lO LOSSES FROM CANALS BY SEEPAGE. 
branch of the Rio Grande canal, known as the North Farm 
lateral or ditch, and on other laterals of the company known 
as the iF and i C laterals. 
The conditions in the San Luis valley are somewhat 
different from those in most places of the state, but the con- 
ditions causing the loss or gain by canals are necessarily the 
same. 
The San Luis valley is one of great extent — nearly the 
size of Connecticut. In Geological times it was the bed of 
a lake. Its surface is of very uniform and moderate slope, 
so that canals often pass for long distances in straight lines. 
The Prairie Ditch, for example extends nearly twenty-six 
miles on a straight line without turn or bend. The fall of 
the country is moderate, though large for canal purposes. 
It decreases from about fourteen feet per mile near the rim 
east and west, to half as much as the center of the valley is 
reached. A map of the valley showing these contour lines 
has been prepared and will be published in connection with 
a bulletin giving further results of investigations in the 
valley. 
A large part of the valley is irrigated by sub-irrigation 
which consists in filling the sub-soil by water from the ca- 
nals and laterals. dTe slope of the land is so uniform and 
gentle that the water does not find low places in which to 
appear in the form of seepage as in an undulating region. 
The general process of irrigation in these regions is to 
run water into the laterals and allow it to soak away, and by 
so doing fill the sub-soil until the water is at a moderate 
distance from the surface, about eighteen inches being de- 
sired during the growing period of the grain crops. The 
soil of the valley is very deep, but is everywhere underlaid 
with coarse gravel which becomes finer as the distance from 
the mountains increases. Most of the ditches are excavated 
into this gravel. 
The irrigated region includes most of the valley east of 
what is known as the “Gun-barrel road” — which extends di- 
rectly north from Monte Vista — and the tract in which sub- 
irrigation shows, includes a portion of this region. In places 
irrigation extends west of the road. It may be expected 
that as long as the surface of the underground water is be- 
low the bottom of the canal there will be loss of water by 
seepage. Where the ground water rises above the bottom 
the canal may then act as a drain and carry awa^ a portion 
of the ground water, and the canal is thus found to increase 
in volume by seepage. 
Circumstances prevented making as extensive measure- 
