i 3 6 THE IMPROVEMENT OF RIVERS. 



All corners should have monuments of stone or concrete marked " U. S.," and the 

 land, if of any value for cultivation, should be fenced. 



Arrangement of Lock and Dam. The question first to be decided is upon which 

 side of the river the lock shall be built. It is not always possible to secure bed-rock 

 upon both sides of the stream. In such cases, other things being equal, the question 

 arises whether to build the lock or the abutment upon the rock. In narrow streams 

 it is usually necessary to place the lock far in the bank in order to avoid restriction of 

 the waterway. If the rock rises out of the water and forms the bank on one side, as 

 is frequently the case, it would be expensive to excavate for the lock and its approaches ; 

 but, on the other hand, this rock would form an excellent protection to the bank upon 

 which the abutment is located, since, both with fixed and movable dams, this bank 

 is the one that is most exposed to undermining from the currents. 



If the lock is located upon the opposite shore, its foundation must be carried down 

 below danger of undermining; this in some cases will also entail much expense, and 

 a careful comparison alone can show the most economical location. 



In movable dams the undermining effect of the water upon the lock foundations 

 is not great, because the pass, which is placed adjacent to the lock, is not opened until 

 the head has been materially reduced by the opening of the weir; but with fixed dams 

 there is a force at work in all rises calculated to disturb the river-bed to considerable 

 depths. 



In situations where the foundation is all good or all bad, the lock should be placed 

 upon that side of the river which will afford the easiest ingress and egress to boats. 



No fixed rules can be made to suit all cases, and each location must be worked 

 out anew, because ideas which would be applicable to one might lead to unfortunate 

 results in another. 



. The arrangement of the principal parts being fixed, it is necessary to determine 

 the elevations and general dimensions. 



Navigable Depth. The minimum depth which should be given a system of slack- 

 water improvements is regulated by the draught of the largest craft when loaded, to 

 which must be added six inches or more for clearance. On rivers of small low- water 

 discharge the pools cannot always be maintained at normal height, because of failure 

 of supply during drouth, and on such streams the clearance should be greater than 

 on those where no lowering of the pools takes place. Another consideration, which 

 is too frequently overlooked, is the requirements of the future commerce which may 

 be developed by the slackwater system. A district whose commerce might have been 

 satisfied by a navigable depth of a few feet may become a producer of coal, ore, etc., 

 which will require for economical transportation an 8-foot stage of water, or, if the 

 stream in question is tributary to one having a greater depth, it may be desirable to 

 extend this depth to the tributary in order not to break bulk in transit. It is much 

 better to provide a depth of water too great for immediate wants than one believed 

 to be just sufficient. Commerce will be quick to take advantage of it, and if the dis- 



