18 CHAPARRAL. 
The amount of water lost by evaporation from open irrigation 
ditches is about 25 per cent and the amount lost from an uncovered 
mountain stream is certainly no less. Often heavy floods completely 
destroy the cover along streams, exposing them to excessive evapora- 
tion. It is a common statement of all who have given the matter 
careful study that during the vears immediately following such a 
disaster the water supply is perceptibly decreased and small streams 
often cease flowing altogether. 
Among the people of southern California the belief is general that 
the chaparral regulates the flow of streams. Even though some 
streams have flood periods and nearly all have intermittent flow, it 
is assumed that the chaparral makes conditions of stream flow much 
better than they otherwise would be. Exact evidence of the extent of 
the chaparral’s influence on the water supply is, however, often lack- 
ing. After a severe fire on a watershed the flow of a stream may so 
alter, its period so shorten, and its value so largely disappear as to 
leave little doubt that the fire has affected the stream. On the other 
hand, but few data have been collected to show whether streams will 
resume their normal flow when the chaparral cover has reestablished 
itself. It is not the purpose of this bulletin to enter into a tech- 
nical discussion of this subject. Accurate scientific data will not be 
available until rain gauges are established over the watersheds and 
measuring weirs placed at proper points to determine the run-off 
and the data from-burned, unburned, and restocked watersheds com- 
pared. It is possible, however, to give the evidence of several close 
observers of conditions in the chaparral region as it bears upon the 
water supply. 
Prof. George Davidson, for 50 years a student of California, said, 
in 1903: 
It must be patent to everyone who has studied this subject that if the whole 
area of the western slopes of the California mountain ranges were bare of 
foliage the rainfall would rush into the waterways and come down into the 
plains with torrential force, just as it does through many of the eastern treeless 
slopes of the Alps into the plains of Italy. All the finer material from the 
whole surface would be quickly brought down, then the heavier materials, 
gravel, and bowlders. We have clear evidence of this in the southern part of 
California. 
But with all these slopes forested and further protected by undergrowth 
these rain waters would be held back; they would first saturate the soil, and 
only a relatively small percentage would promptly reach them directly and by 
percolation. Damaging freshets would occur only after heavy and continuous 
rain storms, and then, in spite of forests and chaparral, the storm waters would 
be hurried forward with torrential force to the plains and be lost in the ocean, 
unless retained by dams and reservoirs. 
During the years from 1881 to 1883, inclusive, fires were set by 
herdsmen back of Los Angeles. The year 1884 saw a number of 
disastrous floods in the region, which ruined much agricultural land. 
