S 6 THE IMPROVEMENT OF RIVERS. 



"Let us consider a navigable stream, or rather a section of this stream comprised 

 between two important affluents. The elements of the regime of this section of the 

 stream are: 



" i st. The total slope of the valley from one extremity of the section to the other 

 and the total development of the stream between the same points which determine 

 its mean slope. 



" ad. The nature of the materials of the bed and banks, and the nature of the material 

 brought from the upper part of the river or from its affluents. 



" 3d. The discharge in low water and in floods, and the slow or torrential character 

 of the latter, etc. 



" From these different constitutive elements the results affecting navigation are 

 the trace of the center line, the width of channel, the depth, and the velocity of current. 

 For the section of stream considered there exists one combination of these results, 

 which is the most favorable from the point of view of navigation, a combination which 

 nature does not generally realize spontaneously, but which works of regularization 

 judiciously conducted permit us to obtain. It is only long experience based on patient 

 observation, and frequently on repeated defeats, that will enable this problem to be 

 solved. It cannot be told a priori, and this explains the diversity that exists in the 

 depths chosen for the different rivers of Germany. There has been adopted for each 

 the greatest depth in low water compatible with other conditions more indispensable. 

 In fact if the navigation should be certain of development on a river (in the contrary 

 case it is best not to be concerned with it) it can only be conducted with facility and 

 security on the condition of having a sufficiently wide channel. When we observed 

 the contractions that have been attempted on the Meuse, we found 35 feet to 39 feet 

 adopted for the width of the normal profile at the bottom. Such a width may suffice 

 to permit the passing of two boats in a canal, but it is incompatible with navigation 

 of any importance in a river. A width of channel from 75 feet to 90 feet should be 

 considered as the minimum. On the Rhine the channel is 450 feet in width. 



"On the other hand the velocity of the current must not exceed certain limits. A 

 rapid current may be an obstacle to economical traction in ascending, and consequently 

 to the development of navigation. 



" We may therefore conclude that works of regularization will only give satisfactory 

 results on streams of large discharge and generally of moderate slope. These two 

 conditions are most often found united in the regime of the rivers of Germany, and 

 it is that which explains the success of works of regulating in that country. They 

 are found also more frequently in the middle or lower parts of streams than in their upper 

 parts; so that works of regularization may be justified for certain sections of some 

 streams and not for others. 



" If the bed of a river is composed of drifting material on which the water can act 

 it naturally results that the depth obtained by diminishing the width of the section 

 carries with it the lowering of the former level of the river above the point considered. 



