Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/289

Rh R1vERs.] powers of small streams where avalanches or an advancing glacier cross a valley and pond back its drainage. The valley of the Dranse, in Switzerland, has several times suffered from this cause. In 1818 the glacier barrier ex’- tended across the valley for more tl1an half a mile, with a breadth of 600 and a height of 400 feet. The waters above the ice-dam accumulated into a lake containing 800,000,000 cubic feet. By a tunnel driven through the ice, the water was drawn off without desolating the plains below. Th-at rivers differ vastly from each other in the amount of material they transport is made evident by the great diver- sities in their relative mud.diness. It should be borne in mind that the actual amount of sediment borne downwards by a river is not necessarily determined by the carrying power of the current. The swiftest streams are not always the muddiest. Tl1e proportion of sed.i1nent is partly dependent upon the hardness or softness of the rocks of the channel, the number of tributaries, the nature and slope of the ground forming the drainage basin, the amount and distribution of the rainfall, the size of the glaciers (where such exist) at the sources of the river, &c. A rainfall spread with some uniformity throughout the year may not sensibly darken the rivers with mud, but the same amount of fall crowded into a few weeks or months may be the means of sweeping a vast amount of earth into the rivers, and send- ing them down in a greatly discoloured state to the sea. Thus the rivers of India during the rainy season become rolling currents of mud. In his journeys through equatorial Africa Livingstone came upon rivers which appear usually to consist more of sand than of water. He describes the Zingesi as “ a sand rivulet in ﬂood, 00 or 70 yards wide, and waist—dcep. Like all these sand-rivers it is for the most part dry; but, by digging down a few feet, water is to be found, which is per- colating along the bed on a stratum of clay. In trying to ford it,” he remarks, “ I felt thousands of particles of coarse sand striking my legs, which gave me the idea that the amount of matter removed by every freshet must be very great. . . . These sand rivers remove vast masses of disin- tegrated rock before it is ﬁne enough to form soil. In most rivers where much wearing is going on, a person diving to the bottom may hear literally thousands of stones knocking against each other. This attrition, being carried on for hundreds of miles in different rivers, must have an effect greater than if all the pestles and mortars and mills of the world were grinding and wearing away the rocks.” The amount of mineral matter transported by rivers can be estimated by examining their waters at different periods and places, and determining their solid contents. A com- plete analysis should take into account what is chemically dissolved, what is mechanically suspended, and what is driven or pushed along the bottom. We have already dealt with the chemically dissolvel ingredients. In determin- ations of the mechanically mixed constituents of river water, it is most advantageous to obtain the proportion ﬁrst by weight, and then from its average speciﬁc gravity to estimate its bulk as an ingredient in the water. The Ganges, according to Everest, contains during the four months of ﬂood earthy matter in the proportion of ¢{—§ by weight or 3%g by volume,——the mean average for the year being 3%; by weight or -1-613-5 by volume. According to Mr Login, the waters of the Irrawaddy con- tain T-,.1m; by weight of sediment during ﬂoods, and 5,1: 5 «luring a low state of the river. The most elaborate measure- ments and calculations yet made regarding this aspect of the operations of a river are those by Messrs Humphreys and Abbot on the Mississippi, who found, as the mean of many observations carried on continuously at different parts of the river for months together, that the average GEOLOGY proportion of sediment contained in the water of the Mississippi is Tglm by weight, or —§-51-0-5 by volume. But besides the matter held in suspension, they observed that a large amount of coarse detritus is constantly being pushed along the bottom of the river. They estimated that this moving stratum carries every year into the Gulf of Mexico about 750 million cubic feet of sand, earth, and gravel. Their observations led them to conclude that the annual discharge of water by the Mississippi is 19,500,000 million cubic feet, and consequently that the weight of mud annually carried into the sea by this river must reach the sum of 813,500 million lb. Taking the total annual contributions of earthy matter, whether in suspension or moving along the bottom, they found them to equal a prism 268 feet in height, with a base of one square mile. 2. Excavating I’ower.—In transporting its freight of sediment a river performs a vast amount of abrasion. In the ﬁrst place it rubs the loose stones against each other, breaks them into smaller pieces, rounds off their edges, reduces them to rounded pebbles and finally to sand or mud. In the next place by driving these loose materials over the rocks it wears down the sides and bottom of its channel which is thereby widened and deepened. The familiar effect of running water upon fragments of rock, in reducing them to smoothed rounded pebbles, is expressed by the common phrase “ water-worn.” Every stream which descends from high rocky ground may be compared to a grinding mill; large boulders and angular blocks of rocks, disengaged by frosts, springs, and general atmospheric waste, fall into the upper end, and only ﬁne sand and silt are discharged into the sea. M. Daubrée has instituted some ingenious experiments for ascertaining the circumstances under which angular fragments are converted into rounded pebbles with the production of sand and mud. Using fragments of granite and quartz, he caused them to slide over each other in a hollow cylinder partially ﬁlled with water, and rotating on its axis with amean velocity of 0'80 to 1 metre in a second. He found that after the_ ﬁrst 25 kilometres (about 153} English miles) the angular fragments of granite had lost {U of their weight, while in the same distance fragments already well-round.ed had not lost more than T%6 to ;%6. The fragments rounded by this journey of '25 kilometres in a cylinder could not be distinguished either in forn1 or in general aspect from the natural detritus of a river bed. A second product of these experiments was an extremely ﬁne impalpable mud, which remained sus- pended in the water several days after the cessation of the movement. During the production of this ﬁne sediment the water acted chemically upon the granite fragments, for after a day or two it was found, even though cold, to have dissolved a very sensible proportion of silicate of potash. After a journey of 160 kilometres, 3 kilogrammes (about 6,15 lb avoirdupois) had yielded 3'3 grammes (about 50 grains) of soluble salts consisting chieﬂy of silicate of potash. A third product was an extremely ﬁne angular sand consisting almost wholly of quartz, with scarcely any felspar, almost the whole of the latter mineral having passed into the state of clay. The sand grains, as they are con- tinually pushed onward over each other upon the bottom of a river, become rounded as the larger pebbles do. But, as M. Daubrée points out, a limit is placed to this attrition by the size and specific gravity of the grains. So long as they are carried in suspension they will not abrade each other, but remain angular ; for he found that the milky tint of the Ilhine at Strasburg in the months of July and August was due not to mud but to a ﬁne angular sand (with grains about {U millimetre in diameter) which constitutes -n,—Q%1;1y 0f_t11B total weight of water. Yet this sandhadtravelled 111 arapidly ' ﬂowing tmnultuous river from the Swiss mountains, a11d had