58 LANDS OF THE ARID REGION OF THE UNITED STATES. 
tions of the farming community for the amount of the increase of the 
streams, but merely for the fact of their increase. The amount is recorded 
in an independent and most thorough manner, by the accumulation of the 
water in Great Salt Lake. 
RISE OF GREAT SALT LAKE. 
A lake with an outlet has its level determined by the height of the 
outlet Great Salt Lake, having no outlet, has its level determined by the 
relation of evaporation to inflow. On one hand the drainage of a great 
basin pours into it a continuous though variable tribute; on the other, there 
is a continuous absorption of its water by the atmosphere above it. The 
inflow is greatest in the spring time, while the snows are melting in the 
mountains, and least in the autumn after the melting has ceased, but before 
the cooling of the air has greatly checked evaporation on the uplands. The 
lake evaporation is greatest in summer, while the air is warm, and least in 
winter. Through the winter and spring the inflow exceeds the evaporation, 
and the lake rises. In the latter part of the summer and in autumn the 
loss is greater than the gain, and the lake falls. The maximum occurs in 
June or July, and the minimum probably in November. The difference 
between the two, or the height of the annual tide, is about 20 inches. 
But it rarely happens that the annual evaporation is precisely equal to 
the annual inflow, and each year the lake gains or loses an amount which 
depends upon the climate of the year. If the air which crosses the drainage 
basin of the lake in any year is unusually moist, there is a twofold tendency 
to raise the mean level. On one hand there is a greater precipitation, 
whereby the inflow is increased, and on the other hand there is a less 
evaporation. So, too, if the air is unusually dry, the inflow is correspond- 
ingly small, the loss by evaporation is correspondingly great, and the 
contents of the lake diminish. This annual gain or loss is an expression, 
and a very delicate expression, of the mean annual humidity of a large 
district of country, and as such is more trustworthy than any result which 
might be derived from local observations with psychrometer and rain gauge. 
A succession of relatively dry years causes a progressive fall of the lake, 
and a succession of moist years a progressive rise. As the water falls it 
