_ Parr IL Scr. ii. §3.] RIVER TRANSPORT. 369 

closes gravel, sand, and even blocks of rock, which, when thaw comes, 
are lifted up and carried down the stream. ‘The rivers of northern 
Russia and Siberia, flowing from south to north, have the ice thawed 
in their higher courses before it breaks up farther down. Much 
_ disaster is sometimes caused by the piling up of the ice, and then by 
the bursting of the impeded river through the temporary ice-barrier. 
In another way ice sometimes vastly increases the destructive 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 extended across the valley for more than 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. | 
The 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. ‘The proportion of 
sediment 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 days or weeks may 
be the means of sweeping a vast amount of earth into the rivers, and 
- sending them down in a greatly discoloured state to the sea. Thus 
the rivers of India, swollen during the rainy season (by sometimes a 
rainfall 25 inches in 40 hours, as at the time of the destructive land- 
slip at Naini Tal in September 1880), 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, 60 or 70 yards 
wide, and waist-deep. 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 percolating 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 disintegrated 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.” 
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 complete analysis should 
take into account what is chemically dissolved, what is mechanically 
suspended, and what is driven or pushed along the ce We have 
B 
