SURFACE WATERS 9 



supplies, water is often obtained from artificial ponds. These are 

 usually formed by throwing up a low embankment across some 

 shallow depression or flood-water channel, behind which the rain 

 water or the brief flood flow accumulates. Such reservoirs are com- 

 monly but a few rods in diameter and a few feet in depth. As a 

 consequence the water becomes heated in summer, is usually kept 

 constantly muddy by the movements of stock, and is highly 

 polluted by them. (See Fig. 2.) As numerous animal diseases 

 may be communicated through drinking water, small ponds of 

 this sort may become a source of great danger. 



In hilly regions it is often possible, by erecting a short dam 

 across some small valley or ravine, to pond the water of some 

 spring or brook, forming a small reservoir from which the water 

 may often be piped to the farm buildings below. If the spring or 

 stream furnishing the supply is protected from cattle, from wash 

 or seepage from pastures, roads, and barnyards and from sources 

 of human pollution, such reservoirs will often provide admirable 

 supplies. This is especially true of the farms in valleys bordered 

 by wooded hills such as abound throughout the Appalachian 

 Mountains of the East and the larger mountain systems of the 

 West. 



Streams. Over a large part of the country streams and 

 rivers form the most available sources of supply, and in thinly 

 settled regions they are usually free from contamination, although 

 even here a tan-bark plant or sawmill may lessen the desirability 

 of the water for domestic purposes. 



Mines, especially coal mines, may likewise discharge their 

 drainage of acid and otherwise polluted waters into the streams 

 with similar effects, but the most common source of pollution is 

 the sewage from towns and cities. In fact, practically all the 

 larger streams, and even many of the smaller ones, are highly 

 polluted by such sewage or by refuse from various manufacturing 

 plants. It is true that such streams become gradually purified 

 and under ordinary conditions may be fairly safe, but the periodic 

 outbreaks of typhoid fever that occur among the users of their 



