SURFACE WATERS 7 



remove the dangerous impurities. At the same time the heavier 

 sediment brought in by the streams is constantly settling to the 

 bottom. As a result of all these processes of purification the 

 water which entered in a polluted state may at last become so 

 changed that on leaving the lake at its outlet it is both palatable 

 and wholesome. 



So rapidly has this purifying action been supposed to take 

 place that before the construction of the Chicago drainage canal 

 it was thought safe by some to draw the supply of the city from 

 Lake Michigan at a distance of only a few miles offshore. Ex- 

 perience proved, however, that while noticeable pollution was con- 

 fined to a relatively small portion of the lake, and while normally 

 the city supply was fairly safe, it was, nevertheless, under certain 

 conditions of wind and currents, liable to pollution. It was thus 

 necessary either to remove the intake to a point several miles 

 farther away or to divert the sewage from the lake. The Chicago 

 drainage canal, by diverting the sewage, has done much to solve 

 the problem. 



Natural lakes are confined almost wholly to the northern por- 

 tion of the country, where they form a belt extending from eastern 

 North Dakota to eastern Maine. In this belt, besides the Great 

 Lakes, there are tens of thousands of smaller lakes, Minnesota 

 alone having several thousand. 



Most of the lakes, especially the smaller ones, are in thinly 

 inhabited regions and afford supplies of high purity. Unfortu- 

 nately, however, owing to the fact that farmhouses, even if in the 

 vicinity of lakes, are usually placed on high ground at some dis- 

 tance from the water it is necessary, if the lake is used as a source 

 of supply, to haul the water required for domestic purposes. Be- 

 cause of the inconvenience thereby entailed few of the lakes are 

 used, although they constitute ideal supplies for horses and cattle 

 and furnish pure water to those cities which lie within convenient 

 piping distance from them. 



Ponds. In the smaller water bodies, varying from mere 

 pools to lakelets several acres in extent, there is less dilution of the 



