GATE STRUCTURES FOR IRRIGATION CANALS. 13 
est factor in favor of the safety of such a structure lies in the design 
and construction of the upper wings and cut-off walls. If water 
tops, goes around, or under these there is little chance of saving the 
gate unless it is paved on top, so that unusual floods may pass over 
it harmlessly. 
Headgate out in the stream channel. — Where the bed of a stream is 
very wide and the low-water discharge very small as compared with 
the floods which have determined the location of the main stream 
banks, it may be necessary to place the river gate out in the stream 
bed. It is then protected from above and below by more or less exten- 
sive cribbing or rock riprap. A canal bed, well protected on the river 
side by riprap, is then built up from the gate to the point where the 
canal line intersects the main bank of the stream. 
This kind of construction also is resorted to in some cases where 
the river bank is more or less precipitous; it is not practicable to 
install a high diversion weir, and the desired canal grade intersects 
the river above the line of flow in time of low water. In this combina- 
tion of conditions, which is quite common throughout the West, it is 
the usual practice to continue the grade of the ditch or canal on up the 
river bed, usually just under the bank of the stream, until the grade of 
the ditch approximates that of the stream, at which point the gate 
may be installed. 
Headgate below upper end of the canal. — The condition spoken 
of in the last paragraph usually is met by constructing the ditch up 
the river bed as just described, but installing the gate at the point 
where the artificially-built bed of the ditch intersects the main bank 
of the stream, at which point a waste gate forms an " L " with the 
river gate and the surplus water is turned back into the stream. 
The conditions of anchorage and foundation may not be favorable 
at the point where the line of the canal intersects the main stream 
bank and then the headgate is installed at some distance down the 
canal with open channel between it and the river water. 
In connection with such a river gate it is most advisable to install 
waste gates of such capacity that the water in the channel leading 
to them will have sufficient velocity to keep the silt moving and not 
choke up the intake channel. 
Headgate on bank of secondary channel. — Some of the most satis- 
factory structures in the West are built at the banks of secondary 
channels to main streams. In most of such cases the water enters 
from the head of the channel and is regulated roughly by logs and 
brush or bowlders at the point where the , secondary channel leaves 
the main stream. In other cases the water Ts made to back up from 
the lower end of the secondary channel and in this way much of 
the silt and sand is confined to the main channel of the stream. The 
structure shown in Plate V. figure 2, is from a secondary channel 
