The Waters oe the Rio Grande 59 
These waters would change the character of the river water if 
they mingled with it, which they appear not to do. 
The brown color is accidental and is due to peaty material dis¬ 
solved out of the aquifers themselves. The presence of fragments of 
wood obtained in sinking the wells and the deportment of these waters 
when submitted to sanitary analysis is taken as proof of the peaty na¬ 
ture of the color. 
The presence of peaty substances and wood in this area is inter¬ 
preted as indicating that this portion of the valley was, in former 
times, low land and probably marshy, i. e., poorly drained. 
The sodic carbonate is considered as originally coming from the 
mineral constituents of the rocks furnishing the sands and clays that 
form the strata now composing the floor of the valley. 
The changes necessary to remove the silicic acid and lime from 
the mountain waters are simple. The small concretions of calcic car¬ 
bonate met with in the sand from the strata passed' through at 550 
feet indicate simple precipitation as the method of removing the lime. 
As the drainage of this portion of the valley has probably been 
just what it is now for the whole period of the existence of the valley 
with but little or no change in its water supply, evaporation alone is 
considered adequate to account for the concentration of the sodic car¬ 
bonate that we find in this section. 
Evaporation at the present time is sufficient to add 145,500,000 
pounds of sodic carbonate to this section of the valley yearly. This 
is on the supposition that the mountain water carries 2^4 grains of 
sodic carbonate in each imperial gallon, or 10 pounds of water evapo¬ 
rated. 
The present agricultural condition of this section of the valley is 
due to the accumulation of this salt, black alkali, rather than to an 
excess of water. 
Local surface drainage is necessary in many small localities. 
The evaporation from the area involved is equivalent to an inflow 
of 2,000 second-feet throughout the year. This is probably a larger 
amount than this section of the valley actually receives, except for a 
very short period in the spring of the year when the direct overland 
inflow may equal or possibly exceed this amount. 
The San Luis Lake water is peculiar in its composition and un¬ 
like either the river- ground- or artesian-waters. 
The deposit of sodic carbonate east of the San Luis Lake is prob¬ 
ably derived from the evaporation of the brown artesian water, and 
has no connection with the lake. 
The conditions which have determined the character of the brown 
artesian waters are still active in determining the agricultural features 
and questions of this section of the valley. 
