PART III. 

 IMPROVEMENT OF RIVERS BY CANALIZATION. 



CHAPTER I. 

 GENERAL DESIGN AND CONSTRUCTION OF WORKS FOR CREATING SLACKWATER. 



General. By the term canalization, or slackwatering, is meant the creation of a 

 series of pools in a river, connected by locks, and affording at all seasons of the year a 

 depth sufficient for navigation. A few cases are found where such pools are formed 

 by natural reefs or bars of rock, but in the vast majority of instances a dam, which may 

 be either fixed or movable, has to be built to obtain the depth required. 



There are certain conditions of existence which are common to all structures used 

 for slackwater navigation, regardless of whether the dams are fixed or movable. These 

 conditions comprise among other things the proper selection of locations, the deter- 

 mination of the level of the sills and of the minimum navigable depth, and the height 

 to be given the lift, or vertical distance from pool to pool. 



Where the dam is fixed, its entire structure is brought to the full height to which 

 it is desired to raise the water-level; when movable, the fixed part is merely a founda- 

 tion on which to erect a suitable superstructure which can be raised or lowered as 

 desired. The lock walls usually are built several feet higher than the crest of the 

 dam in order to enable lockages to be made when the water is considerably above the 

 normal stage of the pool. 



Location. The lift is usually the greatest factor in determining the location of a 

 lock and dam, since the surface of the pool must not only afford the required depth on the 

 sill of the next lock above, but must also provide navigable depth over all obstructions 

 between the two dams. The actual slope of a pool is too uncertain to be taken into account 

 in determining the location, since in seasons of low water it becomes very slight, and is 

 always changing with the discharge of the river. Its flood height, however, must he 

 considered when stationary dams are to be built because of its effect upon adjacent lands 

 and industries, and this is then often of great importance. In movable dams the case 

 is different, because the dam is lowered upon the approach of floods, so that the river- 

 bed has been restored to its natural condition before the water has reached much 

 above the ordinary pool level. For this reason the height of the pools of movaMr 



