14 BULLETIN 155, U. S. DEPARTMENT OF AGRICULTURE. 
On most irrigation systems the head of water carried fluctuates 
more or less and, as a rule, is far below normal for a considerable 
period in the spring and again for a time in the fall. At such times 
siphons and pipe lines may not run full. This condition may ’be 
unfavorable to their life and, as a precaution against it, gates have in 
a few instances been placed at the outlets as a means of throttling 
the discharge so as to keep the pipe full at all times. Such provision 
was made at the outlet of the 84-inch pipe line of the Pueblo Rocky 
Ford Irrigation Co., and the same practice might be followed to ad- 
vantage in many other places. 
HALF FRONT ELEVATION SECTION A-B 

SIDE ELEVATION 



Fic. 3.—Intake of “ Poison Basin’ siphon, King Hill project, Idaho. 
To prevent weeds or coarse débris of any kind from entering 
pipe lines, gratings are usually provided at intakes. However, 
unless carefully watched, the accumulation of weeds at the grating is 
liable to obstruct the entrance so as to cause the water to overflow 
canal banks. The danger of this, and of the serious damage which 
might result in many instances, have led to the removal of gratings 
which could not be inspected frequently. For irrigation service, 
where water is not carried during the winter, iron gratings are very 
satisfactory, but in places where ice is troublesome wooden gratings 
are considered better, particularly if they project above the water, for 
the reason that ice does not form on the wood so readily. 

