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348 STRUCTURAL AND FIELD GEOLOGY 
from superficial sources apart from that obtained from under- 
ground stores, although it is obvious that much of the water 
that circulates in our streams and rivers has come from 
springs. 
Lakes and Impounded Streams.—The character of the 
water of a lake naturally depends upon that of the catchment 
area, for it is needless to say that the level of thewlaicemie 
maintained by the rainfall—in other words, by the springs 
and streams that feed it. Should the rocks within the drainage 
area be largely calcareous the water will be ard; should 
igneous and schistose rocks predominate, the water will be 
moderately soft. Other things being equal, the deeper and 
larger a lake is, the purer and colder must the water be. 
Large and deep lakes occupying mountain valleys, like those 
of our own islands, where there is little or no cultivation and © 
not much chance, therefore, of contamination, are always 
desirable sources of water-supply. But every country is not 
so fortunate in the possession of large natural reservoirs, and 
even when these exist they are often so far removed from 
centres of population as to be practically beyond reach. 
Under such circumstances, engineers are required to form 
artificial lakes by impounding streams. 
Reservoirs.—The formation of reservoirs is purely an 
engineering operation, but, like many other undertakings of 
the kind, it ought to be conducted with a full knowledge of 
the geological conditions. If we have the choice of several 
streams, it is obvious we should select the purest. Those 
which are most likely to yield a desirable water-supply will 
usually occur in sparsely cultivated districts which are not 
likely in the future to attract much population, such as the 
high-lying pastoral regions of our own country. Before 
selecting a stream, however, the character of the rocks within 
the drainage-area should be carefully inspected, for the 
purpose of ascertaining whether these contain deleterious 
ingredients which might unduly affect the character of the 
water-supply. Usually, however, careful chemical analyses of 
the water flowing in the main stream at all seasons of the 
year will determine the suitability or otherwise of the supply. 
A desirable stream having been obtained, the engineer’s next 
care is to select a site or sites for his storage reservoirs. It 

