POLLUTION OF THE POTOMAC RIVER 347 



cuts tlirouoh the successive mountain ridges tlie slope is rapid, 

 but there are no falls of considerable magnitude until the stream 

 has passed the Piedmont plateau and reached the border of the 

 softer Cretaceous rocks. Here, at the fall line, it forms a succes- 

 sion of cataracts, a drop of 90 feet occurring within a short dis- 

 tance at the Great Falls. At about this locality the city of Wash- 

 ington has been placed, its situation being governed ])y the 

 questions of navigation and of water-power. These have been 

 the factors which have contributed largeh' to the growth and 

 development of important cities along what is known as the Fall 

 line, extending from New York to Georgia. Just before the river 

 takes its plunge over the Great Falls a small portion of its water 

 is diverted into an aqueduct, which, following along the north 

 side of the river, delivers by gravity a supply of water to the 

 reservoirs, which in turn feed the water system for the city. 



One of the most notable features is that the river receives only 

 a few short streams from tlie north, the greater portion of the 

 water coming from the south and flowing northeasterly in the 

 nearly parallel narrow valleys l)etween the mountain ridges. 

 The rivers meander through trough-like valleys, cut out mainly 

 in limestone, the bounding ridges being of sandstone or other 

 hard resisting rocks. These ridges rise to heights of 2,000 feet 

 or more and are usually forestclad. The precipitation from these 

 ridges, usually in the form of rain, is })artly eva[)orated or taken 

 up by vegetation, about 50 per cent or a little more flowing down 

 the steep hillsides to the valleys. 



This run-off water is pure and clear, but ui)ou reaching the 

 lowlands it mingles with tiie washings from the rich soils and 

 cultivated fields, and becomes, in time of flood at least, turbid 

 and yellow. The lowlands, especially of the wide valleys such 

 as the Shenandoah, are notably rich, and pros|)erous farms are 

 to be found their entire length. These have induced the growth 

 of villages and towns, some of which, under the stimulus of 

 small manufacturing industries, are rai)idl3^ growing. As a rule 

 these are situated upon some stream, since their location has 

 been primarily determined by a water-power mill or ford. The 

 refuse from all these towns is, as a matter of course, discharged 

 into the stream. 



Potomac river, as the name is commonly applied, results from 

 the union of the North branch, the stream above which Cum- 

 berland, Md., is located, with the South brancli. at a point about 

 12 miles below this citv. The North branch and the main 



