1 8 The Drainage of Fens and Low Lands. 



suspension, but also the roots of the weeds are torn up and 

 are carried by the current out of the drain. If this is done 

 frequently, drains can be successfully kept clear of weeds and 

 deposit ; and may even be deepened at considerably less cost 

 than by dredging, or by spade labour, without any injury to 

 the outfall. Too little advantage is taken of the capacity of 

 the water as a carrying agent in the improvement of rivers. 

 In dredging, the chief expense and difficulty is the removal of 

 the material dredged up. By continually stirring up the 

 matter to be removed, it rises in the form of mud, and the 

 particles are sufficiently small to be moved and carried away 

 in suspension. If the section of the channel is uniform, the 

 velocity of the water will carry the material entirely away, 

 but wherever there are wide places and slack currents, there 

 will be a tendency for the matter in suspension to be de- 

 posited. By frequently and continually running the machine 

 up and down the drain within the defined limits of the water- 

 way a uniform and regular channel can be maintained free 

 from shoals and weeds. 



Transporting Power of Water, — The transporting 

 power of water may be realised by considering the turbid 

 condition and immense quantity of matter carried down by 

 comparatively sluggish streams in times of flood. The 

 quantity of material transported by such rivers as the 

 Humber and the Trent is evidenced by the fact that the 

 warping lands on to which the water is allowed to flow are 

 raised by the alluvium which subsides, as much as two feet in 

 one year. 



Owing to the constant change in the direction or motion of 

 the water causing horizontal and vertical eddies there is a 

 considerable upward vertical action, which counteracts the 

 downward motion of particles of matter of heavier specific 

 gravity carried in suspension. Thus particles of soil are kept 

 suspended, which in still water would fall to the bottom. In 

 addition to the matter carried in suspension, the action of the 



