658 DYNAMICAL GEOLOGY. 



or less, or to the surface, according to the degree of slowness, the 

 eddying currents, and the supply and fineness of detritus. The trend 

 of the shores, pitch of the bottom, and other causes, locate the swifter 

 currents in the channel, and thereby tend to locate the banks or reefs. 

 A stranded log may change the course of the former, and thereby the 

 positions of the latter ; and so may high floods, and also the growth 

 of bars. The lodging of drift-wood on a sand-bar may serve to in- 

 crease the accumulation over it, and so change the bar into a wooded 

 island. Moreover, high floods rob the bars at the same time that they 

 add to them, or they may sweep them away, even if already an island, 

 to form other bars and islands. They push along the movable detri- 

 tus of the river's bottom, and also drop more to keep it generally at 

 the old level. Thus all is movement and change along a river's chan- 

 nel. In a bend, the concave side receives the stroke of the waters 

 and is the deep-water side, and the convex is usually a shallow point 

 made partly of material from the opposite. 



When two rivers unite they often make shoals for one another, one 

 throwing a bar across the channel of the other through the descending 

 detritus of flood waters. The waters of the Upper Mississippi are 

 pushed to the opposite shore by the contributions of a tributary, and 

 a deep, still-water, navigable area made above the junction, and rapids 

 below. Further, the tributary, if not in flood at the same time, will 

 have its mouth filled with sand-bars by the greater river, and often, 

 also, in spite of its floods, if not too rapid in the flood. This subject 

 is well illustrated in Reports on the Mississippi and its Tributaries by 

 General G. K. Warren. 



2. Over the Flood-grounds. — The flood-grounds are under water only 

 in times of floods, and so the flow over them is less rapid than over 

 sand-bars. They have, therefore, a still better chance to get the de- 

 tritus of the flooded stream. Moreover, the laden waters rest long over 

 their lower parts after the flood ceases. The flood-grounds lose from 

 their surface by the floods, and may, in parts, be cut away to open 

 new channels ; but they generally gain as much as they lose, or more. 

 Along the sides of the channel they are usually built up higher than 

 , elsewhere, owing to the material lodged there by the stream, in its 

 frequent occasions of moderate floods when the waters scarcely rise 

 over them, being often detained by bushes. Along the Lower Missis- 

 sippi the pitch of the flood-plain from the river amounts, on an aver- 

 age, to seven feet for the first mile. (H. & A.) 



The deposits of the flood-grounds may be the finest of silt, or the 

 coarsest of gravel and stones, according to the region and the pitch of 

 the stream. The course of a tributary from a mountain region over 

 the flood plain is often marked by a wide bed of stones. 











