114 EMBANKED CHANNELS. FLOODS. [CHAP. artificial embankments, termed also bunds. These suffice to restrain the waters in ordinary seasons, but their effect is not wholly advantageous. The level of the water in the channel is artificially raised, since none can escape; after a time, the bed of the channel also becomes raised by the deposited silt; and in order that the embankments may continue to afford protection, they must be raised in their turn. In the course of many years, a river may thus be raised till it ordinarily flows much above the level of its delta, and even of the houses of the villages around. Such is the case with the river Po in Italy. Should such a river breach its embankments in an extraordinary flood, it can easily be imagined that the destruction caused by it far exceeds that produced by a similar flood in an unembanked river. The Mahanadi is unenviably notorious for its floods. I have already pointed out how the form and character of its drainage basin favour the rapid discharge of the rainfall; and I have now to point out why it happens that, on such occasions, the rice lands of Cuttack and Púri are almost inevitably flooded. Some years since a very careful measurement of all the channels of this delta was made by Captain Harris; and the result showed that, taken altogether, they were capable of carrying away only about half the water that comes down in one of the great floods. The excess must therefore spread over the country around, despite the embankments of the channels. In the delta of the Ganges and Bráhmapútra, the land, for many miles inland from the sea, is a marsh washed by sea water at every spring tide, and covered with the vegetation peculiar to such regions; in which mangroves, the Súndri, the Nipa-palm, &c., are predominant; and beyond the margin of the land, the sea itself, for some miles out, is filled with shoals and sand-banks, partly uncovered at low water; which are in fact a submarine extension of the delta. At some future time, if not submerged deeply by a general sinking of the delta, such as was noticed in Chapter IV. (p. 55), these will be converted into marshy islands; while the VIII.] FORMATION OF DELTAS. 115 present Sundarbans will be gradually raised till they become habitable. Such has been the history of all Lower Bengal. There was a time when the sites occupied by Calcutta, Dacca, Jessore, &c., were covered by the sea; and the shoals, first formed off the mouths of the rivers then existing, have gradually been raised by the continuous deposit of river-silt, in the manner described in the preceding pages. The deltas of the Mahánadi and other rivers of the east coast of the Peninsula are not extending outwards like that of the Brahmaputra. The Mahánadi delta is indeed as yet not quite completed, for not only is its surface still being raised, but it is gradually filling the Chilka Lake, which is a piece of the sea that has been cut off from it and inclosed by a spit of sand. But off the mouth of the Mahánadi there are no great sand shoals, the foundations of future islands, like those that fringe the Súndarbans. The Káveri delta in Trichinopoly and Tanjore represents the final condition, to which that of the Mahánadi is approaching. Very little of this is ever even flooded by the river. It is now so high that it is permanently above the water level, and artificial irrigation is necessary, to enable the land to be cultivated. Not a particle of new land is forming in the sea opposite. In fact it is a finished delta. All the silt, now brought down by the river, is carried away by the sea currents that sweep up or down the coast; and instead of adding to the delta, it is spread abroad over the floor of the Bay. The formation of a river delta may be regarded as a struggle between the land and sea. In a quiet landlocked bay a delta forms rapidly; stretching out its branching channels, indicated by their emerging banks, like the roots of a growing tree. A chart of the Mississippi delta in the Gulf of Florida well illustrates this, and to some extent the same character is traceable in that of the Kistna and Godávarí in the angle of the Indian coast. Even those of the Káveri and the Mahánadi project slightly beyond the general line of the coast. The river, in each case, continually brings down sediment, some of which is deposited wherever the flow of its waters is checked or diminished; 116 BARS. SCOUR OF TIDES. [CHAP. VIII. and the chief check takes place where it enters the sea. Here therefore a deposit is formed; and it depends on the character of the sea, whether this is distributed over a large area, forming shoals separated by shallow channels; or in a single shoal, termed a bar, across the mouth of the river. The former is the case of the Gangetic delta, the latter that of the Mahanadi and all the rivers of the east coast. It is the existence of such bars that renders it impossible for seagoing vessels to enter most of these rivers. A delta ceases to extend outwards, when the permanent currents of the sea are powerful enough to carry away all the sediment that the river brings down. Tidal currents,' on the other hand, have the effect of keeping open the entrance of river estuaries; and their power is increased if the sea is shallow opposite the river mouth. In passing over this shallow, the tidal wave becomes piled up, as was explained in Chapter III.; and as this wave is propagated into the estuary, a large body of water is thrown into the river twice every day, which passes out again at each ebb tide, together with the water the river has brought down in the meantime. A swift current flows therefore in opposite directions twice every day, sweeping away the sediment and keeping the channels open. It is for this reason that the Hooghly remains navigable to the present day. Estuaries such as the Mutlah, which, though at some former time the outlet of some one of the delta rivers, now receives little water from the interior, and therefore little sediment, may be kept open for an indefinite period by the scour of the tidal currents. We have now traced out the history of our river, and incidentally we have compared and contrasted it, in certain of its characteristics, with portions of some other large rivers. But the subject of rivers and their action is too large and too important to be fully discussed in a single chapter, and there are other bodies of fresh and also salt water connected with rivers, which remain to be noticed. These we shall deal with in another chapter. See p. 47. CHAPTER IX. RIVERS AND LAKES. IN the last chapter we studied the case of a river in a tropical climate, fed exclusively by the periodical rainfall and those surface springs that are the outflow of the absorbed rain. We saw that, as a consequence of these conditions and of the rocky and hilly character of its drainage basin, the Mahanadi is very full during the three or four rainy months of the year, and is then sometimes subject to very destructive floods, produced by extraordinary falls of rain; while, during the remainder of the year, it contains so little water, that it can be navigated only by boats of the smallest size. Very different is the case of the Indus; a river, the main stream of which derives its chief supply from the melting of the winter snows on the Tibetan Himalaya; while its larger tributaries bring down the summer rainfall of the outer Himálaya, in addition to the above. After leaving the mountains, it traverses nearly 700 miles of arid plain, in no part of which does the rainfall exceed twenty inches in the year; while by far the greater portion is absolute desert. The Indus, like the Mahánadi, is subject to a regular periodical rise and fall, and occasionally to extraordinary floods; but the causes being different in the two cases, the periods of their occurrence are different also. In Chapter II.1 it was explained how, during the winter season, the cold air over the plains (and, I may add, the 118 FLUCTUATION OF THE INDUS. [CHAP. mountain slopes) of Upper India, flows away to the south, forming the wind known as the north-east or winter monsoon; and how it is replaced by a damp wind, which blows from the south in the higher part of the atmosphere, and descends on the Panjáb and the upper part of the NorthWest Provinces, bringing the winter rains. Over the Himalaya, where the temperature is below the freezing point, the vapour of this wind is condensed as snow, and this accumulates on the mountain summits and in the higher valleys during the winter season. With the increasing warmth in spring, it begins to melt; and the rivers, which in these lofty regions have been silent and frost-bound by the intense cold, are once more set free, carrying down gigantic blocks of ice and rock, with large quantities of mud, in an impetuous and ever increasing torrent. Collecting gradually in the larger tributaries, and then in the main stream, by the month of February the swelling waters reach the plains of the Panjáb; and the stream, which has been shrinking since the previous July, and during the winter months has been represented by a number of shallow, scarcely navigable channels, meandering through the vast bed of the river, begins to swell and sweep away the chars, now covered with grass and bulrushes, which accumulated at the close of the previous season. During the succeeding months, the volume of the river increases, as the snows are more copiously melted; and when, in July, the heat of the Panjáb and the mountain cradle of the river has reached its highest point, and when the summer rains are falling heavily in the Kángra and Hazára Hills, the Indus and its great tributaries are in full flood. After this, the waters decline, and reach their lowest level by October. The extraordinary floods, to which the Indus is subject, appear to be due, not to excessive rain or any unusually rapid melting of the snows, but to some one or other of its larger tributaries being temporarily blocked up, either by the advance of glacier or the fall of a portion of a mountain; so that a barrier is thrown across the valley, and the river waters dammed up behind it. Under such circumstances, |