236 RIVER BENDS. 



of ten miles. At the point D, three miles from the dam, it 

 was found by measurement that the surface E was elevated 

 over 15 inches above D. At a distance of 4 miles above the 

 dam, the surface was elevated by the dam 9 inches.* 



127. River Bends. When rivers flow in narrow val- 

 leys, where the banks do not readily yield to the action of 

 the current, the effect of any variation of velocity is only 

 temporarily to deepen the bed. In wide valleys and allu- 

 vial plains, where the soil of the banks is, more easily worn 

 by the current than the bottom, any increase in the volume 

 of the water will widen the bed ; and if one bank yields 

 more than the other, windings or bends will be formed, and 

 these windings which are thus formed tend to increase in 

 curvature by the scouring away of material from the outer 

 bank and the deposition of detritus along the inner bank. 

 The windings sometimes increase till a loop is formed, with 

 only a narrow strip of land between the two encroaching 

 branches of the river. Finally, a "cut-off" may occur, a 

 waterway being opened through the strip of land, and the 

 loop left separated from the stream, forming a lagoon of 

 marsh shaped like a horse-shoe. 



It is usually supposed that the water, tending to go for- 

 wards in a straight line, rushes against the outer bank and 

 scours it, at the same time creating deposits at the inner 

 bank. This view is considered by many engineers as very 

 incomplete. Prof. James Thomson has given an explana- 

 tion of the action at a bend, which he has completely con- 

 firmed by experiment.! He thinks that the scouring at the 

 outer side and the deposit at the inner side of the bend are 

 due to the centrifugal force, in virtue of which the water 

 passing round the bend presses outwards, and the free sur- 

 face in a radial cross-section has a slope from the inner side 

 upwards to the outer side. 



* D'Aubuisson's Hydraulics, Art. 166. 



t Proc. lust, of Mecb. Engineers, 1879, p. 466 



