THE LATERAL EROSION OF STREAMS 447 



In the upper part of the river, from Fort Benton, Mont., to 

 Sioux City, la., we still have what might be termed the first genera- 

 tion of meanders, that is, they are still in a developmental stage and 

 swing first against one valley wall and then the other. 



The form of these meanders, of such simple history, is significant. 

 Those to the left of the mean axis of the stream are as a rule sharper 

 than those to the right, the river is in contact with the right valley 

 wall more often than with the left, and those reaches where the river 

 is in contact with the right valley are longer generally than those 

 where the contact is with the left valley wall. This latter character- 

 istic becomes more and more notable down-stream. 



In the reach from Fort Benton to Sioux City meanders have been 

 cut off in five cases. Of these, four are on the left side of the stream 

 and one on the right. However, in the case of the individual meander 

 on the right, instead of receding from the loop, as is normal when 

 cut off by differential migration, the river is at present encroaching 

 upon it, making it very probable that the case represents domestic 

 piracy rather than the former process. In each test stronger erosion 

 on the right seems evident. 



The influence of the earth's rotation, then, is to unbalance the 

 lateral erosion of streams, in the northern hemisphere directing the 

 stronger erosion on the right bank, and in the southern, on the left. 

 The deflective force is much stronger in the higher latitudes. Its 

 influence is felt by streams both with and without meandering courses 

 and is most effective in streams with straight courses. Examination 

 of streams in both high and mean latitudes reveals conditions of 

 unbalanced lateral erosion that seem best interpreted as the result 

 of rotational deflection. 



