154 ON THE FRONTIER 



always built upwards from the bed of the stream, where of 

 course their movements cannot be observed, and often 

 thrown across rapid watercourses, several feet deep at their 

 lowest stage ; while, to add to the difficulty, they are com- 

 posed of wood that floats. I will attempt a description of 

 these structures, being very familiar with the subject, it 

 having several times been incumbent on me to superintend 

 the destruction of them, on account of their interfering with 

 mining and irrigation works. Their height varies with the 

 rapidity of the streams they are built across, and where 

 the current is fast the fall of the watercourse is great; 

 and then they require to be high, otherwise the water 

 would not be backed sufficiently far to make a pool of 

 adequate size. In mountain streams about eight feet is 

 their average height. For instance, suppose you stood below 

 one, regarding it, then the dam would stretch across in your 

 front, from bank to bank, eight feet high, and present a 

 perpendicular face of branches, with their butt-ends towards 

 you; these ends varying in size from half-an-inch to two 

 inches in diameter. This is undoubtedly the right way 

 for the sticks to lie, as is well-known by all engineers 

 who have had occasion to make "brush-dams;" and the 

 reason is obvious, for as the smaller twigs and forks on 

 each branch, when laid in such a manner, face the stream, 

 they catch all sediment coming down with the water, 

 which, lodging, helps to make and keep the dam water- 

 tight. 



The edge of the dam is the smooth trunk of some tree 

 that grew on the bank of the stream, which has been felled 

 across it, and its branches all cut off. No water is allowed 



