Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/291

Rh RIVE of sand or shingle at the concave side of each_sharp _bend of the river course. While the main current 1S making a great sweep round the opposite bank, thehwater ilfingelrs along the inner side of the curve and dropst ere its reig it of lobse detritus, which, when laid bare in dry weather, forms the familiar saiid-bank .or shingle bezlich. Agani, when a river, well supplied with sediment, eaves rougi ground where its course has been rapid, and enters a region of level plain,it begins to drop its burden on its bed, which is thereby heightened, till sometimes, as in parts of the courses of the Po, Adige, and Brenta across the plains of Lombardy, it is higher than the surrounding level region. This could not happen were it not that in ﬂoods a river deposits sediment along its banks, which are thus also heiglitened so as to retain the river in its ordinary state. But in such cases, if man lives along the margin of the river, he needs all his skill and labour to keep the banks secure. And even with his utmost efforts the river will now and then break through, sweeping down the barrier which it has itself made, as well as any additional embankments constructed by him, and carrying its ﬂood far and wide over the plain. Left to itself, the river would incessantly shift its course, until in turn every part of the plain had been again and again traversed. It is indeed in this way that a great alluvial plain is gradually levelled and heightened.‘ (c.) On Ifivewbcml-s and Floocl-plaL'ns.—Tl1is deposit is partly implied in the action described in the foregoing paravraph. It is laid down on the level tracts or ﬂood- plaino over which a river spreads in ﬂood, and consists usually of ﬁne silt, mud, earth, or sand, though close to the river it may be partly made up of much coarser materials. When a ﬂooded river overﬂows, the portions of water which spread out on the plains, by losing velocity and consequently power of transport, are compelled to let fall some or all of their mud and sand. If the plains happen to be covered with woods, bushes, scrub, or even tall grass, the vegetation acts the part of a sieve, and ﬁlters the muddy water, which ipayl rejoiii the lmailn stlreanfi (iompi.ir_atively_lclear. 1 Efvery l00L increases tie 1010 it o t ie p ain, unti, part y rom this cause and partly, iii the case of a rapid stream, from the erosion of its bed, the river can no longer overspread it. As the channel is more and more deepened, the river continues, as before, to be liable from inequalities in the material of its banks, sometimes of the most triﬂing kind, to be turned from side to side in wide curves and loops, and cuts into its old alluvium, mak- ing eventually a newer plain at a lower level. Continued erosion carries the channel to a still lower level, where the stream can attack the later alluvial deposit, and form a still lower and newer one. The river comes by this means to be fringed with a series of terraces, each of which repre- sents a former ﬂood-level of the stream. In Britain it is coimiioii to find three such terraces, but sometimes as many as six or seven or cven more may occur. In North America iheriverf gerrtaces exis(on so gra(pd a sofa]: that the ge(<1)- ocrists o ia country iave name one o t e ater perio s of geological history, during which those deposits were formed, the Terrace Epoch. In the attempt to reconstruct the history of the old river-terraces of a country, we have to consider whether they have been entirely cut out ot' older alluvium (in which case, of course, the valleys must have been as deep as now before the formation of the terraces); whether they afford any indications of havina been formed during a period of greater rainfall, when the rivers were larger than at present ; whether they point to any upheaval of the interior of the country which would accelerate the erosive action of the streams, or to any depression of the ’ It in_ the north of Italy that the struggle between man and nature in this department has been most persistently waged. See on this sub_}ect Lombardini in Ann. dcs Ponts et Cizaussées, 1847. GEOLOGY 277 interior or rise of the seaward tracts, which would diminish that action and increase the deposition of alluvium. Pro- fessor Dana has connected those of America. with the ele- vation of the axis of that continent. (d.) In La/I-es.—W hen a river enters a lake its current is at once checked, and its sediment begins to gather over the lake bottom. If the lake be long enough in proportion to the volume of the river, the whole of the detritus may be deposited, so that, at its outﬂow, the river becomes as clear as when its infant waters began their course from the springs, snows, and mists of the far mountains. Thus the Rhone enters the Lake of Geneva turbid and impetuous, but it escapes at Geneva as blue translucent water. Its sediment is laid down on the ﬂoor of the lake, and chieﬂy at the upper end. Hence, lakes act as ﬁlters or sieves to intercept the sediment which is travelling in the rivers from the high grounds to the sea. If we look down from a height even upon a small lake among mountains, we observe that at the mouth of each torrent or brook which enters it there lies a little tongue of ﬂat land (a true delta), through which the streainlet winds in one or more branches before ming- ling its waters with those of the lake. Each of these tongues consists of alluvium, laid down in obedience to the same law which governs the formation of river alluvium elsewhere, and continually creeping further out from the land as the deposit of sediment advances. Two streams entering a lake from opposite sides may join their alluvia so as to divide the lake into two, like the once united lakes of Thun and Brienz at Interlakeii. Or the lake may be ﬁnally ﬁlled up altogether, as has happened in innumerable cases in all mountainous countries; the hilly tracts of Britain, for example, furnish abundant illustrations of every stage in this process. Where a large river with abundant sediment enters an important lake (as the Rhone at the head of the Lake of Geneva), the accumulation of its alluvium or delta may quite rival that of a great river in the sea, as described in paragraph below. (6.) Bars and Lagoon-Barrz'ers.—If we take a broad view of the degradation of the land, we must admit that the deposit of any sediment on the land is only temporary; the inevitable destination of all this material is the ocean. Most rivers which enter the sea have their mouths crossed by a bar of gravel, sand, or mud. The formation of this barrier results from the conflict between the river and the ocean. Although the muddy fresh water ﬂoats on the heavier salt water, its current is lessened, and it can no longer push along the mass of detritus on its bed. It has been ascertained, moreover, that, though fresh water can retain for a long while ﬁne mud in suspension, this sediment is rapidly thrown down when the fresh is mixed with saline water. Hence, apart from the necessary loss of transporting power by the checking of the river current at the mouth, the mere mingling of a river with the sea must of itself be a cause of the deposit of sediment. But a large body of fresh water may ﬂoat for a long dis- tance before it is thoroughly mingled with the heavier water of the ocean. Ultimately, however, the ﬁne detritus dropped by a river, together with the coarser materials on the bottom, are arrested by the sea. Moreover, in many cases the sea itself piles up great part of the sand and gravel of the bar. Heavy river—ﬂoods push the bar farther to sea, or even temporarily destroy it ; storms from the sea, on the other hand, drive the bar farther up the stream. Another remarkable illustiation of the contest between the alluvium-carrying streams and the land-eroding ocean is shown by the vast lines of bar or bank which, both in the Old World and the New, stretch along the coast. The streams do not ﬂow straight into the sea, but run sometimes for many miles parallel to the coast, accumulating behind the barriersinto broad