IRRIGATION WATERS AND THEIR .EFFECTS. 7 
must be above the land to be irrigated and are not as a rule in 
low places, but they are natural collecting basins, many of them 
having been small lakes before they were converted into reser¬ 
voirs. These conditions suggest that they might now receive 
larger quantities of seepage water which in some instances is un¬ 
doubtedly the case. 
These stored waters sometimes suffer as great changes as the 
river water. It is understood that the water stored is taken from 
the river, much of it directly and some of it, the seepage water, in¬ 
directly in that this water has been taken from the river, ap¬ 
plied in irrigating land and has reappeared as seepage water. A 
small portion has fallen as snow or rainwater. 
In studying the changes in the reservoir waters it is not easy 
to determine just how much is to be attributed to the several 
causes contributing to them. If the waters were found to be quite 
pure, with an increase of only 0.5 of a grain per gallon, the gain 
could justly be attributed to evaporation from the surface of the 
reservoir. This would be the exact amount in the case of Terry 
lake. But we find an increase in this instance of upwards of 130 
grains per gallon instead of 0.5 of a grain and the amount of salts 
indicated by this small amount, 0.5 grain per gallon, can be whol¬ 
ly neglected without affecting our final results in the least. The 
only rational explanation that we can offer for this increase is the 
seepage, together with whatever quantity of soluble salts may be 
furnished by the bed of the reservoir. 
The amount of salts actually present in some of these reser¬ 
voirs is rather surprising to the layman, and to others too, who are 
not cognizant of the facts in the case. 
In the instance of Terry lake, which presents the most strik¬ 
ing results of the four reservoirs which I have studied in anything 
like detail, the amount of salts held in solution was in round 
numbers 27,000 tons. The samples on which this estimate is 
based were taken just before they began to draw off the water and 
I think were as good as could be gotten. A volume of Poudre 
river water equal to the content of Terry lake, 9,000 acre-feet, 
would contain about 500 tons of mineral matter, leaving 26,500 
tons as having been brought in by seepage. The other lakes, res¬ 
ervoirs, examined gave smaller figures but indicate the same gen¬ 
eral fact. 
A peculiar fact is that there was a slight increase in the per¬ 
centage of potash which, for reasons that would take too much 
space to enter into in this place, we believe to indicate that much 
of this increase was due to the solution of alkalies by waters flow¬ 
ing over the surface of seeped ground. 
The changes which took place in this instance are so patent 
that they cannot be misinterpreted; the carbonates, relatively 
