78 



OxV CHANGES OF THE SEA LEVEL. 



[1854. 



hin-her than those at a distance fi'om it, and therefore that fig. 

 2 would also represent the transverse section of slow rivers gen- 

 cralhj. The similarity of the physical features presented by 

 the lower parts of all rivers was particularly remarked by 

 Hutton.* 



It has been observed by engineers,t that in all rivers in this 

 country the large quantities of silt brought into them by win- 

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

 period of the year, former accumulations of deposit are actu- 

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

 although winter-freshets bring down silt with them, they carry 

 into the sea a larger quantity than they have introduced into 

 river channels. J If it were allowable to assume that the un- 

 equal supply of water at different seasons of the year produces 

 effects in the channel of the Mississippi similar to those just 

 described on our own streams, the following consequences 

 might be deduced from the fact that winter freshets remove 

 more detritus than they bring down. 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 

 favors 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 the small elevations amounts to more than the 

 height of the banks, above last year's level, but floods leave a 

 deposit of silt, &o., upon the banks they pass over, which in- 

 creases the capacity of the channel ; and until new deposits 

 has again reduced the area of the stream below its proper size, 

 inundation 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 enlarge- 

 ment 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 allu- 

 vial 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 

 identical sediment that had fallen in summer might be removed 

 asrain in winter. It is, however, well known that river-chan- 



* Theory of the Earth, vol. ii, p. 205-211. 



\ On this and the following points sec First Report of the Tidal 

 Harbors' commission, above referred to, "which contains the opinions 

 of our most celebrated engineers on the phenomena presented by Tidal 

 and other rivers. 



J The author has not met with any explanation of the causes that 

 produce changes in river-cliannels, although the constant alterations 

 taking place in tliem have been repeatedly alluded to. 



nels are deep on one side and shallow on the other. The prin- 

 cipal deposit therefore takes place on the shallow or quiet side, 

 and the principal removal occurs from the deep side where the 

 current runs more quickly. 



This may explain why the traveller on the Mississippi sees for 

 hundreds of miles a caving bank on one .side and an advanc- 

 ing sandbar on the other (Lyell). When the action of the river 

 is also unequal on its two banks in different places along its 

 course, a channel consisting of curves in.stead of straight lines 

 must be produced. When each curve, however, had assumed4.he 

 complete horse-shoe form, the water, by travelling round the 

 outer circumference of the bend, will have its effective speed 

 reduced to that on the inner or shallow side. The current 

 would thus become more nearly equal in all parts of the 

 channel, and necessarily the deposit likewise ; and in winter it 

 would have a nearly equal tendency to excavate the banks on 

 both sides, which condition of equilibrium might last for some 

 time. 



Hutton, in 1795, has remarked, that there is evidence of 

 denudation in every country where at any time of the year the 

 streams carry off any particles of the superficial soil.* The 

 Mississippi must derive it vast supplies of mud for thousands 

 of such tributaries ; for it could obtain them from no other 

 source, unless we suppose it abstracts them from its own plains. 

 Certainly in many places soil is being removed fi'om one part 

 or other of its plains ; but an equal quantity must be added 

 to some other part, for the river could not ma^e a permanent 

 inroad into its plains without enlarging its channel. This it 

 does not do, or it would be able to carry off the winter-freshets 

 without overflowing, and the present artificial^ bank would be 

 unnecessary. 



I have thus briefly referred to observations made by British 

 engineers which may throw some light on the causes of per- 

 iodical floods and changes of channel in rivers, and also upon 

 the formation of alluvial plains along their course. These ques- 

 tions need not further be entered into, because the limited growth 

 of alluvial plains and deltas may be best illustrated by tracing 

 the alteration in the mean level of a large part of North America 

 that would be consecjuent upon a denudation sufiBciently ex- 

 tensive to furnish the alluvium said to exist in the valley of the 

 3Iississippi. On the borders of the Gulf of Mexico at the 

 present time marine strata are forming within a short distance 

 of the fluviatile, and frequently alternate with them, because 

 spaces of the sea-shore are enclosed by banks of river-mud and 

 converted into lakes ordinarily communicating with the river, 

 but sometimes with the sea after high tides. 



The present marine or fluvio-marine deposits must be com- 

 posed of mud that has passed the mouth of the river, or wash- 

 ed up by the sea, while the freshwater strata must be entirely 

 formed from sand and mud carried over the river banks, or de- 

 posited on the bottom of lakes supplied by the stream before it 

 enters the Gulf of Mexico. An idea of the amount of denu- 

 dation that has taken place in the interior of North America 

 might be either obtained from the extent of the marine deposits 

 formed of mud that had passed the mouth of the river, or 

 from that of the purely fluviatile and contemporaneous depos- 

 its formed from mud which had never entered the Gulf of 

 Mexico. 



* Our clearest streams run muduy in a flood. The great causes, 

 therefore, for the degradation of mountains never stop as long as there 

 is water to run ; although, as the heights of mountains diminish, the 

 progress of their diminution may be more and more retarded. Op. cit. 

 vol. ii, p. 2-50. 



