Sivut kuvina
PDF
ePub

towers, necessarily erected as at once signals and defences for the entrance of the river, marks the gradual and secular prolongation of the banks forming the mouth of the Rhone. The custom of erecting such structures is mentioned by Strabo. On the left bank the towers of Mauleget, St. Arcier, Parade, and Beloare bear witness to secular changes. On the right, below the towers of Mondovi, Vassale, and Le Graux, exist the tower of Sampau, built in 1614, that of St. Ernest, built at the embouchure of the Bras-de-Fer, or old Rhone, in 1656, and that of St. Louis, built in 1737. This last semaphore tower was erected on the shore. It is now more than seven kilometres distant from the sea.

While data such as the above bear unmistakable evidence as to river deposit, the general problem is complicated by the effects of storm waves and of littoral currents. The predominating action of the sea in the Gulf of Lyons beats from the south-east. The direction of the prevailing winds, and of the most violent storms, is a point or two further towards the west. The south-easterly wind blows for from five to six times the number of days during which the south-westerly gales prevail, and, indeed, for more than eight months out of twelve. The littoral current from east to west attains a velocity of from .06 metres to :3 metres per second in calm weather, and from 1.5 metres to 2 metres, and even to 3 metres in storms. Under this influence actual erosion of the shore of the Camargue, or Rhone delta, is in progress. The lighthouse of Faraman was built in 1836, at about 700 metres from the sea. It is now condemned. A semaphore was placed, in 1852, at 30 metres in advance of the lighthouse. It has been destroyed for two years. There is a depth of 25 metres of water at the spot occupied a hundred and fifty years ago by the Pointe de Faraman; and although the advance of the sea is less rapid than formerly, it is still maintained at the rate of 15 metres per annum.

The semaphore is drowned; the Pharos is not more than 50 yards from the sea ; in three or four years more it will no longer exist.

It is matter rather of special than of general interest, to trace the varied action of the river and the sea to the controlling causes. The chief interest of the phenomena of the delta of the Rhone to the engineer, the historian, or the statesman, concerns not so much the local movement, as the light thrown by such movement on the general laws of the deposit made by large rivers in tideless seas. As to this, the detailed study of M. Lenthéric is of no little value, although in the parallel which he attempts to establish between the action of the European rivers and that of the Nile, he omits the due consideration of that important element, the littoral current, which we have just seen to play so important a part in the erosion of the shore of Faraman, and the filling up of the Gulf of Fos. Unresting activity is the great characteristic of the delta-forming power of the Rhone. The steady growth of land, and retrogression of the sea, are the result of this activity. But such growth and retrogression are not simple and regular. They do, indeed, follow certain controlling laws; but the application of those laws not only differs in each locality, but varies according to the effects produced by the position of the deposits themselves. The general course pursued by a river in the formation of its delta is, briefly, this. When the descending current has reached the level of the sea, and the channel has been permanently formed down to what becomes the point of diramation, the check the movement of the stream causes the precipitation of a cone of sand. The river, parted by this constantly accumulating obstacle, continues to form its own banks on either side, and thus lines its course as it advances through the sea with constantly extending walls. With the variations in height caused by floods the river overflows these newly-formed barriers, and thus precipitates a layer of sand or mud sloping gently outwards from the stream. At points determined mainly by the littoral currents the formation of the bank is checked, and the material deposited is partly swept away by the current, and either spread over the bottom of the sea, or deposited in a cordon, spit, or belt of sand at an angle to the direction of the river. These cordons, increased by the action of the waves, especially during storms, shut off pools from the main sea, which at first are open to internal navigation, then gradually become filled up by deposits from the river floods; then encourage a rank fluviatile and marshy vegetation, and finally are warped up into rich and productive soil. The whole series of phenomena-formation of berge or river bank, of cordon, of étang or marsh, and finally of reclaimed soil -- which the French engineers include under the term of growth of the appareil littoral, may be traced in various stages of their progress at the embouchure of each of the great rivers which enter the Mediterranean and its affluent lake, the Black Sea.

The locality in which the action of the English rivers in effecting an alteration of the shore-line may perhaps be studied with most advantage is the remarkable lagoon formed by the confluent streams of the Avon and the Stour, immediately below Christchurch. The ancient bluff of Hengistbury Head, still furrowed by the defensive lines of the old Saxon invaders, stands out in the long, hollow range of coast reaching from Hurst Castle to Studland Bay, and on to Durlston Head, causing the shore line to present the plan of a double curve, somewhat similar to that marked in the air by the wings of a large bird. When the ordnance survey of this part of England was completed, in 1811, the area immediately to the north of the promontory presented a plan closely resembling that of a Roman post. A true lagoon then reached for the 11 mile of distance from the confluence of the two streams to the bar thrown up by the tide, which ran in a north-easterly direction from the end of Hengistbury Head to a promontory on the opposite mainland. In the middle of this bar was an opening, which looks on the survey exactly like an artificial entrance between two well-built walls. Within was a capacious basin, into which, however, thin lines of sand protruded from the mouths of the rivers, like the berges we have before described in the cases of the Rhine and of the Nile. But, by .1848, when the fourth sheet of the survey of the south coast of England was completed by Captain Sheringham, R.N., the appareil littoral had undergone a marked change. The greater part of the former lagoon had been transformed into marsh or into meadow, through which the confluent streams ran in a distinctly marked and curving channel. The central opening in the bar had disappeared, having been entirely choked by the action of the waves, and the escape of the water now takes place through a mouth more than a mile eastward, below Highcliff castle. Thus the formation of the berge, that of the cordon or bar, that of the lagoon, and that of the ultimate marsh and meadow, are illustrated in this beautiful spot by careful and exactly dated surveys. The face of the shore within half a century has undergone far more change than is apparent on the secular walls of the noble priory church that has looked down for 800 years on the activity of the rivers.

pure and dry is the air that the graceful decorations carved by the Roman masons on the pannelled walls of the sacred building are as sharp and clear as if they had been cut within the century. It is the work of man here that assumes permanence, while that of nature undergoes such comparatively

sudden change.

An approximate estimate of the area of the gathering grounds of the Rhone and its affluents has been given by Professor Ansted, in a paper on Lagoons and Marshes, which was read at the Institute of Civil Engineers on February 16, 1869. This paper gave the fullest account of the Rhone delta that

VOL. CXLV. NO. CCXCVII.

[ocr errors]

we have met with before the publication of the work of M. Lenthéric, and the detailed account of the lagoons may still be read with interest. The watershed drained by the Rhone is stated in this paper at 37,000 square miles; but a note gives a correction to the effect that French geographers have lately given as the drainage area, in France alone, 45,884 square miles. A discrepancy of this amount in a special study of the subject is, at all events, a proof that the subject is not yet thoroughly mastered. If we may rely on Mr. Ansted's figures, the area covered by lagoons and marshes is in the proportion of a little more than two acres to every square mile of watershed basin ; or in round numbers, about the three-hundredth part of the larger area. As to the rainfall, the information is but fragmentary. From 1857 to 1864, the mean rainfall at Montpellier was 36.58 inches. Over the Camargue the rainfall is said to be about one-fifth less. But what occurs in the upper part of the river's course is unknown. A long series of observations, carried on at properly distributed points, is necessary in order to arrive at clear information on a subject so deeply affecting the well-being of France. That showers and storms of great violence occasionally burst on the cradle of the Rhone and its affluents is well known. In October 1868, as much as ī inches of rain is said to have fallen in 24 hours in the neighbourhood of Montpellier. If we assume the English average of 36 inches as that of the water-shed basin of the Rhone, we shall find that the annual rainfall over that area gives a total quantity of 150 milliards of tons, or 66 per cent. more than the measured volume poured into the Mediterranean by the Nile. The chief value of this comparison is the lesson which it points as to the need for ascertaining discharge, as well as rainfall. From its confluence with the Atbara the Nile runs for 24 degrees of latitude without receiving a single affluent. Its loss by evaporation in that distance materially reduces its volume. Were its course sufficiently prolonged, not a drop of its water would reach the sea, except in the season of flood. Yet no doubt can be entertained that the rainfall over the watershed of the Nile must be enormously greater than that of the basin of the Rhone.

Some valuable hydrometric observations on the River Tiber have been abstracted, in the Foreign Transactions of the Institution of Civil Engineers, from the Giornale del Genio • Civile' of 1875. From observations taken for a period of eight years, Signor Venturoli has calculated that the mean amount of the water brought down by the Tiber is 10,000 cubic feet per second. In 1870 the total average flood of water in the valley of the Tiber was 213,900 feet per second; the flood water being calculated to be double that of the Po in relation to the area of its basin. The rainfall area of the Tiber is estimated at 6,455 square miles. The rainfall registered at Perugia is considered to be equal to the average fall over the whole basin of the Tiber. This is stated by Venturoli at 34:8 inches; one-fifth of which is deducted for loss by evaporation and otherwise, leaving an annual supply of 27•3 inches for feeding the river. The advance of the delta of the Tiber is measured by the obliteration of the ancient ports of Trajan and of Claudius. According to plans collected by Sir John Rennie, the retrogression of the sea here is at the rate of about two yards per annum.

But this is not so much the advance of a projecting delta, as the gradual augmentation of a line of sea-board of undetermined length, lying within the great curve of 110 miles of coast, stretching from Capo Farnesio to Capo D'Anzo. The solid deposit of the Tiber is not estimated in the paper cited.

The action of the river that drains the great LombardoVenetian basin possesses an importance, not only from engineering considerations but from historic associations, scarcely inferior to that of the movement of the Nile itself. A frequent feature in river systems is the confluence of one stream with another, often at an obtuse angle to its course, and often near its mouth. Not unfrequently it is the case that the affluent stream drains a different description of country from that which feeds the principal river. In such cases, the junction is that of a torrent with a stream of permanent flow, as in the instances of the Parana and the Uruguay, and of the Nile and the Atbara. In the Lombard plain a somewhat different arrangement has been effected by the engineering of nature. The Tanaro, rising in the Maritime Alps; the Po, springing from Monte Viso; the Dora Riparia, reaching from Mont Cenis; the Dora Grossa, descending from Mont Blanc and Mont St. Bernard, and the Sesia, flowing from Monte Rosa, converge above the confluence, near Pavia, of the Ticino with the united streams. A fan-shaped network of water-ways is thus formed, extending over a circle, roughly measured, of some 80 miles radius from a point near Vercelli, and draining a basin girded by the loftiest summits of the Alps, and covering more than 20,000 square geographical miles of ground. The lakes of Como, Iseo, and Garda send down their surplus waters from the north and north-west to swell the main stream of the Po. But below Mantua, and through the area of that ancient Eridanic delta within which the lake of Comacchio, as well

« EdellinenJatka »