ia THE IMPROVEMENT OF RIVERS. 



the regimen of those parts would not vary; but wherever the bottom had previously 

 been raised these elevations would act more and more as if they were dams that closed 

 the whole width of the river. The water would flow over them, would attack them, 

 and dig out a new low-water channel which would have a width and a depth adapted, 

 according to hydraulic principles, to the nature of the obstructions, to the fall (from the 

 upper level to the lower), and to the low- water discharge. 



" When the water again rose, the swell, starting at points above, with a current 

 increased with the slope, would bring down new material and fill up the low-water channel 

 already formed, and thus reconstruct the bar to its former height. This work of lower- 

 ing and reconstructing bars is repeated at each freshet. The longitudinal profile of a 

 river taken during low water shows that it is composed of a succession of pools where 

 the fall is generally less than the mean fall, and also that the pools are separated from 

 each other by bars where the fall is greater than the mean fall of the river. 



"We have taken a purely theoretical river for an example, and we have supposed 

 that in the beginning it had a regular bed situated in a plain where the soil was homo- 

 geneous. If we now take into account the diverse topographical features, and the geo- 

 logical complexity which ordinary river valleys present, we shall have that variety of 

 cross-sections, of surface slopes, and of more or less pronounced bends which are found 

 in free rivers. 



"Our attention is thus called to the intimate relationship that exists between all 

 those phenomena which at first view appear so entirely distinct from each other. No 

 one can anywhere interfere with the curvature of a river, its slope, or the depth of its 

 cross-section, without immediately causing, either above or below the point, some change 

 in the pre-existing conditions of its equilibrium. This equilibrium, we must not forget, 

 is not a static, but a dynamic equilibrium, and, therefore, in the present condition of 

 the science it is very difficult to determine its exact conditions in advance. 



"We have shown that the want of solidity in the soil is the natural regulator of 

 the rapidity of the current. This lack of solidity, consequently, leads to the forma- 

 tion of bends and bars, which re-establish the equilibrium between bed resistance and 

 velocity of current. 



"The status, the general character of a river, therefore, depends on the united 

 action of these three factors: the discharge, which is variable; the magnitude of the 

 slope, which is likewise variable ; and the nature of the soil, which is variable in different 

 localities. 



"For a river to be navigable it is necessary that it should have a sufficiently 

 deep channel throughout its entire length. A river may have much water, but if the 

 fall is considerable and the soil unstable it cannot have a deep channel. On the other 

 hand, there are rivers with a relatively small discharge and great fall which are yet quite 

 suitable for navigation owing to their hard bottom." 



Shoals. Shoals, as generally understood, result from a natural disposition of the 

 river-bed which has resisted all erosion, and thus the water is, of necessity, compelled 

 to pass over them. They are, in fact, natural dams of greater or less height and width. 



