THE LATERAL EROSION OF STREAMS 445 



strength and effectiveness depend upon the amount of difference 

 in velocity in a given distance. 



The lateral shifting of the locus of maximum velocity and the 

 downward deflection of the stronger upper currents adjacent to the 

 outer bank on curves mark these points as loci of maximum erosion. 

 But equally important in the development of meanders is the deposi- 

 tion of the debris resulting from the cut, normally on the inside of 

 the next curve below the cut, and on the same side. Early in the 

 development of a meander the lateral currents are weak. The debris- 

 laden currents are not shifted far toward the center of the stream 

 before the reversal occurs, their carrying power is changed, and 

 deposition takes place. In more developed meanders the lateral 

 currents being stronger, the loaded parts of the currents are shifted 

 farther and carry their burden a greater distance. In development 

 of meanders, then, the debris is deposited farther and farther from 

 its source, but normally on the same side. When the meanders 

 are strongly enough developed, the load should be shifted entirely 

 across the channel and back again on subsequent curves, and in this 

 may lie an important factor in controlling the width of meander 

 belts. 



If, however, in the case of a stream having a straight course, 

 currents are set up by a force other than that due to curvature, the 

 debris will be shifted constantly in the same direction. There will 

 be no deposition below the cut on the same side, and consequently 

 no deflection of the stream. The lateral currents, being constantly in 

 one direction, will mark one side of the stream for dominant cut, 

 and the other for maximum fill. 



Whether due to F^ or F^, the selective cut and fill due to the 

 lateral currents introduce and maintain an asymmetry in the channel 

 profile which, in turn, further modifies the distribution of mass and 

 consequently of velocities in the stream cross-section. The result 

 is to accentuate the selective cut and fill due directly to the lateral 

 currents. The slightly increased depth on the side of the stream 

 toward which the lateral force acts furnishes still another, though 

 probably small, factor in favor of the same selective erosion. 



If the foregoing principles are true, we have an adequate explana- 

 tion of the peculiarities shown by the Yukon and other Alaska streams. 



