374 WOOD TRANSPORT BY WATER. 



The slower the current, the further back the slack-water 

 extends ; in sluggish streams, weirs may reduce the velocity 

 of the stream too much for floating, and are useful only for 

 diverting mill-streams from the main watercourse. Wherever, 

 on the other hand, the current is rapid, it is evidently advan- 

 tageous to keep the water back as much as possible; for then, 

 independently of the advantages of a moderate current, the 

 banks and works to improve the floating are secured much 

 better against erosion, and the depth of the stream is 

 rendered much more suitable for floating, an important 

 matter when it contains much gravel and boulders. 



The most suitable places for weirs are narrow valleys between 

 rocky banks, as in such places the water cannot damage the 

 banks of the channel and cause inundations, even when its depth 

 is considerably increased. 



In such places generally several consecutive weirs are 

 required, so that the watercourse in certain cases becomes 

 regularly terraced, with a succession of falls. As a rule, 

 the number of weirs should be proportional to the rapidity 

 of the current and the quantity of gravel and boulders in the 

 stream. These weirs are not constructed all at the same time, 

 but by degrees, as the space between any two of them becomes 

 filled with silt and gravel, and therefore a new weir becomes 

 necessary. 



Besides the above-mentioned weirs, others also are required 

 wherever any side-channel leaves the main stream to supply a 

 mill, etc. Booms for collecting the floating wood also are 

 erected frequently on weirs. The more remote the point where 

 the water from a side-channel is required, the higher must be 

 the weir which supplies it. 



It is evident that sand, gravel and boulders accumulating 

 behind the weirs constantly raise the bed of the stream, so 

 that the water will in time overflow its banks unless they are 

 sufficiently high. This is dangerous not only for the banks, 

 but also for the wood which is being floated and tends to leave 

 the stream and become stranded. If then a rush of high water 

 follows, much damage may be done to the riparian properties, 

 for which the manager of the floating will be held respon- 

 sible unless he has taken proper precautions. In all cases, 





