VIII.] FLOODS OF THE MAHÁNADI. 109 back into the air. Another part, after soaking into the ground, is absorbed by the roots of trees and the smaller vegetation which covers the surface; and much of this also, evaporating from the surface of the leaves and herbage, passes back into the air : the remainder is converted into wood and leaves. Perhaps about half of the whole rainfall is disposed of by evaporation and in contributing to the growth of the vegetation. The residue reaches the river in two ways. When the rain is heavy and the ground has already been soaked by previous rain, the larger part runs at once into the nearest streamlet, carrying with it earth and sand, dead leaves and other light bodies lying on the ground. This it is that produces freshets and floods, filling the channels, and, when the rain is very heavy and extensive, even causing the streams to overflow their banks; in which case the latter are cut away, and whole trees and sometimes houses and living animals' are swept away by the swollen waters. The Mahánadi is notorious for the magnitude and destructive character of its floods, and this is due to the generally hilly and rocky character of the country drained by it in consequence of which, when heavy rain falls over a large part of the basin, the water rapidly runs off the steep hill slopes, and is carried into the main channel. In a great flood which occurred in July 18552 and lasted for seven days, the quantity of water discharged by the river was, in round numbers, 761,770 millions of cubic feet, or rather more than five cubic miles. If this quantity of water were distributed equally over the 45,000 square miles of the basin, it would cover it to a depth of 6 inches. But the actual rainfall must have been greater than this. Captain Harris estimates that a great rain storm in which 9 inches of rain should fall (in addition apparently to the ordinary rainfall of the season) would suffice to produce this flood, but it seems probable that a smaller quantity would be ample. : In a great flood in July 1872, large numbers of cattle, human bodies, and even elephants, were carried down past Cuttack. 2 These data, together with the area of the basin above given, are taken from a paper by Captain Harris in Vol. XXX. of the Asiatic Society's Journal. SURFACE SPRINGS. [CHAP. Great floods, such as this, occur only once in three or four years but all through the usual rainy season, a very large body of water flows down the Mahánadi and all rivers similarly situated. During the dry months of the year, especially from February to May, the river is low; but even then, more than five hundred millions of cubic feet of water on an average pass down every day. Whence comes this great volume of water? There is but little rain, even in the hills, in these months; an occasional thunderstorm is the only contribution; and at Sambhalpúr, the quantity thus received does not amount, on an average, to one inch of rain in a month; which, falling on dry ground, must be in great part absorbed. It is in this absorption of rainwater by the ground that we have the explanation of this constant supply. However dry the surface of the ground may be, there is always water at a certain depth; a depth which depends on the frequency of rain. Even hard rock, when freshly quarried, contains some water; and any excavation, such as a mine or a well, carried to the depth of thirty or forty feet, in a country such as that drained by the Mahánadi, will be partly filled with water at all times of the year. When absorbed by the surface soil, the rainwater gradually penetrates deeper through the crevices, which exist in all rocks; through the mass of the rock itself, if it be porous and permeable to water; and at a depth which varies with the season and the dryness of the country, a level is reached, below which the rocks and all their cavities and crevices are fully charged with water. So long as this level is higher than the bottoms of the lowest valleys, the water will gradually trickle out of the rock at some lower level, where it can find an outlet ; in some places, where the outlet and supply are large, as a permanent spring; at other places merely oozing through the soil. The accompanying diagram will help to elucidate this description. The figure represents an ideal section of the ground, across a little valley, drained by the stream at s. On the one side a, the hill is formed of some hard rock, not itself permeable to water, but much cracked and broken, so that water can find its way from the surface through the crevices. с The water thus absorbed will slowly, and in the course of time, trickle out at the hillside at o, about at the level of the stream; but this will go on very slowly; and, except perhaps after a quite unusual drought, there will always be a supply of water in the rock above this level. After heavy rain, we may suppose the dotted liner will indicate the level below which the rock is saturated with water; while, after a moderately long drought, the level of saturation will have fallen to d. On the other side of the valley the rocks are different. A bed of gravel forms the upper part of the hill; and below this is a bed of clay, which water cannot penetrate. In this case, the porous bed of gravel will hold a large quantity of absorbed rainwater; which, being stopped by the clay bed at c, will gush out at sp; forming a perennial spring if the bed of gravel be extensive, or a temporary spring, yielding only after a fall of rain, if it be small, so that the supply is soon exhausted. This supply from subsoil water is extremely important in a climate like that of the greater part of India, where the rainfall is very heavy during certain months of the year, and very scanty, or altogether wanting at other seasons; and it becomes a question of national importance to prevent its diminution or loss. Unhappily, the destruction of the forests, which has been going on rapidly in many parts of India for many years past, has this effect. Trees shade the ground and shield it from the scorching rays of the sun. At the same time the soil is held together by their roots and 112 EFFECT OF FORESTS. THE DELTA. [СНАР. those of the smaller vegetation that springs up in a forest; and the soil, thus held and sheltered, serves to absorb rain and transmit it to the rocks below. The effect of clearing the forest is, then, to expose the soil to the baking heat of the sun, and to be washed away by heavy rains. On steep rocky ground, where the soil is scanty, this is soon effected; and the spontaneous re-establishment of the forest is rendered impossible; while, the absorbent covering of the rocks having been destroyed, every shower of rain rushes at once off the hard surface, flooding the rivers for the time, and leaving no provision for that subsoil drainage, which, amid forest-clad hills, furnishes a constant supply to the rivers throughout the rainless season. In parts of France, more especially that known as the Côte d'Or, large tracts of country have been rendered sterile and uninhabitable, in consequence of the ignorant destruction of the forests; and there are few parts of India that have not suffered more or less already, from the same cause. The preservation of forests from general destruction is then a matter of national importance. Once cleared, it is the work of many generations to restore them. the We must now turn our attention to the lower part of the river, the delta1 and its channels. The character of this part of the river, and of the country through which it runs, is very different from that hitherto described. No sooner does the river enter the plain, than its current is retarded; slope, down which it runs, being so small as to be imperceptible to the eye. Consequently, it is no longer able to carry with it all the sand and other sediment that it has washed away from the hill slopes in the upper part of its course, and it begins to deposit the excess on the bed of its own channel. Moreover, the country around being on nearly the same level, it is no longer restricted to one The term delta was originally given to the lower part of the Nile, which, below Cairo, divides into two principal branches, now known respectively as the Damietta and Rosetta branches. The triangular tract of flat land, between them and the Mediterranean Sea, resembles in form the Greek letter A delta. Hence the name. 2 See Introduction, p. 5. VIII.] RIVER DELTAS. 113 channel, but divides up into several, branching out over the plain. Even these cannot contain all the water when the river is flooded; and then they overflow their banks and spread their waters abroad over the low lands around. And now follows a noteworthy result. As soon as the waters have left the river, their motion decreases. More sand and silt is deposited; and it is deposited most abundantly close to the margins of the channel, raising the banks higher than the general level of the plain. A section across a river in its delta is, therefore, such as is represented in the accom FIG. 16.-Section of a Deltaic river. panying Fig. 16; which contrasts strongly with that of the stream with its valley represented in Fig. 15. In Cuttack, the banks of each stream are further artificially raised, to preserve the country from flooding. But even where this has not been done, the banks of the rivers in a delta are higher than the land round about. In Calcutta, for instance, the drainage of the town is carried not into, but away from, the Hooghly, down the natural slope of the ground to the Salt Lake; and in the eastern districts of Bengal, where the rivers are not restricted by artificial embankments, when the whole intervening country is flooded in the rains, the banks of the river channels generally remain more or less above water. In the delta of the Mahánadi, as in that of the Ganges, and indeed all large rivers, the ordinary level of the water in the channels is not very much lower than that of the land around. When the rivers are flooded, it is frequently much higher; and I have already observed that, on such occasions, the waters spread over the intervening low lands, and gradually raise them, by depositing their silt. But in thicklyinhabited countries, such as Orissa and the Midnapore districts, where the land is under cultivation, it is frequently sought to confine the rivers to their channels by dykes or B.G. I |