446 HENRY M. EAKIN 



The Tanana River, the largest tributary of the Yukon, in the last 

 200 miles cuts bed-rock only on the right side and has almost all of 

 its extensive flood plain on the left side. The lower Koyukuk has 

 broad flats to the left and usually rock-cut banks on the right. Spurr 

 notes a similar condition in the Kuskokwim, and Maddren in the 

 Innoko. Though lacking the absolute data on which to base definite 

 measurements of the effect of rotational deflection, the general 

 knowledge of the streams mentioned above seems sufficient to estab- 

 lish the fact of its notable operation in high latitudes. That other 

 causes may produce asymmetrical valleys is not denied. If a larger 

 amount of rock debris is supplied by the tributaries on one side than 

 on the other, the tendency is certainly to crowd the main stream to 

 the opposite valley wall. The thawing of the bank most exposed to the 

 sun would favor greater erosion in that direction. A tilting of the 

 landmass would favor a widening of the meanders on the down- 

 tilted side, but any moderate tilting would have but little direct 

 effect on the lateral erosion of a stream. In affecting the carrying 

 power of the tributaries, however, a notable shifting in the direction 

 of the down-tilting should result. 



However, these processes, singly or combined, cannot account for 

 all the peculiarities observed in the Alaska streams and in the lower 

 Yukon, the most striking instance of unbalanced erosion, there is 

 no evidence of their operation at all. 



Reasoning that, if such notable difference in lateral erosion should 

 be effected in high latitudes, rivers in lower latitudes should show the 

 result of the same influence to some lesser degree, the Missouri 

 River was chosen for comparison. The Missouri has a well-developed 

 flood plain, meanders broadly, and has been engaged in progressive 

 de-gradation probably since late Pleistocene time. The summary 

 results of all the factors directing its erosion during this period of 

 time should be expressed in the present condition of the river and its 

 valley. 



The distribution of the flood plain with respect to the river was 

 measured on the compiled Index Map of the Missouri River Commis- 

 sion, between Sioux City, la., and Kansas City, Mo. The results 

 were: 



Between the river and the left valley wall, 1,370 sq. miles. 



Between the river and right valley wall, 312 sq. miles. 



