ti6 THE IMPROVEMENT OF RIVERS. 



to provide a regulating-weir whose closing apparatus shall always be under control, 

 and which can be maneuvered with certainty, night or day, by a watchman. The 

 proper location for such a weir is away from the lock, so that the water passing 

 through it will not disturb navigation. It should, therefore, be near the abutment, 

 and may be separated from the main dam by a masonry pier. This regulating- weir 

 should be of sufficient length and depth to safely pass all the drift usually found 

 upon the stream up to the stage of water at which the dam is lowered, and to discharge 

 the surplus water at medium stages, but it should not be of such dimensions as to 

 render its maneuvers uncertain. 



Abutment and Protection of Banks. The banks below the abutment and the lock 

 must be protected as descrilxnl in the chapter on Fixed Dams, since they are simi- 

 larly exposed to washing, and the design and construction of the abutment should be 

 based on similar principles. 



Remarks. Movable dams, although solving in principle the problem of slack- 

 watering rivers, are open to several objections in comparison with fixed dams, the chief 

 of which arc greater expense in establishment, and sometimes in operation and in 

 maintenance, and greater liability to injury from drift and ice, especially on Ameri- 

 can rivers. The problem of drift has been overcome so far without serious injury 

 resulting, by constant watchfulness on the part of the dam-tenders, and as the dis- 

 tricts along the rivers become settled and the timber cleared away, this danger will 

 gradually decrease. The problem of ice, however, will always remain a serious one, 

 as the dams have to be lowered when it begins to run, or the drifting pieces will catch 

 in the trestles and other parts, and pile up until lowering becomes impossible, and the 

 force of the river will eventually wreck the dam. Because of this, movable dams on 

 northern rivers have always to be lowered at the approach of winter and kept down 

 until the spring, when the danger is past. Should the season be one of low water, navi- 

 gation has to stop, the dam being then of no benefit. In certain cases, as where the 

 dam is just below a city, additional inconveniences will result, because all harbor navi- 

 gation comes to a standstill, and factories which are largely dependent on the river for 

 coal or other supplies, are put to much expense in procuring them elsewhere. This 

 state of affairs has happened more than once with the Davis Island dam on the Ohio, 

 just below Pittsburg, and on certain occasions, at the urgent insistence of the manu- 

 facturing interests, the wickets have been raised in winter-time in order to provide 

 sufficient depth for harbor navigation. The result, however, has usually been disas- 

 trous, as ice and high water coming suddenly have wrought havoc with the dam. 



When some means have been devised of overcoming such a disadvantage mov- 

 able dams will be of greater benefit than they are now. The problem will in all like- 

 lihood be solved by the adoption of a closure of the bear-trap or drum type, which 

 can be operated from the shore and by the natural forces present, and which will allow 

 a free overflow along the crest for drift and ice, with no parts below in which these 

 can become entangled. 



