THE LIFE OF AN INDIAN RIVER. 21 in on the other side by the MEGHNA, the most easterly of the mouths of the Ganges; or rather the vast estuary by which the combined waters of the Brahmaputra and Gangetic river systems find their way into the Bay of Bengal. In order, therefore, to understand the plains of Northern The part India, we must have a clear idea of the part played by the played by the great great rivers; for the rivers first create the land, then fertilize rivers. it, and finally distribute its produce. The plains of Bengal were in many parts upheaved by volcanic forces, or deposited in an aqueous era, before the present race of man appeared. But in other parts they have been formed out of the silt which the rivers bring down from the mountains; and at this day we may stand by and watch the ancient process of land-making go on. A great Indian river like the Ganges has three distinct Three stages in its career from the Himalayas to the sea. In stages in the life of the first stage of its course, it dashes down the Himalayas, a river. cutting out for itself deep gullies in the solid rock, ploughing First up glens between the mountains, and denuding the hillsides stage; of their soil. In wading over the Sutlej feeders among the hills in the rainy season, the ankles are sore from the pebbles which the stream carries with it; while even in the hot weather, the rushing sand and gravel cause a prickly sensation across the feet. The second stage in the life of an Indian river begins at the Second point where it emerges from the mountains upon the plains, stage. It then runs peacefully along the valleys, searching out for itself the lowest levels. It receives the drainage and mud of the country on both sides, absorbs tributaries, and rolls forward with an ever-increasing volume of water and silt. Every torrent from the Himálayas brings its separate contribution of new soil, which it has torn from the rocks or eroded from its banks. This process repeats itself throughout more than ten thousand miles; that is to say, down the course of each tributary from the Himalayas or Vindhyas, and across the plains of Northern India. During the second stage of the life of a Bengal river, therefore, it forms a great open drain, which gradually deepens itself by erosion of its channel. As its bed thus sinks lower and lower, it draws off the water from swamps or lakes in the surrounding country. Dry land takes the place of fens; and in this way the physical configuration of Northern India has been greatly altered, even since the Greek descriptions 2000 years ago. As long as the force of the current is maintained by a second stages of First and sufficient fall per mile, the river carries forward the silt thus supplied, and adds to it fresh contributions from its banks. Each river acquires a character of its own as it advances, a character which tells the story of its early life. a great river, as a silt-collector. Loss of power. Thus, the Indus is loaded with silt of a brown hue; the Chenab has a reddish tinge; while the Sutlej is of a paler colour. The exact amount of fall required per mile depends upon the specific gravity of the silt which it carries. At a comparatively early stage, the current drops the heavy particles of rock or sand which it has torn from the Himálayan precipices. But a fall of 5 inches per mile suffices to hold in suspension the great body of the silt, and to add further accretions in passing through alluvial plains. The average fall of the Ganges between Benares and the delta-head (about 461 miles) is nearly 5 inches per mile. In its upper course its average declivity is much greater, and suffices to bear along and pulverize the heavier spoils torn from the Himalayas. By the time the Ganges reaches its delta in Lower Bengal carrying (Colgong to Calcutta), its average fall per mile has dropped to 4 inches. From Calcutta to the sea the fall varies in the numerous distributaries of the parent stream, according to the tide, from 1 to 2 inches. In the delta the current seldom suffices to carry the burden of its silt, except during the rains, and so deposits it.1 Third stage of an Indian river, as a landmaker. In Lower Bengal, therefore, the Ganges enters on the third stage of its life. Finding its speed checked by the equal level of the plains, and its bed raised by the deposit of its own silt, it splits out into channels, like a jet of water suddenly obstructed by the finger, or a jar of liquid dashed on the ground. Each of the new streams thus created throws out in turn its own set of distributaries to right and left. The country which their many offshoots enclose and intersect forms 1 The following facts may be useful to observers in Bengal who wish to study the most interesting feature of the country in which they live, namely the rivers. Ten inches per mile is considered to be the fall which a navigable river should not exceed. The average fall of the Ganges from the point where it unites with the Jumna at Allahábád to Benares (139 miles), is 6 inches per mile; from Benares to Colgong (326 miles), 5 inches per mile; from Colgong to the delta-head, where the Bhagirathí strikes off (about 135 miles), 4 inches per mile; from the delta-head to Calcutta (about 200 miles), also 4 inches per mile; from Calcutta to the sea via the Húgli (about 80 miles), 1 to 2 inches per mile, according to the tide. The fall of the Nile from the first Cataract to Cairo (555 miles), is 6 inches per mile; from Cairo to the sea, it is very much less. The fall of the Mississippi for the first hundred miles from its mouth, is 180 inch per mile; for the second hundred miles, 2 inches; for the third hundred, 2.30 DELTAIC CHANNELS OF GANGES. 23 the delta of Bengal. The present delta of the Ganges may be The delta of Bengal. taken to commence at a point 1231 miles from its source, and 326 from the sea by its longest channel. At that point the head-waters of the Húglí break off, under the name of the Bhagirathí, from the parent channel, and make their way south to the sea. The main volume of the Ganges pursues its course to the south-east, and a great triangle of land, with its southern base on the Bay of Bengal, is thus enclosed. deltaic Between the Húglí on the west and the main channel on The the east, a succession of offshoots strike southward from the distribuGanges. The network of streams struggle slowly seaward taries; over the level delta. Their currents are no longer able, by reason of their diminished speed, to carry along the silt or sand which the more rapid parent river has brought down from Northern India. They accordingly drop their burden of silt in their channels or along their margins, producing how they almond-shaped islands, and by degrees raising their banks banks and channels above the surrounding plains. When they spill above surrounding over in time of flood, the largest amount of silt is deposited country. on their banks, or near them on the inland side. In this way not only their beds, but also the lands along their banks, are gradually raised. SECTION OF A DELTAIC CHANNEL OF THE GANGES. raise their a. The river channel; bb the two banks raised by successive deposits of silt from the spill-water in time of flood; c c. the surface of the water when not in flood; d d. the lowlying swamps stretching away from either bank, into which the river flows when it spills over its banks in time of flood; e e. the dotted lines represent the ordinary level of the river surface. inches; for the fourth hundred, 2:57 inches; and for the whole section of 855 miles from the mouth to Memphis, the average fall is given as 4 inches to the mile. The following table, calculated by Mr. David Stevenson (Canal and River Engineering, p. 315), shows the silt-carrying power of rivers at various velocities: Mile per Hour. 0170 will just begin to work on fine clay. 0'340 will list fine sand. 04545 will lift sand as coarse as linseed. 06819 will sweep along fine gravel. 13638 will roll along rounded pebbles 1 inch in diameter. size of an egg. canals. Delta The rivers of a delta thus build themselves up, as it were, rivers build into high-level canals, which in the rainy season overflow their themselves up into banks and leave their silt upon the low country on either side. high-level Thousands of square miles in Lower Bengal receive in this way each summer a top-dressing of new soil, carried free of cost for more than a thousand miles by the river currents from Northern India or the still more distant Himálayas--a system of natural manuring which yields a constant succession of rich crops. At Goálanda, about half-way between the delta-head and sea, the Ganges unites with the main stream of the putra, and Brahmaputra, and farther down with the Meghna. Their comMeghná. bined waters exhibit deltaic operations on the most gigantic scale. They represent the drainage collected by the two vast river systems of the Ganges and the Brahmaputra, from an aggregate catchment basin of 752,000 square miles on both sides of the Himalayas, together with the rainfall poured into the Meghna from the eastern Burmese watershed. Junction of Ganges, Brahma Their combined delta. Deltaic swamps, the The forces thus brought into play defy the control even of modern engineering. As the vast network of rivers creeps farther down the delta, they become more and more sluggish, and raise their beds still higher above the adjacent flats. Each set of channels has a depressed tract or swamp on either side, so that the lowest levels in a delta lie about half-way between the rivers. The stream constantly overflows into these depressed tracts, and gradually fills them up with its silt. The water which rushes from the river into the swamps has sometimes the colour of pea-soup, from the quantity of silt which it carries. When it has stood a few days in the swamps, and the river flood subsides, the water flows back from the swamps into the river channel; but it has dropped all its silt, and is of a how filled clear dark-brown hue. The silt remains in the swamp, and by up by silt. degrees fills it up, thus slowly creating new land. The muddy foliage of the trees which have been submerged bears witness to the fresh deposit. As we shall presently see, buried roots and decayed stumps are found at great depths; while nearer the top the excavator comes upon the remains of old tanks, broken pottery, and other traces of human habitations, which within historic times were above the ground. Last scene in the life of an Indian river. The last scene in the life of an Indian river is a wilderness of forest and swamp at the end of the delta, amid whose malarious solitude the network of tidal creeks merges into the sea. Here all the secrets of land - making stand disclosed. The river channels, finally checked by the dead weight of the sea, deposit most of their remaining silt, which emerges THE DELTA OF BENGAL. 25 from the estuary as banks or blunted headlands. The ocean currents also find themselves impeded by the outflow from the rivers, and in their turn drop the burden of sand which they sweep along the coast. The two causes combine to build up breakwaters of mingled sand and mud along the foreshore. In this way, while the solid earth gradually grows outward into Landthe sea, owing to the deposits of river silt; peninsulas and making islands are formed around the river mouths from the sand estuary. dropped by the ocean currents; and a double process of landmaking goes on. in the the Nile.' the 'Gift The great Indian rivers, therefore, have not only supplied new solid ground by draining off the water from neighbouring lakes and marshes in their upper courses, and by depositing islands in their beds lower down. They are also constantly filling up the low-lying tracts or swamps in their deltas, and are forming banks and capes and masses of low-lying land at their mouths. Indeed, they slowly construct their entire deltas by driving back the sea. Lower Egypt was thus 'the Egypt, the gift of the Nile,' according to her priests in the age of Hero- Gift of dotus; and the vast Province of Lower Bengal is in the strictest scientific sense the gift of the Ganges, the Brahma- Bengal, putra, and the Meghná. The deltas of these three river of the systems are in modern times united into one, but three Ganges.' distinct delta-heads are observable. The delta-head of the Brahmaputra commences near the bend where the river now twists due south round the Garo Hills, 220 miles from the sea as the crow flies. The present delta-head of the Ganges begins at the point where the Bhágírathí breaks southward from the main channel, also about 220 miles in a direct line from the sea. The delta of the Meghná, which represents the heavy southern rainfall of the Khasi Hills together with the western drainage of the watershed between Bengal and Independent Burma, commences in Sylhet District. delta. The three deltas, instead of each forming a triangle like the Size of the Greek A, unite to make an irregular parallelogram, running Bengal inland 220 miles from the coast, with an average breadth also of about 220 miles. This vast alluvial basin of say 50,000 square miles was once covered with the sea, and it has been slowly filled up to the height of at least 400 feet by the deposits which the rivers have brought down. In other words, the united river systems of the Ganges, Brahmaputra, and Meghná have torn away from the Himalayas and North-eastern Bengal enough earth to build up a lofty island, with an area of 50,000 square miles, and a height of 400 feet. |