274 Mr. A. Tylor on Changes of the Sea-Level effected by 



country the large quantities of silt brought into them by winter 

 freshets do not tend to choke the channels, but that, at that 

 period of the year, former accumulations of deposit are actually 

 removed by the force of the stream ; and therefore, that althougn 

 winter-freshets bring down silt with them, they carry into the 

 sea a larger quantity than they have introduced into river chan- 

 nels. If it were allowable to assume that the unequal supply of 

 water at different seasons of the year produces effects in the chan- 

 nel of the Mississippi similar to these just described on our own 

 streams, the following account would represent the course of 

 events. The diminution of the speed of the current of rivers 

 assists the deposition of silt upon their beds, as much as its 

 increased speed in the winter season favours its removal. The 

 summer deposit, however thin it may be, cannot occur without 

 contracting the sizes of the channel. 



Winter-freshets following a sudden fall of rain would raise the 

 water-level of rivers rapidly, and carry it above the banks before 

 the augmented current has time to scour the river-channel and 

 raise it to its former capacity. Accumulations of silt, small at 

 any one place, must each raise the water a little above its proper 

 level, and the point of overflow will be where the sum of these 

 small elevations amounts to more than the height of the banks 

 above last year's level. But floods leave a deposit of silt, &c. 

 upon the banks they pass over, which increases the capacity of 

 the channel ; and until new deposit has again reduced the area 

 of the stream below its proper size, inundations will not occur. 



As each flood raises only the part of the bank it flows over, it 

 is easy to see that the point of overflow will be changed from 

 time to time ; and every part of the alluvial plains through which 

 a river flows will be visited in turn by floods, provided there are 

 no artificial banks. These banks assist the scouring power of 

 rivers in winter, because they retain more water in the river ; 

 but, on the other hand, silt that would have been carried over 

 the banks is kept within the channel, and this may be the reason 

 why the beds of all navigable rivers have become so much elevated 

 during the historical period. The contraction of water-channels 

 in summer, and their enlargement in winter, is thus directly 

 traced to the unequal supply of rain at different periods of the 

 year. 



This being admitted, we have an explanation of the manner in 

 which rivers may, by a succession of floods, build upon alluvial 

 deposits along their courses, at the same time raising their beds 

 in proportion to the height of their plains. 



If river-channels were perfectly symmetrical in form, the iden- 

 tical sediment that had fallen in summer might be removed again 

 in winter. It is, however, well known that river-channels are 



