WATER SUPPLY. 73 
which fall on the lake, we deduce a total annual evaporation of about 80 
inches of water; but for the present purpose it will be more convenient to 
consider the former figure. 
The extent of the Salt Lake basin is about 28,500 square miles. The 
western portion, amounting to 12,500 miles, sends no water to the lake, 
yielding all its rainfall to evaporation within its own limits. The remain- 
ing 16,000 miles includes both plains and mountains, and its tribute is 
unequal. To supply 664 inches annually to the whole area of the lake, 
2,125 miles, it must yield a sheet of water with an average thickness of 
8.83 inches. In former times, when the lake had an area of only 1,820 
miles, the yield of the same area was 7.43 inches. The advance from 7.43 
to 8.83, or the addition of 1 inch and 4 tenths to the mean outflow of the 
district, is the phenomenon to be accounted for. 
All the water that is precipitated within the district as rain or snow 
returns eventually to the air, but different portions are returned in different 
ways. -Of the snow, a portion is melted and a portion is evaporated with- 
out melting. Of the melted snow and the rain, a part is absorbed by vege- 
tation and soil, and is afterward reabsorbed by the air; another part runs 
from the surface in rills, and a third part sinks into the underlying forma- 
tions and afterward emerges in springs. The streams which arise from 
springs and rills are again divided. Part of the water is evaporated from 
the surfaces of the streams and of fresh water lakes interrupting their 
courses. Another part enters the adjacent perous soils, and either meets in 
them the air by which it is slowly absorbed, or else so saturates them as to 
produce marshes from which evaporation progresses rapidly at the surface. 
The remainder flows to Great Salt Lake, and is in time evaporated from its 
surface. The lesser portion of the precipitation enters the lake; the greater 
is intercepted on the way and turned back to the air. Whatever man has 
done to clear the way for the flowing water has diminished local evapora- 
tion and helped to fill the lake. Whatever he has done to increase local 
evaporation has tended to empty the lake. 
The white man has modified the conditions of drainage, first, by the 
cultivation of the soil; second, by the raising of herds; and, third, by the 
cutting of trees. 
IO AR 
