COLORADO IRRIGATION WATERS AND THEIR CHANGES. 19 
ligible, but the amount of salts with which we shall have to deal 
is so large that in subsequent statements we will take no note of 
this factor. 
§ ^ eva P orat ion from an unprotected surface of water 
u ^ rt T° lnS * s a ^ out f° r ty inches per annum, or, considering 
the Poudre water to carry 2.9 grains per imperial gallon, there 
would be added to the remaining water 400 pounds of inorganic 
salts for each acre of surface exposed, and if the average depth of 
the re ^ er voir were, as is the case in Terry lake, about twenty 
teet, the difference would be about 0.5 grain per gallon This in¬ 
crease may be attributed to evaporation, but it is too high, for the 
water does not remain in the reservoirs the whole year, as here 
supposed; at least the reservoirs are not full, and the actual in- 
crease due to this cause is less than 0.5 of a grain per imperial 
gallon 1 hat the aggregate amount of inorganic or mineral mat- 
ter is large is evident. There is, however, but little profit in in¬ 
dulging in calculations of this sort, as they are modifications of 
the one already made in regard to the amount of dissolved matter 
daily brought down from the mountains by the Pondre river. 
§ 37 * The reservoir known as Terry lake has, when full a 
surface area of 470 acres, from which forty inches of water evap¬ 
orate annually, leaving nearly 400 pounds on each acre, or an a°*°re- 
gate of 94 tons of mineral matter for the entire reservoir, which has 
been dissolved out of the granite of the mountains where the 
snows have melted. But when this quantity is compared with 
the figures which we shall have to use to represent the ao-o-regate 
of salts carried by these reservoir waters, it will be realized that 
this increase in the mineral matter in such waters due to evapor- 
ation can be neglected. 
§ 38. I shall give the reservoir waters in order, going down 
the valley, beginning with the Larimer & Weld, or ‘ Terry lake. 
This may not be the best order, or rather it might be well to omit 
Terry lake altogether, because it is not typical of the chants 
which I most desire to set forth, but presents them in so extreme 
a form as to overshadow the less extreme but probably more rep¬ 
resentative results shown by the others. This reservoir, however, 
is one of the oldest and stores 9,000 acre-feet of water, and al¬ 
though the changes presented by the water of this reservoir may 
be greater than in the other cases, it may alone serve to o-i ve a 
more adequate idea of the real extent and importance of the solv¬ 
ent action of water upon the soils and of the supply of the soluble 
salts contained and formed therein than the others. 
LARIMER & WELD RESERVOIR (TERRY LAKE). 
§ 39.. This reservoir is situated about two miles north of 
Fort Collins and is filled principally by water taken from the 
