Chalk stream

“ Chalk trout, then, are large not merely because the water is swift.
Whether trout have not a specific fondness for lime; whether water of some dozen degrees of hardness is not necessary for their development? are questions which may be fairly asked. Yet is not the true reason this; that the soil on the banks of a chalk or limestone stream is almost always rich—red loam, carrying an abundant vegetation, and therefore an abundant crop of animal life, both in and out of the water? The countless insects which haunt a rich hay meadow, all know who have eyes to see ”

“ On the sides of the chalk hills, though not on their summits, these streams cut narrow gullies and glens. Wherever, in fact, there is hilly, broken ground, the little rills form these broken ravines and gullies, often only a few yards in width from side to side. Usually these brooklet valleys are choked with brambles or fern, and filled with rank undergrowth. Often the stream is overhung and invisible, or dammed and left in soak, breeding frogs, gnats, and flies. The trees are always tall and beautifully grown, whatever their age, for the moisture and warmth force vertical growth ”

“ To the angler, however, some of these chalk streams are exceedingly precious—as they indeed ought to be, when a rental varying from fifty to a hundred pounds per mile per annum is gladly paid (and taken) for the [Pg 2] right of fishing with rod and line. Such choice preserves are stocked with trout of aristocratic quality, trout which can only be reared in streams issuing from the chalk; their water, when unpolluted by contact with towns, is crystal clear ”

“ The surface of the subterranean water would be shown by an inclined line, fixed at the river, and more or less elevated or depressed towards the ridge according as the stock of water is augmented by the replenishment of the chalk stratum, or reduced by the natural drainage. Thus in chalk districts the level at which water is found may be accurately ascertained. ”

“ The consequence of this absorbent power of the soil is that the streams and rivers in a Chalk country are not liable to floods, and moreover that their flow is but little affected at the time by rain; they being almost entirely dependent on perennial springs, which, during the driest of summers, still continue to deliver the water that in the course of the preceding winter, or even previously, has accumulated in the body of the Chalk. ”

“ To a thinking mind few phenomena are more strange than the state of little ponds on the summits of chalk hills, many of which are never dry in the most trying droughts of summer. On chalk-hills I say, because in many rocky and gravelly soils springs usually break out pretty high on the sides of elevated grounds and mountains: but no person acquainted with chalk districts will allow that they ever saw springs in such a soil but in valleys and bottoms, since the waters of so pervious a stratum as chalk all lie on one dead level, as well-diggers have assured me again and again. ”

“ Fig. 185 shows the changes recorded in the channel of the river at this point during the year 1883. It shows that the scour-and-fill during this year amounted to almost 40 feet. All 196streams similarly situated do a like work. The material thus eroded is shifted down-stream, some of it for short distances only, and some of it to the sea. Even an aggrading stream therefore is not without erosive activity; it is a stream whose fill exceeds its scour, not one which has ceased to erode. ”

“ In tracing this vast collection of water-worn materials, we find that it forms a considerable hill behind the town, and that it joins to the side of the range of hills called the South Downs.
In Alum bay in the Isle of Wight, however, the stratum immediately next to the flinty chalk, and consequently deposited upon it, consists of a white chalk marl without flints. Its nature is sufficiently shewn by its pulverizing with the frost; and the rains wash it down, so that its situation is marked by a deep hollow. ”

“ We have in England a kind of rock which constitutes at once an exceedingly clean recipient and a natural filter, and from which we can obtain water extremely free from mechanical impurities. I refer to the chalk formation, in which large quantities of water are held in store. Our chalk hills are in most cases covered with thin layers of soil, and with very scanty vegetation. Neither opposes much obstacle to the entry of the rain into the chalk, where any organic impurity which the water may carry in is soon oxidised and rendered harmless. ”

“ The dam and the uncompleted power-house, still figuring to the eye as skeleton masses of form timbering, lay just below them, and on the hither side the flooding torrent thundered through the spillway gates, which had been opened to their fullest capacity. Between the quarry and the northern dam-head ran the smooth concreted channel of the main ditch canal, with the water in the reservoir lake still lapping several feet below the level of its entrance to give assurance that, until the spillways should be closed, the charter-saving stream would never pour through the canal. ”

“ Along the course of the larger streams huge chambers occur; for the ever-eddying water, bearing sand along in its course, eats out the sides of its channel, or, revolving stones in its bed, carves out the pothole by friction. Or some pendent mass of rock has its support undermined and comes crashing into the streamway, only to be broken up and carried away by the ceaseless energy of the stream, so ever enlarging the chambers upwards towards the light of day. But whilst this action is going on underground, a more potent factor is at work where the subterranean stream first sees the light. ”

“ The rapidity of the increase is accounted for by the fact that every field it passes whose surface inclines towards it is a watershed from which an unseen but considerable drainage takes place. When no brook passes through the fields the water stands and soaks downwards, or evaporates slowly: directly a ditch is opened it fills, and the effect of a stream is not only to collect water till then unseen, but to preserve it from evaporation or disappearance into the subsoil. ”

“ In the course of time, the lakes and ponds in the depressions made or occasioned by the drift will be destroyed by drainage. Remembering how valleys develop (p. 46) it is readily understood that the heads of the valleys will sooner or later find the lakes, and drain them if their bottoms be not too low.
Drainage is hostile to lakes in another way. Every stream which flows into a lake brings in more or less sediment. In the standing water this sediment is deposited, thus tending to fill the lake basin. ”

“ Rain percolates through the chalk rapidly at all times, it being greatly fissured and cavernous, and finds vent at the bottom of the hills, in ordinary seasons, in the perennial springs which issue there, at the top of the chalk marl, or of the galt (the clay so called) which underlies the chalk. But when long-continued rains have filled the fissures and caverns, and the chinks and crannies of the ordinary vents below are unequal to the drainage, the reservoir as it were overflows, and the superfluity exudes from the valleys and gullies of the upper surface ”

“ When the river is essentially free from sediment, the surface of the limestone near the bank beneath it is sometimes distinctly green from the presence of the one-celled plants (fresh-water algæ) which grow upon it. The whole force of the mighty torrent is not able to sweep them from their moorings. Were the stream supplied with a tithe of the sand which it is capable of carrying, it would not take many hours, and perhaps not many minutes, to remove the last trace of vegetation. This illustration furnishes a clue to the method by which the erosion of solid rock in a stream’s bed is effected. ”

“ The principal water-bearing formations, utilized in Great Britain by means of deep wells, are the Chalk and the New Red Sandstone. The Upper and Middle Chalk are permeable almost through their mass. They hold water like a sponge, but part with it under pressure to fissures by which they are intersected, and, in the case of the Upper Chalk, to ducts following beds of flints. A well sunk in these formations without striking any fissure or water-bearing flint bed, receives water only at a very slow rate ”

“ Waves, whether due to wind or other agency, may cause scour, especially of the banks. Their effect on the bed becomes less as the depth of water increases, but does not cease altogether at a depth of 21 feet, as has been supposed. Salt water possesses a power of causing mud, but not sand, to deposit.
Arts. 2, 3, and 6 of this chapter refer to action on the bed of a stream. Action on the sides will be considered in Art. 7.
Weeds usually grow only in water which has so low a velocity that it carries no silt to speak of, but if any silt is introduced the weeds cause a deposit. ”

“ With the deepening of the channels, there comes an increase in the number of gulleys on the slopes of the channel; the gulleys grow into ravines and these into side valleys, joining their master streams at right angles (La Noë and Margerie) . With their continued development, the maturity of the system is reached; it is marked by an almost complete acquisition of every part of the original constructional surface by erosion under the guidance of the streams, so that every drop of rain that falls finds a way prepared to lead it to a stream and then to the ocean, its goal. ”

“ The capture of a tributary, however, leads both to the diminution of its main and to the increase of its captor, and the weakened stream may ultimately fall a prey to the one which is strengthened.
The processes of adjustment go on until the streams flow as much as possible on the weaker beds, and as little as possible on the stronger, when adjustment is complete. This amounts to the same thing as saying that the outcrops of the hard layers tend to become divides. In many cases an area is so situated that there is no escape for its drainage except across resistant rock. ”

“ These are especially characteristic of arid lands, where the streams wither as they flow out upon the thirsty lowlands and are therefore compelled to lay down a large portion of their load. In humid climates mountain-born streams are usually competent to carry their loads of waste on to the sea, and have energy to spare to cut the lower mountain slopes into foothills. In arid regions foothills are commonly absent and the ranges rise, as from pedestals, above broad, sloping plains of stream-laid waste. ”

“ At the bottoms of steep slopes, temporary streams build alluvial cones or fans. Along its flood-plain portion, a stream deposits more or less sediment on its flats. The part played by deposition in building a river flat has already been alluded to. A depositing stream often wanders about in an apparently aimless way across its flood plain. At the bends in its course, cutting is often taking place on the outside of a curve while deposition is going on in the inside. The valley of the Baraboo illustrates this process of cutting and building. ”

“ There is a stiff loam, in some places twenty feet deep, on a bottom of chalk. Though the grass grows so finely, there is no apparent wetness in the land. The wells are more than three hundred feet deep. The main part of the water, for all uses, comes from the clouds; and, indeed, these are pretty constant companions of these chalk hills, which are very often enveloped in clouds and wet, when it is sunshine down at Burghclere or Uphusband. They manure the land here by digging wells in the fields, and bringing up the chalk, which they spread about on the land ”

“ They also show that in small streams the proportion of mud in the flood-flows is greater, and the average duration of floods is shorter, than in larger streams. In other words, the differences between flood- and low-water flows are greatest in small streams, and gradually become less as the size of the stream increases.
When a stream is used for water-works purposes in the usual way, a certain quantity of water is taken from the stream each day, which quantity is nearly constant, and is not dependent upon [Pg 126] the condition of the stream, or the volume of its flow. ”

“ Where several neighboring streams flow by similar routes to their common base level, those which discover a harder rock will advance their headwaters less rapidly into the upland and so will be at a disadvantage in extending their drainage territory. A stream which is not thus hindered will in the course of time rob the others of a portion of their territory, for it is able to erode its lower reaches nearer to base level and thus acquire for its upper reaches, where erosion is chiefly accomplished, an advantage in declivity. ”

“ Whether, in a particular case, a header alone will be sufficient to cut off the flow of water from the higher land, or whether, in addition to the header, an open catch-water may be required, must depend upon the quantity of water likely to flow through or upon the land. An under-drain might be expected to absorb any moderate quantity of what may be termed drainage-water, but it cannot stop a river or mill-stream; and if the earth above the tiles be compact, even water flowing through the soil with rapidity, might pass across it. ”

“ In a chalk country, such as that through which the Somme flows, each cubic foot of flint, gravel or sand, represents the removal of at the very least twenty cubic feet of chalk, all of which, as we have already seen, must have been removed from the present area of drainage. In considering, therefore, the formation of these upper and older gravels, we must not picture to ourselves the original valley as it now is, but must, in imagination, restore all that immense mass of chalk which has been destroyed in the formation of the lower level gravels and sands. ”