FRESH-WATER STREAMS. 637 



the ocean than in the interior of the country, hecause of the easy- 

 discharge through the mouth. Owing to the height of the waters, 

 which often cover the banks, the course loses some of its minor bends ; 

 and the whole distance is therefore less ; while the inequalities of pitch 

 between the still water and more rapid portions tend to disappear in 

 a broad open channel. When a river runs through a narrow, rocky- 

 gorge, the waters above the entrance of the gorge are partially held 

 back, and have less pitch during freshets than at low water ; and con- 

 sequently the pitch through the course of the gorge is increased. 



3. Flow of a Stream. — The above conditions affect directly the 

 velocity of the stream, as this varies with the pitch and depth of 

 water. The sudden expansion in size and depth of a river-channel, as 

 when a lake intervenes, also affects the velocity, often producing seem- 

 ingly a state of nearly perfect quiet. The water-level becomes for 

 the interval nearly horizontal. The quiet at the whirlpool, in the 

 rapids below the Falls of Niagara, is accounted for on the ground of 

 the great increase of depth and the abrupt expansion in breadth. 



The movement of a stream is most rapid near the surface. The 

 bottom, sides, and air retard by friction the layer in contact with 

 them ; and other adjoining layers are retarded through the cohesion 

 between the particles of the water. The velocity is greater, the less 

 the extent of the upper (or air) and bottom surfaces, — the surfaces 

 of friction. When two streams unite, the waters have the surfaces of 

 friction of one stream instead of two, and there is consequently an 

 increased rate of flow; and, besides, owing to the greater velocity, 

 the united waters do not occupy a space equal to the sum of those 

 which they occupied before the union. 



Other characteristics of rivers are brought out in the following: 

 pages. 



2. Mechanical Effects of Rivers. 

 The mechanical effects of fresh waters are, — 



1. Erosion, or wear. 



2. Transportation of earth, gravel, stones, etc. 



3. Distribution of transported material, and the formation of frag- 

 mental deposits. 



1. Erosion. 



1. General Statement of the Effects of Erosion. — The effects of erosion 

 are seen, first, in the imprint of the falling rain-drop, — a trifling 

 matter to most eyes, but not so to the geologist ; for it remains among 

 the records of the earliest and latest strata, to show that it rained 

 then as now, and to teach us where the lands at the time lay above 



