194 



GEOLOGY. 



brought the part of the flood-plain (though not necessarily the bot- 

 tom of the channel) adjacent to the stream above the level of that 

 farther from it (Fig. 178). The change is likely to be effected in time of 

 flood. 



Flood-plains often attain great size. That of the Mississippi below 

 the Ohio (Fig. 179) has a width ranging from rather more than 20 miles at 

 Helena (Ark.), to something like 80 miles in the latitude of Greenville 

 (Miss.).^ Below the Ohio its area is something like 30,000 square miles, 

 and its entire area has been estimated at about 50,000 square miles.^ 



Theoretically, the rotation of the earth should affect the erosion of 

 streams, increasing it on the one bank (the right in the northern hem- 

 isphere and the left in the southern) and decreasing it on the other.^ 

 The streams doubtless accommodate themselves to the rotation of 

 the earth in the original development of their gradation-plains and 

 flood-plains, and the later effects of rotation are usually inconspicuous. 



Fig. 184. — Meanders of Trout Creek, Yellowstone Park. (Walcott, U. S. Geol. Surv.) 



Scour-and-fiU. — It has already been shown that aggrading streams 

 cut laterally at the same time that they build up their plains. It is 



1 According to map published by the Mississippi River Commission in 1887. 



2 Russell. Rivers of North America, p. 114. 



3 Gilbert. Am. Jour. Sci., Vol. XXVII, 1884, pp. 427-34. 



