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~ Parr IL Szcr. ii. § 3.) TRANSPORT BY RIVERS. 367 
1. Transporting Power.1A—One of the distinctions of river water, as 
compared with that of springs, is that, as a rule, it is less transparent, 
in other words, contains more or less mineral matter in suspension. 
A sudden heavy shower or a season of wet weather suffices to 
render turbid a river which was previously clear. The mud is 
washed into the main streams by rain and brooks, but is partly pro- 
duced by the abrasion of the water-channels through the operations of 
- the streams themselves. The channels of the mountain tributaries of 
_a river are choked with large fragments of rock disengaged from 
cliffs and crags on either side. Traced downwards the blocks become 
gradually smaller and more rounded. They are ground against each 
other and upon the rocky sides and bottom of the channel, getting 
more and more reduced as they descend, and at the same time 
abrading the rocks over or against which they are driven. Of the 
detritus thus produced, the finer portions are carried in suspension, 
and impart the characteristic turbidity to rivers; the coarser sand and 
gravel are driven along the river bottom. ? 
The presence of a moving stratum of coarse detritus on the bed 
of a brook or river may be detected in transit, for though invisible 
beneath the overlying discoloured water, the stones of which it is 
composed may be heard knocking against each other as the current 
sweeps them onward. Above Bonn, and again a little below the 
Lurelei Rock, while drifting down the Rhine, the observer by laying 
his ear close to the bottom of the open boat, may hear the harsh 
erating of the gravel stones over each other as the current pushes 
them onwards along the bottom. On the Moselle also, between 
Cochem and Coblentz, the same fact may be noticed. 
_ The transporting capacity of a stream depends (a) on the volume 
and velocity of the current, and (6) on the size, shape, and specific 
eravity of the sediment. (a) According to the calculations of 
Hopkins,* the capacity of transport increases as the sixth power of 
the velocity of the current; thus the motive power of the current is 
increased 64 times by the doubling of the velocity, 729 times by 
trebling, and 4096 times by quadrupling it. Mr. David Stevenson * 
1 On the abrading and transporting power of water, see Login, Nature, i. pp. 629, 
654; ii. p. 72. 
2 These operations of running water may be studied with great advantage on a small 
scale where brooks descend from high grounds into valleys, rivers, or lakes. A single 
flood suffices for the transport of thousands of tons of stones, gravel, sand, and mud, 
even by a small streamlet. At Lybster, for example, on the coast of Caithness, as the 
author was informed by Mr. Thomas Stevenson, C.E., a small streamlet carries down 
annually into a harbour, which has there been made, between 400 and 500 cubic yards 
of gravel and sand. A weir or dam has been constructed to protect the harbour from 
the inroad of the coarser sediment, and this is cleaned out regularly everysummer. But 
by far the greater portion of the fine silt is no doubt swept out into the North Sea. The 
erection of the artificial barrier, by arresting the seaward course of the gravel, reveals to 
us what must be the normal state of this stream and of similar streams descending 
from maritime hills. The area drained by the stream is about four square miles; con- 
sequently the amount of loss of surface, which is represented by the coarse gravel and 
sand alone, i8 ys4p5 Of a foot per annum. 
3 Q. J. Geol. Soc. viii. p. xxvii. 
* “Canal and River Engineering,” p. 315. 
