378 WOOD TEANSPOET BY WATEE. 



In some cases wattle-work is applied round the piles, other 

 rows of piles are driven-in a few feet from the first rows, and 

 the interval filled with stones, branches and sand. Finally 

 solid parallel lines of masonry may be constructed, which are 

 no other than dams running parallel to the stream and united 

 to the old banks by wings ; they may be looked upon as artificial 

 banks to the stream. 



The top of these dams must be of about the average level 

 of the stream so that all flood-water passes over it, carrying 

 with it silt and gravel which gradually fills-up the site of 

 the slack-water. Sometimes where there is an extensive tract 

 of slack-water, it may be covered with a network of dams 

 crossing one another, which gets filled with silt, etc. ; if these 

 dams are raised gradually as the spaces between them become 

 filled, the slack- water may disappear entirely, and the lateral 

 dams be overflowed no longer at high water. 



ii. Strengthening the Bed of the Stream. 



The bed of a stream requires artificial improvement much 

 less frequently than the banks. This is, however, sometimes 

 requisite, in the case of mountain-torrents with stony beds, 

 and usually consists in blasting-away the rocks and removing 

 stones, which otherwise might cause holes to form behind them 

 in the bed of the stream, and thus catch the floating logs. The 

 best season for these operations is the autumn, or whenever 

 the water is lowest ; the stones removed from the stream 

 may be utilised to improve its banks. It is, however, easy to 

 do too much in the way of removing obstacles from the bed of 

 a rapid stream : for if a floating-channel be freed from all 

 impeding rocks and stones, which form so many natural weirs 

 in its course, the stream often becomes torrential and its banks 

 may be broken and inundations or other disastrous conse- 

 quences ensue. 



Kapids may occur where the bed of a channel is narrow and 

 steep, where the stream runs between rocks in passing from a 

 higher stage in the valley to a lower one, and there is then 

 likely to be difficulty in floating the timber. If in such places 

 the bed be terraced (Fig. 251), floating will be much expedited 



