870 



SCIENCE 



[N. S. Vol. XXIX. No. 752 



the position of the western bank of the river 

 in 1883 and the continuous line represents the 

 position of the western bank in 1903. Loca- 

 ting the letters X, X' and X" at the inter- 

 sections of the old and new channels ; then, all 

 the territory between the old channel and the 

 new and between X and X' has been washed 

 from the Nebraska side, while the correspond- 

 ing territory between X' and X" has been 



added to the Nebraska side. From the point 

 a' to the point 6 is 5,000 feet, aU of which 

 has accumulated in twenty years, or an annual 

 increase of 250 feet. The actual increase has 

 varied, however, from 50 to 500 feet annually. 

 At no time during the twenty years has the 

 channel between X' and X" encroached upon 

 the Nebraska shore. On the other hand, the 

 river has annually encroached upon the Ne- 

 braska shore between the points X and X' for 

 the same period of time. 



The river, at Peru, has a descent of about 

 eleven inches per mile and the rate of flow 

 varies greatly. During high waters, in the 



spring months when the principal washings 

 take place, it is found that at the points a and 

 a" the water along the western shore has an 

 elevation of 20 to 50 inches above that of the 

 eastern shore, directly opposite. This piling 

 up of the water along the western shore causes 

 a part of the water to turn up stream, pro- 

 ducing immense eddies at times a mile or 

 more in circumference. The principal part of 

 the water, however, is directed down the 

 stream diagonally toward the opposite shore 

 where the water is at a much lower level. 



The most rapid erosion usually takes place 

 in early spring at the time the ice breaks in 

 the river; frequently, however, the June 

 freshets play great havoc with the banks. The 

 rate of erosion at the upper half of the curve 

 is not the same as the deposition at the lower 

 end of the same curve. For example, in 

 twenty-four hours there may be from ten to 

 twenty-five acres washed from the shore aX', 

 while the accretion to the shore line X'a" is 

 very gradual. As the channel encroaches 

 upon the upper portion of the curve it shifts 

 farther from the lower portion. As the main 

 channel recedes from the shore the space is 

 usually transformed into eddies which supply 

 the necessary conditions for depositing large 

 quantities of detritus. In the shifting of the 

 channel the symmetry of the curve is usually 

 quite well preserved with reference to upper 

 and lower halves. 



Another factor enters our problem : the gen- 

 eral direction of the river is southward and 

 it is gradually flowing over larger and larger 

 circles of the earth. Hence, the water piled 

 up at a starts off in the direction of X' with 

 the impetus given it from the piling up of 

 the water. This rebounding force is counter- 

 acted by the influence of the rotary action of 

 the earth by the time the water reaches X'; 

 so, beyond the point X' the rotary action of 

 the earth becomes the dominant factor and 

 the water gradually falls behind until it again 

 cuts the western blufi's at a". Furthermore, 

 the height to which the water is elevated at a 

 determines, in a large measure, the distance 

 aX' while the distance X'a" determines the 

 height to which the water is piled up at a". 



