Hydrology

“ In considering the principles of well supplies it is important to bear the following facts in mind. The crust of the earth, so far as it is permeable and above the sea-level, receives from rainfall its supply of fresh water. That supply, so far as it is not evaporated or absorbed by vegetation, passes away by the streams or rivers, or sinks into the ground. If the strata were uniformly porous the water would lie in the rock at different depths below the surface according to the previous quantity and distribution of the rainfall. ”

“ Geologic activity.—Of all geological agencies water is the most obvious and apparently the greatest, though its efficiency is conditioned upon the presence of the atmosphere, upon the relief of the land, and upon the radiant energy of the sun. Through the agency of rainfall, of surface streams, of underground waters, and of wave action, the hydrosphere is constantly modifying the surface of the lithosphere, while at the same time it is bearing into the various basins the wash of the land and depositing it in stratified beds. ”

“ The water imbibed by the soil in winter sinks until it meets a more or less impermeable or a saturated stratum, and then, by unseen conduits, slowly finds its way to the channels springs, or oozes out of the ground in drops which unite in rills, and so all is conveyed to the larger streams, and by them finally to the sea. The water, in percolating through the vegetable and mineral layers, acquires their temperature, and is chemically affected by their action, but it carries very little matter in mechanical suspension. ”

“ Water arising by land-floods brings along with it much of the most soluble parts of the manure from the higher lands to the lower ones. River-water in its clear state and those springs which are called soft are less beneficial for the purpose of watering lands, as they contain less earthy or saline matter; and water from dissolving snow from its slow solution brings but little earth along with it, as may be seen by the comparative clearness of the water of snow-floods. ”

“ If it shall be found that nearly all the rainfall should go into the soil and thence into the under-drainage, coming out slowly and steadily by seepage and by springs into the streams, clear and pure, these streams should present nearly ideal conditions for water-food, for water-power and for stream-navigation. An ideal solution of the soil problem may therefore solve the greater part of the whole complex of problems of which navigation is the last term. ”

“ The former part of the volume is occupied with discussion of the laws that govern stream-flow, rainfall, evaporation, ground storage, surface storage, and surface or flood flows. Gauging flows and the method of computing them are next considered. The local water systems are then described. The latter part of the book is devoted to generalizations as to water supply, chemical analyses, public water supplies, water power, evaporation, ground storage, effects of vegetation, and stream-flow; and a list of the developed water powers and the drainage systems is given in the appendix. ”

“ Estuary and Shore Deposits.
Rivers which discharge themselves into the ocean, where tides and currents break with a certain regularity the quiet of its waters, exhibit always at their mouths, and often along the lower part of their channels, another set of phenomena.
Where the tide enters a river's mouth, and periodically combats the freshes, these are "backed" to certain distances, their motion is nearly destroyed for a time, and the sediment, which was only suspended by the agitation of the water, is dropped in the interval of quiescence. ”

“ But the magnitude of a stream is more especially regulated by the extent of country which forms the declivities of its basin, though there is no invariable proportion here, for a small basin in a humid region will yield a greater quantity of water than one much more considerable in a different situation. High mountains, a humid climate, and a wide superficial drainage, are the three physical circumstances which lead to the accumulation of vast bodies of water, the magnitude of which will be proportionate to the degree in which these causes are in combined operation. ”

“ But it seldom happens that pure water is employed for the purposes of irrigation. The water of rivers, more generally, is diverted from its course, more or less loaded with mud and other finer particles of matter, which are either gradually filtered from it as it passes over and through the soil, or in the case of floods subside naturally when the waters come to rest. ”

“ In fact, its operators are successful only in regions in which ground water occurs in definite sheets in porous material, or in more or less clayey deposits, such as pebbly clay or till. In such regions (which are extremely common) few failures can occur, for wells can get water almost anywhere. Ground water occurs under certain definite conditions, and just as surface streams may be expected wherever there is a valley, so ground water may be found where certain rocks and conditions exist. ”

“ On the other hand, if the volume of water abstracted is great, its removal deprives its basin of an equalizing and moderating influence; for large bodies of water take very slowly the temperature of the air in contact with their surface, and are almost constantly either sending off heat into the atmosphere or absorbing heat from it. Besides, as we have seen, lakes in elevated positions discharge more or less water by infiltration, and contribute it by the same process to other lakes, to springs, and to rivulets, at lower levels. ”

“ River-water is in general purer than spring-water; as the neutral salts washed down from the earth decompose each other, except perhaps the marine salt; and the earths, with which spring-water frequently abounds, is precipitated; yet it is not improbable, that the calcareous earth dissolved in the water of many springs may contribute to our nourishment, as the water from springs, which contain earth, is said to conduce to enrich those lands, which are flooded with it, more than river water. ”

“ In later times, when an abundant and various soil covered the earth, when every river brought down to the ocean, not only its yearly tribute of mud or clay or lime, but the débris of animals and plants that lived and died in its waters or along its banks, when every lake and pond deposited at its bottom in successive layers the lighter or heavier materials floating in its waters and settling gradually beneath them, the process by which stratified materials are collected and gradually harden into rock is more easily understood. ”

“ The necessary conditions for the formation of extensive salt beds include arid climate and bodies of water which are essentially enclosed—either as lakes, as lagoons, or as arms of the sea with restricted outlets,—where evaporation exceeds the contributions of fresh water from rivers, and where circulation from the sea is insufficient to dilute the water and keep it at the same composition as the sea water. ”

“ The water of rivers must possess the powers of a chemical solvent like rain and springs, though its actual work in this respect can be less easily measured, seeing that river water is directly derived from rain and springs, and necessarily contains in solution mineral substances supplied to it by them and not by its own operation. Nevertheless, it is sometimes easy to prove that streams dissolve chemically the rocks of their channels. ”

“ Its fresh air, and the air which follows the descent of the water-table, carries oxygen to the organic and [pg 025] mineral parts of the soil, and hastens the rust and decay by which these are prepared for the uses of vegetation. The water itself supplies, by means of their power of absorption, the moisture which is needed by the particles of the soil; and, having performed its work, it goes down to the level of the water below, and, swelling the tide above the brink of the dam, sets the drains running, until it is all removed. ”

“ Any newly formed land surface, like a recently drained lake bed or part of the marginal sea bottom which has been raised into land, has a drainage system developed upon it. In the early or youthful stage of such a new land area lying well above sea level, under ordinary climatic conditions a few streams only form and these tend to follow the natural or initial slope of the land. ”

“ If precipitation takes place no faster than the water can sink through the soil, all the water may become ground-water. (3) The topography of the surface has much to do with determining the proportion of rainfall which becomes ground-water. If the surface be flat, more will sink in; if it be sloping, more of it will run off before it has time to sink. Other things being equal, the steeper the slope the larger the proportion of the rainfall which will run off over it. ”

“ In arid regions of which the declivities are sufficient to give thorough drainage, the absence of vegetation is accompanied by absence of soil. When a shower falls, nearly all the water runs off from the bare rock, and the little that is absorbed is rapidly reduced by evaporation. ”

“ The proportion borne by the available to the total rainfall varies very much, being affected by the rapidity of the rainfall and the compactness or porosity of the soil, the steepness or flatness of the ground, the nature and quantity of the vegetation upon it, the temperature and moisture of the air, which will affect the rate of evaporation, the existence of artificial drains, and other circumstances. ”

“ Taking a few centuries or a few thousand years, no doubt, we can attribute to the action of rivers, whether in ordinary flow or in flood, little power of modifying the region which they drain. But taking that wider survey (in time) of fluviatile work which modern science requires, dealing with this form of aqueous energy as we deal with the earth’s vulcanian energies, we perceive that the effects of river action in the course of long periods of time are not limited to the course which at any given time a river may pursue. ”

“ If the spring is located higher than the grounds and buildings to be supplied, a simple gravity supply line may be carried from it, with pipes of good size, thus avoiding undue friction in the line, and stoppages. If lower than the house, the water from the spring must be raised by some pumping method.
All water found underground owes its origin to the rainfall. If concealed water is returned to the surface by natural processes it is called spring water, but if recovered by artificial means it is called well water. ”

“ Such water, one must suppose, is obtained from ground water which has fallen upon the earth, has filtered through it, and is slowly flowing towards the river. The purifying agent in these cases is mainly the living humus which lies upon the surface, although the subsoil cannot be without some effect. These facts must alter our attitude towards surface wells, and must teach us what to a great extent has been admitted—that the purity of surface wells must depend more upon the mode of construction and the surroundings of the well than upon its depth. ”

“ BRAIDED CHANNELS. A river actively engaged in aggrading its valley with coarse waste builds a flood plain of comparatively steep gradient and often flows down it in a fairly direct course and through a network of braided channels. From time to time a channel becomes choked with waste, and the water no longer finding room in it breaks out and cuts and builds itself a new way which reunites down valley with the other channels. Thus there becomes established a network of ever-changing channels inclosing low islands of sand and gravel. ”

“ These are the natural consequences which must follow if the land has long been worked upon by streams, and no other process has ever been suggested which is competent to produce them. We must conclude that valley systems have been formed by the river systems which drain them, aided by the work of the weather; they are not gaping fissures in the earth’s crust, as early observers imagined, but are the furrows which running water has drawn upon the land. ”

“ When this mechanical power of running water is considered, we are prepared for the transportation before alluded to of large quantities of gravel, sand, and mud, by torrents which descend from mountainous regions. But a question naturally arises, How the more tranquil rivers of the valleys and plains, flowing on comparatively level ground, can remove the prodigious burden which is discharged into them by their numerous tributaries, and by what means they are enabled to convey the whole mass to the sea? ”

“ When a river is embanked at a given point, and, consequently, the water of its floods, which would otherwise spread over a wide surface, is confined within narrow limits, the velocity of the current and its transporting power are augmented, and its burden of sand and gravel is deposited at some lower point, where the rapidity of its flow is checked by a dam or other artificial obstruction, by a diminution in the inclination of the bed, by a wider channel, or finally by a lacustrine or marine basin which receives its waters. ”

“ The lakes which originate by the isolation of portions of the sea are salt at the outset. If inflow exceeds evaporation, they become fresher and may ultimately become fresh; otherwise they remain salt. If evaporation exceeds inflow they diminish in size and their waters become more and more salt or bitter.
Indirect effects of lakes.—Lakes tend to modify the climate of the region where they occur, both by increasing its humidity and by decreasing its range of temperature. They act as reservoirs for surface-waters, and so tend to restrain floods and to promote regularity of stream flow. ”

“ Rivers are to the globe what the veins are to the animal organization. They pick up the surplus moisture not needed in the growth of vegetation and for the sustenance of animal life, and carry it on, together with the débris that it gathers in its course, to the great reservoirs, the seas and oceans, where it is redistilled and purified by the action of the sun's rays. From here it is carried back in the form of invisible moisture and again precipitated in the purified state, to help carry on the great operations of growth—animal and vegetable. ”