CHAPTER II. 

 SURFACE WATERS. 



Sources. Under surface waters, as the term is here used, are 

 included those waters which occupy basin-like depressions in the 

 surface, giving rise to lakes and ponds, or flow as streams down 

 its valleys. The waters of such lakes and streams do not repre- 

 sent simply the collected rainfall from the surface, by far the 

 greater part coming, as a matter of fact, from the ground and rep- 

 resenting the surplus precipitation which was first absorbed by 

 the soils and rocks and later set free to swell the surface drainage. 



Springs are intermediate between ground water and surface 

 supplies, and are classed sometimes with the one and sometimes 

 with the other, according as the ground water origin of the supply 

 or the surficial situation of the spring is considered as most sig- 

 nificant. In the present discussion they are considered in a 

 chapter by themselves. 



Besides the natural surface-water bodies, which include the 

 lakes, ponds, pockets and "tanks" on the one hand and the 

 brooks, streams and rivers on the other, there are the artificial 

 pools, ponds, and reservoirs, all of which, under certain circum- 

 stances, are in common use as sources of farm supplies. . 



Lakes. - - The waters of lakes, which here include all fresh- 

 water bodies a mile or more in diameter, 'are generally good, 

 except when they are polluted by the drainage or sewage from 

 cities or large towns on their shores or on the lower courses of 

 tributary streams. In ponds and in the smaller lakes such pollu- 

 tion may render the entire body of water unsafe for domestic use. 



Sunlight, however, has a marked purifying action, tending to 

 destroy the dangerous germs, while, under the action of winds, 

 wave action and circulation are induced in the water, favoring 

 aeration (mixture with air), which, by oxidation, likewise helps to 



