | | Richmond Water | Hydrant at | |
Nashville, | Trenton, | | Works, Va. | Los Angeles,| |
Tenn. | N. J. | | | Cal. | |
--------------+--------------+---------------+----------------+-------------+--------------+
| | | Oct. 24. 1876, |Sept. 8, 1878| |
| | |after light rain| | |
| | | | | |
N. T. Lupton | H. Wurtz | C. F. Chandler| W. H. Taylor | W. J. Jones |C. F. Chandler|
| | | | | |
Am. Chemist, |Geol. of N.J.,| Public Health | Ann. Rept. | Rept. Cal. | Report of |
July 16, 1876,| 1868, p. 702 |Papers, Vol. I,|Board of Health,| State Board | Toledo Water |
p. 16 | | Am. Pub. | Richmond, Va., | of Health, | Works, 1881 |
| | Health Ass. |1876 | 1878 | |
--------------+--------------+---------------+----------------+-------------+--------------+
| | | | | |
.01032 | .00072 | .00244 | .00234 | .02968 | .00162 |
| | | | | |
.00050 | .00178 | .00058 | .00251 | ...... | .00309 |
| | | | | |
.02987 | .01104 | .02220 | .01284 | .01750 | .02645 |
| | | | | |
.00280 | .00435 | .00465 | .00377 | .02097 | .00443 |
| | | | | |
.00299 | .00121 | .00581 | .00105 | .01044 | .00250 |
| | | | | |
.05727 | .02552 | .07278 | .02954 | .05635 | .04438 |
| | | | | |
.00563 | .00175 | .01257 | .00363 | .05724 | .01401 |
| | | | | |
...... | .00172 | | Trace | .02638 | |
| | | | | |
.00511 | ...... | ...... | .00231 | ...... | ...... |
| | | | | |
Trace | .00852 | .00698 | .01024 | .02005 | .00724 |
| | | | | |
| } { | | .00041 | .00171 | ...... |
| } .00047 { | | | | |
...... | } { | ...... | | | .00100 |
| | | | | |
.00671 | | .00120 | ...... | | |
| | | | | |
| | | .00072 | ...... | |
| | | | | |
| ...... | | | .00443 | |
| | | | | |
| Trace | ...... | | | |
| | | | | |
| | .00121 | | | |
| | | ...... | | |
| | | | | |
...... | } { | ...... | .00001 | | ...... |
| } .01087 { | | | | |
.01666 | } { | .01197 | .00299 | | .00499 |
| | | | | |
...... | ...... | ...... | ...... | ...... | ...... |
| | | | | |
--------------+--------------+---------------+----------------+-------------+--------------+
.13786. | .06795 | .14238 | .07246 | .24475 | .10971 |
--------------+--------------+---------------+----------------+-------------+--------------+

----------------+----------------+---------------+--------------+------------+--------------+
Mississippi | Ottawa | Passaic | Rio Grande | Sacramento | St. Lawrence |
| | | del Norte | | |
Hydrant, City | St. Ann’s Lock,| 4 miles above | Fort Craig, | Hydrant, | South side |
Water Works, | Montreal, Can. | Newark, N. J. | New Mexico | Sacramento,| Point des |
New Orleans, La.| | | | Cal. | Cascades |
| Mar. 9, 1854 | 1851 | 1873 |Sept., 1878 | Mar. 30, 1863|
| | | | | |
W. J. Jones | T. S. Hunt |E. N. Horsford | O. Loew | W. J. Jones| T. S. Hunt |
| | | | | |
Rept. La. State |Geol. of Canada,|Geol. of N. J.,| U. S. Geog. | Rept. Cal. | Geol. of |
Board of Health,| 1863, p. 567 | 1868, p. 708 |Surv. west of |State Board | Canada, |
1882, p. 370 | | |100th M., Vol.| of Health, | 1863, p. 567 |
| | | | III. p. 576| 1878 |
| | | | | |
----------------+----------------+---------------+--------------+------------+--------------+
.0310 | .00239 | .02357 | .03220 | .00200 | .00513 |
| | | | | |
...... | .00139 | .00163 | .00063 | ...... | .00115 |
| | | | | |
.0372 | .00992 | .01459 | .01633 | .01279 | .03233 |
| | | | | |
...... | .00161 | .00404 | .00123 | .00121 | .00585 |
| | | | | |
.0480 | .00076 | .03192 | .03604 | .00242 | |
| | | | | |
.0383 | .02255 | .02634 | .01025 | .00887 | .06836 |
| | | | | |
| .00194 | .01716 | .04700 | .00397 | .00831 |
| | | | | |
| Trace | | Faint trace | .01794 | Trace |
| | | | | |
| ...... | Trace | | | |
| | | | | |
| .02060 |} {| Trace | .03167 | .03700 |
| |} {| | | |
| Trace |} .01342 {| Trace | .00120 | Trace |
| |} {| | | |
| |} {| | | |
| | | | | |
| | | | ...... | |
| | | | | |
| ...... | | ...... | .01088 | ...... |
| | | | | |
| Trace | | Trace | | Trace |
| | | | | |
| Trace | | | | Trace |
| | | | | |
| | | | ...... | |
| | | | | |
| | | | .02431 | |
| | | | | |
| | | Trace | | |
| | | | | |
...... | | | .01392 | | |
| | | | | |
.0154 | ...... | ...... | ...... | ...... | ...... |
----------------+----------------+---------------+--------------+------------+--------------+
.1699 | .06116 | .13267 | .15760 | .11484 | .16055 |
----------------+----------------+---------------+--------------+------------+--------------+

------------------+------------------+------------------+----------------+--------------+----------------+
Humboldt | Truckee | Walker | Jordan | Mohawk | Genesee |
| | | | | |
Battle Mt., Nev. | Lake Tahoe, Nev. | Mason Valley, | Utah Lake | Utica, N. Y. |Rochester, N. Y.|
| | Nev. | | | |
| | | | | |
Dec., 1872 | Oct., 1872 | Oct., 1872 | Nov., 1873 | | |
| | | | | |
T. M. Chatard | F. W. Clarke | F. W. Clarke | F. W. Clarke |C. F. Chandler| C. F. Chandler |
| | | | | |
U. S. Geol. Surv.,|U. S. Geol. Surv.,|U. S. Geol. Surv.,| Bulletin No. 9,| Johnson’s | Johnson’s |
Monograph XI, | Monograph XI, | Monograph XI, | U. S. Geol. | Cyclopedia, | Cyclopedia, |
p. 41 | p. 42 | p. 40 | Surv., p. 29 | Vol. IV | Vol. IV |
------------------+------------------+------------------+----------------+--------------+----------------+
| | | | | |
.0467 | .0073 | .0318 | .0178 | .0036 | .0044 |
| | | | | |
.0100 | .0033 | Trace | ...... | .0009 | .0023 |
| | | | | |
.0489 | .0093 | .0228 | .0558 | .0318 | .0417 |
| | | | | |
.0124 | .0030 | .0038 | .0186 | .0069 | .00896 |
| | | | | |
.0075 | .0023 | .0131 | .0124 | .0023 | .0024 |
| | | | | |
[3].1544 | [3].0287 | [35].0576 | .0608 | .0569 | .0646 |
| | | | | |
.0477 | .0054 | .0284 | .1306 | .0187 | .0431 |
| | | | | |
| | | | ...... | |
| | | | | |
...... | | ...... | ...... | ...... | ...... |
| | | | | |
.0326 | .0137 | .0225 | .0100 | .0067 | .0014 |
| | | | | |
.0013 | | | | | |
| | | | | |
| | | | | |
| | | | ...... | ...... |
| | | | | |
| | | | .0013 | .0014 |
| | | | | |
| | | | | |
| | | | | |
| | | | | |
| | | | | ...... |
| | | | | |
| | | ...... | ...... | ...... |
| | | | | |
| | | ...... | ...... | ...... |
| | | | | |
| | | ...... | ...... | ...... |
| | | | | |
| | ...... | ...... | .0234 | .0250 |
| | | | | |
...... | ...... | ...... | ...... | ...... | ...... |
------------------+------------------+------------------+----------------+--------------+----------------+
.3615 | .0730 | .1800 | .3060 | .1525 | .19526 |
------------------+------------------+------------------+----------------+--------------+----------------+

The following table[36] gives the percentage of material carried in suspension by various rivers:

| | | | |Height |Thickness | | Mean | | |in Feet |of Sediment | | Annual | | |of Column |in Inches | Drainage |Discharge | | Ratio of |of Sediment |if Spread | Areas in |(in Cubic | |Sediment to |with a Base | over | Square |Feet.) per| Total Tons | Water by | of One |Drainage River. | Miles. | Second. | Annually. | Weight. |Square Mile.| Area. -----------+-----------+----------+-------------+------------+------------+-------- Potomac | 11,043 | 20,160 | 5,557,250 | 1 : 3,575 | 4.0 | .00433 Mississippi| 1,244,000 | 610,000 | 406,250,000 | 1 : 1,500 | 241.4 | .00223 Rio Grande | 30,000 | 1,700 | 3,830,000 | 1 : 291 | 2.8 | .00116 Uruguay | 150,000 | 150,000 | 14,782,500 | 1 : 10,000 | 10.6 | .00085 Rhone | 34,800 | 65,850 | 36,000,000 | 1 : 1,775 | 31.1 | .01075 Po | 27,100 | 62,200 | 67,000,000 | 1 : 900 | 59.0 | .01139 Danube | 320,300 | 315,200 | 108,000,000 | 1 : 2,880 | 93.2 | .00354 Nile | 1,100,000 | 113,000 | 54,000,000 | 1 : 2,050 | 38.8 | .00042 Irrawaddy | 125,000 | 475,000 | 291,430,000 | 1 : 1,610 | 209.0 | .02005 Mean | 334,693 | 201,468 | 109,649,972 | 1 : 2,731 | 76.65 | .00614 -----------+-----------+----------+-------------+------------+------------+--------

The composition of rain-water falling near London, as determined by
analysis, was as follows:[37]

Organic carbon .99 part in 1,000,000 of water.
Organic nitrogen .22 „ „ „ „ „
Ammonia .50 „ „ „ „ „
Nitrogen as nitrates and nitrites .07 „ „ „ „ „
Chlorine 6.30 parts in „ „ „
Total solids 39.50 „ „ „ „ „

A comparison of the composition of rain-water with that of springs
and rivers gives some idea of the solvent work of water. From a
study of the water of nineteen of the principal rivers of the world
Murray has compiled the following table[38] showing the amount of
mineral matter in average river water:

_MATERIAL IN SOLUTION IN ONE CUBIC MILE OF AVERAGE RIVER WATER._[39]

Constituents. Tons in a Cubic Mile.
Calcium carbonate (CaCO₃) 326,710
Magnesium carbonate (MgCO₃) 112,870
Calcium phosphate (Ca₃P₂O₈) 2,913
Calcium sulphate (CaSO₄) 34,361
Sodium sulphate (Na₂SO₄) 31,805
Potassium sulphate (K₂SO₄) 20,358
Sodium nitrate (NaNO₃) 26,800
Sodium chloride (NaCl) 16,657
Lithium chloride (LiCl) 2,462
Ammonium chloride (NH₄Cl) 1,030
Silica (SiO₂) 74,577
Ferric oxide (Fe₂O₃) 13,006
Alumina (Al₂O₃) 14,315
Manganese oxide (Mn₂O₃) 5,703
Organic matter 79,020
-------
Total dissolved matter 762,587

Murray also estimates that the aggregate amount of water flowing
into the sea annually is about 6528 cubic miles, which, on the
above basis, would carry about 4,975,000,000 tons of mineral matter
in solution.

A large number of analyses of waters of rivers from the United
States and Canada give an average of about .15,044 part in a
thousand of mineral matter in solution, more than one-third being
CaCO₃. The average amount of mineral matter in solution in 48
European streams cited by Bischoff[40] is .2127 part in a thousand,
of which CaCO₃ is rather more than half. The average mineral matter
in solution in 36 rivers cited by Roth[41] (including some of those
tabulated by Bischoff) is .2033 part in a thousand, of which CaCO₃
is slightly less than one-half.

An average for American and European rivers, so far as determinable
from data at hand, is about .1888 part in a thousand in solution,
of which CaCO₃ is slightly less than one-half. These last figures
are probably not very far from an average for river water in
general.

The following table shows the total amount of solids carried in
solution by the rivers indicated:[42]

Rhine 5,816,805 tons per year.
Rhone 8,290,464 „ „ „
Danube 22,521,434 „ „ „
Thames 613,930 „ „ „
Nile 16,950,000 „ „ „
Croton 66,795 „ „ „
Hudson 438,000 „ „ „
Mississippi 112,832,171 „ „ „

ECONOMIC CONSIDERATIONS.

Certain considerations of human interest in connection with river erosion are worthy of note. When a drainage system has reached its mature stage its basin has the roughest topography which it will have at any time during that cycle of erosion. At that stage, therefore, road construction is relatively difficult. If the relief be great, roads must follow the valleys, or the crests of the ridges between them, if they would avoid heavy grades. In such regions roads are usually few and crooked.

The stage of development of valleys has an influence on the navigability of their streams. Streams well advanced in life are much more readily navigable than young ones, because their grades are lower and their volumes of water greater. Old streams, on the other hand, are sometimes depositing sand or silt along their lower courses to such an extent as to interfere with navigation.

At certain stages of their development the power of streams is more easily utilized than at others. Young streams, depending as they do for their supply on the rainfall of a limited area, are likely to be fitful in their flow, and therefore unreliable as a source of power. This is especially true where the precipitation is unequally distributed, and where the slopes are steep and free from forests. Because of their great volume, old and large streams, though sluggish, have great power, but it is less easily controlled. Where streams are large enough to be navigable industrial considerations often prevent the utilization of their power, the streams being more serviceable as highways than as sources of power. Other things being equal, it follows that streams are most available for water-power when they are large enough to have a moderately steady flow, and not so large as to be beyond ready control, or to be valuable for purposes of navigation.

Streams are subject to more disastrous floods in some stages of their development than in others. Floods resulting from heavy rains are likely to be greatest where the slopes above the drainage lines are on the whole greatest, for this is the condition under which the water is most quickly gathered into the drainage channels. The most disastrous floods, humanly speaking, are those which affect wide-bottomed valleys, where the flats are settled. In such cases a relatively slight rise may flood very extensive areas. In such valleys the most disastrous floods are generally in the spring, when the waters from the melting snows of the preceding winter are being discharged.[43] Many other considerations enter into the problem of floods. The presence of forests and other forms of vegetation on the slopes retards the flow of water into the valleys, and so tends to prevent floods, or at any rate to make them less severe. Porous soil and subsoil, or in their absence porous rock, absorb the rainfall, and prevent its prompt descent into the valleys and so tends to prevent or diminish floods.

The acreage of arable land within a given area stands in some relation to its drainage development. At an early stage in its erosion history, before an upland has been dissected by valleys, nearly all of it may be arable. Later, when drainage is at its maturity, and when hillsides and ridge slopes constitute a large part of the area, there is probably the least acreage of arable land. This is especially true if the slopes are so steep as to allow the soil to be readily washed away. At a still later stage, when the valley bottoms have become wide and the slopes of the ridges and hills so reduced as to be available, the area of cultivable land is again increased.

Marshes, ponds, and lakes have some bearing on the resources and industries of a region, and they stand in a more or less definite relation to the stage of erosion in which a region finds itself. In its youth ponds and lakes may occupy much of the surface; in its maturity they will have been largely drained.

These suggestions are sufficient to show that the topography of a region, even in so far as shaped by erosion, touches human interests at many points.

ANALYSIS OF EROSION.[44]

Erosion is the term applied to all the processes by which earthy matter or rock is loosened and removed from one place to another. It consists of three sub-processes, namely, _weathering_, _transportation_, and _corrasion_.

_Weathering._

The term weathering is applied to nearly all those natural processes which tend to loosen or change the exposed surfaces of rock. The lettering of inscriptions on exposed marble becomes fainter and fainter as time goes by, and finally disappears, because the rock in which the letters were cut has weathered away. Some of it has crumbled off as the result of the expansion and contraction induced by changes of temperature, and some of it has been dissolved by the rain which has fallen upon it. In this case the weathering is effected partly by the atmosphere and partly by water. These are the chief, but not the only agents concerned in the general processes of weathering. Those phases of weathering which are the result of the activities of the atmosphere, whether physical or chemical, have been discussed in connection with the atmosphere (pp. 42 and 54).

The rain which falls upon the surface of exposed rock, and that which sinks through the soil to the solid rock below, dissolves, even if slowly, some of the rock constituents. Each constituent of a rock composed of several minerals may be looked upon as a binding material for the others. When one is dissolved the rock crumbles, much as mortar does when the lime which cements the sand is dissolved.

The solution of mineral matter by ground water, as well as the other chemical changes it effects, is greatly augmented by the impurities, especially carbonic and other organic gases, dissolved by the water from the atmosphere and the soil. The commonest chemical changes effected by the joint action of water and air, oxidation and carbonation, have been referred to in Chapter II. Hydration is more exclusively the work of water, and is one of the commonest processes of rock change, and often of rock disintegration. Numerous other less simple chemical changes resulting from the activities of ground water are constantly in progress, and in so far as they lead to the disintegration of rock are processes of weathering. Many chemical changes involve notable changes in volume of the mineral matter concerned. Merrill has calculated that in the conversion of the granitic rock of the vicinity of Washington, D. C., into soil, its volume has been increased 88 percent., largely as the result of hydration.[45] Even when the chemical changes do not themselves directly involve the disintegration of the rock, the accompanying increase of volume is sometimes sufficient to cause its physical disruption. This also may be regarded as a phase of weathering.

The weathering accomplished by water, or under its influence, proceeds at rates which vary with the composition of the rock, the amount and composition of the water, the temperature, and certain other factors less susceptible of brief statement. The weathering effected by ground water has a wider range both in area and depth than that due to changes of temperature, for while the latter is effective only where temperature changes are considerable, and where coherent material lies at the surface (p. 45), the former is operative to all depths to which water sinks.

There are other processes of weathering not due directly either to the atmosphere or to water. The roots of trees and smaller plants frequently grow into cracks of rocks, and, increasing in size, act much like freezing water (p. 45) in similar situations. This wedge-work of roots is a phase of weathering.

From the faces of steep cliffs masses of rock frequently fall. However dislodged, their descent is effected by gravity. The quantities of débris at the bases of many cliffs, forming slopes of _talus_ (Fig. 94), testify to the importance of the action of gravity in getting material from higher to lower levels. Another phase of gravity-work is shown in Fig. 95. Here, under the influence of gravity and expansion and contraction, due to freezing and thawing and wetting and drying, the surface material is creeping down slope. In the process the rock is being broken. The process illustrated by the figure involves weathering as well as other factors.

The foregoing are among the commoner processes of weathering, although they do not exhaust the list. The more active and tangible processes by which surface rocks are broken up, such as wave wear, river wear and glacier wear, are processes of corrasion. The mechanical wear effected by wind-driven sand might be considered either as corrasion or as weathering. It is more likely to be regarded as corrasion if the amount of wear is considerable enough to be obvious. Rock is sometimes decomposed by the chemical action of hot vapors, gases, and waters rising to the surface from considerable depths. This is often seen in volcanic regions. A conspicuous illustration is seen in the canyon of the Yellowstone in the National Park. Decay of this sort is perhaps not properly weathering, but is not always readily distinguished from it.