248 FORESTS, RESERVOIRS, AND STREAM FLOW 
and that the spray works intermittently. If the showers are small in 
volume and the intervals between them long, the sand may retain 
nearly or quite all of the individual showers and give them off in 
evaporation, so that there will be no run-off whatever. 
Between these two extreme conditions, the covered area will exert 
a greater or smaller regulative effect upon the run-off. The retentive 
power of the sand will be less as the slope of the surface upon which 
it rests increases, or it will be greatest when the surface is nearly 
horizontal and least when it is nearly vertical.* 
Now in Nature this ideal illustration is never fully exemplified in 
the cleared land and the forest. There is nearly everywhere a marked 
retentive capacity in the bare soil. In newly plowed ground it is 
probably greater than in the forest. Moreover, certain crops, like 
heavy grass or grain, obstruct the flow of water almost as much as the 
forest cover. On the other hand, the furrows of cultivated fields, 
drainage ditches, roads and, particularly, the pavements and roofs of 
towns, greatly accelerate the run-off; so that, while the full contrast 
of the ideal example does not exist in Nature, the principle of the 
illustration applies perfectly. That is, there are times when the per- 
centage of retention in the forest bed is 0, and there are other times 
when it is 100; or, there are times when so much water comes that 
the forest bed can hold none of it and there are times when so little 
comes that it holds it all. Between these extremes there are periods 
when it holds more or less and gives up less or more and exercises a 
*Since the above was written the writer has noticed, in the report of the hear- 
ing on House Resolution 208 before the Committee on the Judiciary, that Gifford 
Pinchot, Assoc. Am. Soc. C. E., Chief of the Forest Service, used an illustration very 
similar to that given above, except that he failed to carry it to its logical conclu- 
sion. Addressing the Committee, February 27th, 1908, he said: “I have in my 
hand here a photograph of a denuded hillside. After the forest has been removed 
rain falls on that hillside and runs off rapidly, as the water I drop upon the photo- 
graph does now, and disappears instantly (illustrating). If, on the other hand, I 
place a forest cover on the hillside, that is exactly analogous in texture and effect 
with this piece of blotting paper, and drop the water slowly upon it, we would find 
that, instead of running off slowly at the bottom, the water is held (illustrating with 
blotting paper). Part of it runs off, but, as soon as the absorbent quality of the 
paper or the forest floor has time to take effect, the water is kept and drips grad- 
ually for a considerable length of time off the hill into the stream. This is an exact 
eo of the way in which the forest controls the stream flow on that hill- 
side. 
Mr. Pinchot should have completed his illustration. He should have continued 
to sprinkle the paper long enough and heavily enoughto have saturated it 
completely in order to show that the water would then flow from the paper as 
rapidly as from the uncovered area; and he should then have explained that this 
condition represents what always happens in the forest in times of great flood. 
Then he should have sprinkled the paper intermittently in small quantities, and at 
such long intervals that the warm air of the room would evaporate all of the ab- 
er wae oe bias ‘none whatever would flow away. He should then have ex- 
plaine a is condition represents what always takes pl i i 
Fe great Arouche y: Place in the forest in times 
