DISCUSSION : FORESTS, RESERVOIRS, AND STREAM FLOW 367 
course, extreme conditions determine certain elements, such as the Mr. Swain. 
height of levees. A few great, but not extreme, floods may do more 
damage than one extreme flood, and may necessitate more expenditure 
for dredging and other purposes. 
From every point of view—power, navigation, erosion, irrigation— 
it is of even more importance to diminish the frequency of great 
floods than the height of extreme floods which occur only at long 
intervals, and to keep up the usual low-water flow rather than the ex- 
treme low-water flow which seldom occurs. 
If the writer fully understands the argument with reference to 
extreme floods, it appears to be this: Great floods are always the 
result of combinations from various tributaries, occurring after periods 
of long-continued and widespread precipitation; the forest bed be- 
comes completely saturated, its storage capacity exhausted, and when 
this point is reached the water flows from the forest surface without 
restraint. 
With reference to this it is, of course, evident that, when the forest 
bed has become fully saturated, it cannot hold any more water; but, 
even then, it will hinder its discharge, because the soil will not be 
washed away and the torrents will not flow down in gullies. In this 
sense the forest floor never discharges such torrents as come from un- 
forested areas. Moreover, it is certainly a gain if forest floors can 
absorb even a part of the rainfall and pay it out gradually to the soil 
beneath, and it seems quite erroneous to urge that because their storage 
capacity is not unlimited it is therefore not valuable. The author states 
that the fact that the forest bed has retained a portion of the rainfall 
and will yield it up later may produce a worse condition than if the 
country were clear, and asserts positively, but without proof, “that the 
forest does promote tributary combinations there would seem to be no 
question, and that it may therefore aggravate flood conditions neces- 
sarily follows.” Perhaps the writer does not fully understand what is 
intended, but he is perfectly ready to admit that a case may be imag- ; 
ined in which the statement of the author would be true. Suppose, for in- 
stance, a storm is moving in the direction shown by the arrow on Fig.4, It 
