The annual swelling and overflowing of the Ganges, is attributable as much to the rain water that falls in the mountains contiguous to its source, and to the sources of the great northern rivers that fall into it, as to that which falls into the plains. The dissolution of mountain snow adds but little to us increase ; for it rises but 151 feet out of 32 (the sum total of its rise) by the latter end of June, and it is well known that the rainy season does not com mence in most of the low countries till about that time.
In the mountains, the rains begin early in April, and by the latter end of That month, when the rain water has reach ed Bengal, the river begins to rise, but by very slow de grees, the increase being only about an inch a day for the first fortnight. It then gradually augments to two or three inches, before any quantity of rain falls in the low countries. On the rain becoming general, the increase on a medium is five inches per day.
The following Table shews the gradual increase of the Ganges, and its branches, according to observations made at Jellinghy and Dacca.
Its daily increase is nearly in the following proportion. During the latter half of August, and all September. from three to four inches ; from September to the end of Novem ber, it diminishes gradually from three inches to one and a half inch ; and at a medium a half inch per day, from No vember to the latter end of April. These proportions re late to such parts of the river as arc not affected by the tides. By the latter cud of July, all the lower parts of Bengal are overflowed contiguous to the Ganges and Brah rnapootra, forming an inundation of more thz.n 100 miles in width, nothing appearing but villages and trees, and here and there the site of an abandoned village, resembling an island. Owing to the quantity of rain that falls in Bengal, the lands are generally overflowed to a considerable height long before the bed of the river is filled, the ground adja cent to the bank, to the extent of some miles, being more elevated than the rest of the country. Dikes, kept up at an enormous expence, guard particular tracks from inunda tion, yet these are liable to he damaged, owing to the want of tenacity in the soil of which they are composed. It is cal culated that the length of these dikes, collectively, exceeds 1000 miles. The inundation is nearly at a stand in Bengal for some days preceding the 15th of August, when it be gins to run off, though great quantities of rain still con tinue to fall during August and September ; but by this time a decrease of rain having taken place in the moun tains, a consequent deficiency of supply to keep up the in undation ensues. Of the increase of the Ganges, Rennell remarks that there is a difference in the quantity of this in crease at places more or less remote from the sea, the height of the periodical increase diminishing gradually from where the tide reaches to the sea, until it totally dis appears at the point of confluence. The ocean preserving the same level at all seasons, (unde• similar circumstances of tide) necessarily influences the level of all waters that communicate with it, unless precipitated in the form of a cataract. At Luckipoor, there is a difference of about six feet in the height at different seasons ; at Dacca, and places adjacent, 14 ; and at Custee of 31 feet. The last place is about 240 miles from the sea, by the course of the river ; and the surface of the river there is, in the dry season, 80 feet above the level of the sea at high water, The quantity of water discharged by the Ganges, in one second of time, in the dry season, is 80,000 cubic feet ; but the river, when full, having twice the volume of water in it, and its motion being accelerated in the proportion of 5 to 3, the quantity then discharged is 405,000 cubic feet. Taking the medium of the whole year, it will be nearly 180,000 cubic feet per second of time.
Europeans view with wonder the remarkable alterations in the course of the Ganges, and the other rivers of Bengal, although the natives of the country, who have long wit nessed the encroachments and deviations of their streams, behold these changes without surprise. The greatest in jury is sustained during the periodical floods, and while the waters are draining off ; and when it is considered that at this season, at the distance of 200 miles from the sea, there is an increase of more than 25 feet in the perpendicular height of the water, some idea may be formed of the velo city with which it will run off, and of the havoc which it will make on the banks. Accordingly, it is not unusual to
find, when the rainy season is over, large portions of the banks precipitated into the channel, and the devastation ex tended over fields and plantations ; even trees which, with the maturity of a century, have acquired strength to resist the most violent storms, have been suddenly undermined, and swept away by the stream. But the encroachments are as often carried on gradually, and in the dry season ; in this case the natives have time to remove their effects, and change their places of abode, if too near the banks. Whole villages are thus seen deserted, whose inhabitants had retired to safer situations. Along the banks of the Ganges, where the depredations of the flood are most to be apprehended, the people are so accustomed to removal, that they make use of light materials only in huts, and such as, on an emergency, can be easily transported. These effects are attributable, in a great measure, to the looseness of the soil ; but this destructive operation of na ture is in some degree compensated by the formation of new lands, either by alluvions on the opposite shore, or by islands which emerge in the middle of the stream, and ultimately become connected with the main land, by the closing up of one of the channels. The Ganges gives birth to numerous islands, which are of an extent propor tional to the bulk of its waters. The rapidity with which these islands have been thrown up, and the size to which they have swoln, appear objects of astonishment to those who have opportunity of observation. When the inundation has subdivided, and the river found its ordinary level in the dry season, considerable sand banks are seen in places where, the preceding year, the channel had been navigable. The collection of sand becomes sometimes so great, as to divert the main stream into a new and general ly more direct course ; for it is only by encroachments on the banks that inflexions in the stream are produced, while the sudden alluvions, and frequent depositions of sand, have a tendency to fill up the channel, into which it had been di verted, and to restore the straightness of its course. Such of the islands as are found on their appearance to have any soil, are immediately cultivated ; and water melons, cu cumbers, and sum soo or mustard, are the produce of the first year. Even rice is seen growing on those parrs, where a quantity of mud has been deposited near the wa ter's edge. Some of these islands, before they have ac quired sufficient stability to resist the force of toe cut are swept away ; but when, by repeated additions of soil, they appear to be firm and consolidated, the natives no longer hesitate to take possession of them, and the new acquisitions become immediately a subject of nhercation. The settlers transport their families, cattle, and effects. The highest spots are selected for the villages, and dwell ings arc raised with as much confidence as on the main land ; and though the foundation be sandy, the stratum of soil which is uppermost being interwoven with the roots of grass and other plants, becomes hardened by the sun, and at length sufficiently lirm to resist the attacks of the stream. Thus these islands are liable to destruction only by the same process of undermining and encroachment to Nv hich the banks of the river are subject. When an island is found too extensive for cultivating the whole of it, it is soon overrun with reeds and long grass, forming impene trable thickets, and affording shelter to tigers, buffaloes, deer, and other wild animals. The rest of the lands pro duce good pasturage, and feed thousands of cattle. The subsequent inundations fertilizing the soil, to which the burning of the grass greatly contributes, the inhabitants are induced to extend the limits of their cultivation, and settle permanently. The islands of the Ganges are distin guishable from the mainland, by their having few or no trees, even after a communication has been formed by the dosing up of one of the channels, which generally happens in a few years. Dera Khowaspour, one of the largest, has continued longer in an insulated state than any other, ow ing to its peculiar situation, immediately below the con of the Ganges and Doosa rivers. It is 9 miles in length, and at the greatest breadth 21. broad, contain ing about 20 square miles of land, mostly cultivated, with several villages. In the highest floods the inhabitants are obliged to erect temporary huts on pillars of wood, or stages ; but they arc seldom reduced to this necessity.