20 BULLETIN 300, U. S. DEPARTMENT OF AGRICULTURE. 
This is undesirable, unless the fall be slight, since in working up- 
stream dams must be built behind the boat to maintain the necessary 
water level. The cost of excavation increases with the amount of 
face, or exposed surface of ditch side. This should not be over 
2 feet, and any extra expense required to maintain the water at 
that level means increased cost of excavation. In working down- 
stream the ditch remains full, and the dredge, floating high, can dig 
a much narrower bottom than if working upstream in shallow water. 
Moreover, when floating low the dipper may not properly clear the spoil 
bank. Again, in working downstream, any material dropping from 
the dipper into the ditch will be washed ahead of the dredge and 
picked up later, whereas if working upstream any material dropped 
or any silt washed behind the dredge is left to settle in the bottom of 
the ditch. 
The dams may be constructed in various ways. A common method 
is driving two lines of sheet piling about 6 feet apart directly across 
the ditch, the piling being held in place by longitudinal timbers across 
the channel. The space between the piling is filled with earth. The 
second line of piling is sometimes omitted, and dirt is banked directly 
against the one line, a method requiring more earth than the other. 
Some operators cover the piling with canvas to prevent leakage; in 
this case the piling must be strongly braced. Hopper, or V-shaped, 
dams are also built. These are constructed by inclining the piling 
and filling the space between with earth. Some operators provide 
spillways in dams to take care of any unusual rise of water caused 
by heavy rains. Boxes of dynamite are often buried in these tempo- 
rary dams for demolishing them after they have served their purpose. 
The floating dipper dredge moves itself ahead by means of the 
dipper. The spuds are first loosened from their bearings, and the 
dipper is run ahead of the machine and rested on the natural ground 
surface in front of the ditch. The spuds are then raised, and the 
engines operating the backing drums are started; the dredge, being 
free, is thus pulled ahead. The spuds are then lowered, the dredge 
pinned up, and excavation resumed. 
In timbered country the right of way must be cleared. In many 
cases the timber cut will supply sufficient fuel for the dredge. The 
removing of logs from the right of way by the dredge decreases the 
output materially; therefore they should be cut in lengths of 16 to 20 
feet, so that they can be handled readily. Although it is possible to 
excavate stumps with the larger floating dredges without first blast- 
ing them, it is preferable to shatter them with dynamite to avoid the 
strain on the machinery, which shortens the life of the dredge. Spe- 
cial care must be taken to loosen stumps near the banks of the new 
ditch, for the dirt can be dug away from but one side of them, 
