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appareil littoral is almost identical in these very different rivers. The Rhone, one of the most rapid rivers in Europe, rises at the height of 5,772 feet above the level of the sea. In its first descent (of 112 miles out of its entire course of 520 miles), into Lake Leman, it falls no less than 4,555 feet; a descent which gives a slope of 7•4 met. per kilometre, or seven and fourtenths in a thousand. From Lake Leman to Bellegarde the river continues to roll down rocks and large blocks of stone. Below this point commences the gravelly bed, the inclination being reduced to 1.009 m. per kilometre. From Lyons to Beaucaire the inclination of the bed (with the exception of some rapids) varies from 0.5 to 0:3 m. per kilometre, and the velocity of the river is from 105 m. to 2.5 m. per second in ordinary flow, rising to as much as 4 m. per second during freshets. Between Beaucaire and Arles the inclination of the bed of the Rhone is reduced to 0:123 m. per kilometre. The grinding action of the current is by this time complete. The material brought down by the river is reduced to the state of sand or mud; the latter being chiefly contributed by the affluent stream of the Durance, which enters the Rhone near Avignon. At Arles the Rhone divides into two branches : the Grand-Rhone on the left and the Petit-Rhone on the right. The level of the surface of the river here is 1.03 above that of the sea, to reach which it has to flow for some 30 miles further. Velocity and inclination progressively diminish from this point; the mean fall hence being less than 0.03 per kilometre. Thus the formation of thirty miles of delta has not produced a greater elevation, or banking up of the ordinary level of the low-water mark of the Rhone at Arles, than is equivalent to about an inch and a third per mile. This inclination is considerably less than the minimum which is considered necessary to ensure the flow of water through the dykes of our own fen districts. We thus have a proof, at once, of the soundness of the hypothesis above suggested as to the cause of the diramation of the river, and of the absence of change in the level of the Mediterranean itself since the commencement of the historic delta of the Rhone.
We say historic delta, because a pre-historic, or geological period has left evidence of its occurrence at a time when the action of the Rhone and its affluents appears to have been of a more violent nature than has been the case within recent times. Over the vast triangle of which Beaucaire, Cette, and Fos form the angles, stretches a vast deposit of boulders, which is known as the Alpine diluvium. We need not now enter into the question of the mode of formation of this great slope, which gradually loses itself beneath the waters of the Gulf of Lyons. The continuation of the incline beneath the sea is shown by the gradual increase of depth. The line of 50 metres sounding is nearly parallel with the coast at a distance of about 15 miles. A more irregular curve, lying about 3 miles seaward of the 50-metre line off Cape Couronne, and stretching thence towards the Pyrenean promontories, leaving a distance of 24 miles between the centres of the two curves, is bounded by the depth of 100 metres. At some 12 miles, again, to the south, this depth is doubled. The actual Delta of the Rhone is a triangular island of 250 square miles in area, contained between the two previously mentioned arms, which are known as the Grand-Rhone and the Petit-Rhone, in the centre of which is the Etang, or marshy lake of Valcarès, possessing a superficies of somewhat under 30 square miles, and a depth of from one to two metres. The effect of the waters of the Durance, which, rushing through the defile of Lamanon, falls almost at right angles into the Rhone, appears to have determined the extension of the diluvial delta towards the west. The deposit of diluvium, even limiting its area to the space landward of Cette and of Fos, covers seven times the area of the historic delta, formed of the sands and mud of the rivers. It may throw some light on the progress of secular change to notice, that the Etang of Valcarès covers about an eighth part of the present delta.
The bulk of solid matter annually brought down by the Rhone is estimated by M. Surell at seventeen millions of cubic metres. M. Lenthéric does not present us with the data for this calculation, nor with any estimate of the volume of the Rhone, the extent of the area which it drains, the average rainfall over that area, or the proportion of solid matter held either in suspension or in solution by the waters of the river at any portion of its course. Determinations of these data are requisite to enable the engineer to make any calculation as to the relative activity displayed in the zone of erosion and in the zone of deposit, and thus to estimate how much of the annual deportation of the river goes to the formation of visible delta, and how much to the raising of the bottom of the sea, over a larger or smaller area. These questions, indeed, may not assume an European interest in the case of the Rhone. As to the deposits of the Po, the Adigo, the Brenta, and the Danube, they are, however, of very great importance; and in the case of the Nile, the largest of all the delta-forming rivers of the inland seas of Europe, the determination of the disposition of the deposit is a point upon which depends the ultimate maintenance of the
line of communication opened, by the Isthmus of Suez Canal, for the maritime intercourse of Europe and the East. These great rivers are spoken of by the French writers as fleuves travailleurs. It is not, however, the case that the work performed by a river in abrading and eroding its mountain cradle, pulverising its spoil, and bearing down the material to form bars and islands at its mouth, is to be measured by the visible growth of the latter. The collaboration of another workman has to be taken into account. The Thames is not less of a workman river' than the Tiber. But the strong tides of the Channel prevent an accumulation which is normal in the quieter waters of the Mediterranean. It is thus needful to study the destructive and transporting work of a river, independently of any estimate of its activity which may be formed from the growth of its delta. Taking the latter as the sole basis of calculation, it would follow that the deposits of the Nile were now only about one-tenth of their
average annual amount for the entire historic period; and not only so, but that they are considerably less than the actual deposits of the Rhone. M. Lombardini, cited by M. Lenthéric, estimates the annual deposit of the Nile at 40,000,000 metres cube; that of the Rhone being, as above stated, 17,000,000 metres cube; that of the Po 40,000,000 metres cube; and that of the Mississippi 644,000,000 metres cube. The annual growth, or prolongation, of the mouth of the Grand-Rhine is given at 50 metres; that of the Po at 80 metres ; that of the Nile is said now not to exceed from 3 to 4 metres per annum. We shall return to the subject of the formation of the Delta of the Nile. It is, however, apparent from the above figures, apart from any question of rectification, that the measurement of delta mouths alone is far from being enough to give information as to the efficacy of a river as a denuding and degrading agent.
Even in the case of the Rhone alone, it is evident from the facts accumulated by M. Lenthéric, that the action of waves and currents demands as careful and minute a study as does the evidence of actual and visible deposit. Two great promontories mark the angles of the Delta of the Rhone; the Pointe de l'Espignotte, to the west of the present embouchure of the western branch, and the Pointe de Beauduc, to the west of that of the Grand-Rhone. These points advance into the sea at the mean annual rate of 70 metres. But in the coast-line of more than 24 miles which lies between these two promontories, not only is there no corresponding advance, but actual retrogression of the shore is in some parts taking place. A double line of
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 thap 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 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 given to 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