144 THE IMPROVEMENT OF RIVERS. 



If it is desirable to remove the timber at once, the trees must be deprived of their 

 branches, and the trunks cut into lengths of ten or fifteen feet, so they will pass out 

 of the river without forming snags. As with deadened timber, they must be cut off 

 close to the ground, so there will be no hidden obstructions left. Above the level of 

 the new pool it will only be necessary to remove the timber that overhangs the river, 

 and in wide streams even this is not required. Where the trees are large, cross-cut 

 saws are more economical for their removal than axes. 



Dangerous stumps, if the ground is hard, can be removed by Judson powder, a 

 mixture of black powder and dynamite, and which must always be fired with exploders 

 or percussion-caps. It is very useful also in removing broken or creviced rock, as it 

 can be poured into the cracks, thus saving drilling. 



Any dangerous bowlders or reefs should be blasted out, if possible to eight or ten 

 feet below the new pool level. For this it is best to use a high grade of dynamite, as it 

 will shatter the rock more and make its removal easier. 



The cost of clearing a pool, where the timber is heavy and snags and bowlders 

 plentiful, may reach as high as $800 per mile 



General Remarks on Design. In designing the various parts of a lock and dam, 

 the engineer should bear in mind that repairs to works of this nature arc always costly 

 and tedious, not only because the presence of water has to be contended with, but also 

 because the lock or dam may have to be placed out of operation, thus interrupting 

 navigation. For this reason especial attention should be given to making all parts 

 as simple as possible, and in addition, if the parts are connected with the operation, 

 such as valves, pintles, etc., they should be so designed that they can be easily removed 

 and replaced when worn or broken. This is a matter which is unfortunately too 

 often overlooked, and forms one of the unnecessary difficulties to be contended with 

 in repairs to existing structures. 



The same principles apply to movable dams with still more importance. In the design 

 of works of this class, where material of temporary life must be employed, such as wood 

 or iron, a large excess of strength should be provided, not only because wear and rust 

 will weaken them, but because they are subject to blows and twisting from drift and 

 ice, and occasionally from craft, the forces of which must always remain indetermi- 

 nate. As far as experience in this country has shown, the life of the trestles and wickets 

 of a Chanoine dam appears to be about twenty years. In that time the water will 

 practically wear out the timber of the wickets and will eat up the ironwork of the 

 trestles, and the latter process appears considerably more active on steel than on iron. 

 It has not been found practicable to protect such ironwork adequately with paint, as 

 the scour of sediment and the long immersion soon damage and undermine the coat- 

 ing, and once an entrance has been gained the rusting will continue beneath slowly 

 and insidiously, and will create a surface so rough that it can never be thoroughly 

 protected again. 



All journals, pins, or other movable parts which are to be subject to immersion 



