WORKS FOR CREATING SLACKWATER. 139 



ered also. If navigation alone is to be consulted then the fewer the dams the better, 

 within certain limits, because each is an obstruction which causes more or less delay 

 to transportation, beside expense for maintenance, and by increasing the lifts in estab- 

 lishing a system of slackwater the number of locks and dams can be reduced. It 

 must be seen to, however, that the sizes of the parts to be maneuvered are well within 

 the power available, and the effect which the flow of a great volume of water will have 

 on the works themselves and on the banks below, with the consequent danger from 

 undermining or cutting around, must also be considered. The effects upon the regime 

 of the river, in causing shoals or bars which may become obstructions, must not be 

 omitted from the study. With proper care in the selection of the locations, and proper 

 study in the design of the various parts of the works, these objections can usually be 

 overcome. 



In France, where practically all the dams are movable, the lift is generally small, 

 although in the later dams it has been increased considerably. In this country, where 

 nearly all the dams are of the fixed type, the lifts are rarely less than ten feet and in 

 some cases as much as eighteen feet. Where the banks along the stream are high and 

 of firm character, there is no objection to making the lift high, provided proper pre- 

 cautions are taken in the construction. With movable dams the lifts are usually 

 small in this country also, rarely being over eight feet. 



Height of Lock Walls. The height of the walls above the crest of the dam should 

 be such that but little time will elapse between the time they are overflowed and the 

 time when boats can pass over the dam. In France they are usually built to the level 

 of the highest navigable water, and this rule is followed on some streams in this country 

 where navigation ceases at a moderately high stage, but it is not applicable to our 

 larger rivers where boats run at stages of fifty or more feet above low water. 



With movable dams, the coping is usually placed four to six feet above the upper 

 pool; with fixed dams it is made from nine to twelve feet, except where the flood 

 range of the river is very small, when it is placed lower. The object of having a good 

 height of wall technically known as the guard- wall, or guard above the upper 

 pool, where the dam is fixed, is to allow the river to rise to a considerable height before 

 it can drown the lock and thus interfere with navigation. Theoretically, when this 

 occurs, boats should be able to pass over the dam, but practically it is a condition not 

 always realizable except at an expense quite incommensurate with the benefits to be 

 secured, since it may require an inordinate height of wall. This is especially the case 

 with rivers from the mountains, where the rises are very rapid. However, in such 

 streams, by the time the locks are drowned, the current has usually become too swift 

 and dangerous to allow boats to run, and, at the most, navigation is only suspended 

 at " drowning-out " stages for a few days each year. 



In some constructions the lock walls are built a few feet lower at the lower end 

 than at the upper end, for the purpose of cheapening construction. 



When the water-way has been restricted in the construction of the dam, it is 



