SURFACE CONDITIONS AND STREAM FLOW 



373 



eration of the flow of any stream should 

 therefore take particular note of the soil 

 and of the changes in its capacity for water 

 storage. 



"Scai'cely separable from the soil itself 

 is another factor of equal or greater im- 

 portance in the disposition of water. This 

 is the ground cover, or, to speak of it more 

 particularly, the condition of the surface 

 of the ground; that is, whether it is barren 

 rock or clay, pasture land, cultivated land, 

 or forest. 



The condition of the surface does not 

 influence very much, so far as known, the 

 amount of water which falls. It is claimed 

 by some that the precipitation is greater in 

 the forest than on unforested land, and 

 the figures given in the table at the begin- 

 ning of this paper seem to indicate a ten- 

 dency toward lessened precipitation on 

 those watersheds which will presently be 

 shown as having a decreased forest area. 

 However, it is not intended at present to 

 lay any stress upon that point. If the in- 

 fluence of the various factors which affect 

 stream flow have been correctly stated, 

 none of them is suflScient to have caused 

 the increased floods. The natural conclu- 

 sion is that the increase must be due to 

 changes in the ground cover, or to the 

 ground cover in conjunction with changes 

 in the soil. Is the influence of the ground 

 cover and of the soil suflScient to have 

 caused the results? 



"It is generally agreed by those who 

 have studied the subject most thoroughly 

 that the forest offers the best conditions 

 for absorption and underground storage. 

 Next to the forest comes well-cultivated 

 farm land; then meadow and pasture land; 

 while the worst conditions of all are to 

 be found on barren surfaces of stone, clay, 

 or gravel, which because of inferiority are 

 unable to support growth of any kind. 



"The reasons why the forest offers the 

 best conditions for absorption and ground 

 storage are several. It does so, in the first 

 place, because the foliage of the trees forms 

 a storage place from which water drips 

 slowly to the ground for a considerable 

 length of time after each rain; because the 

 complex layer of brush, leaves, weeds, 

 mosses, and vines, all in a more or less 

 advanced stage of decay, becomes filled 

 with moisture with each heavy rain and 

 holds it for a considerable time; because, 

 also, the surplus water so stored continues 

 to be absorbed by the upper humus-filled 

 layers of earth; because, further, the tem- 

 perature is lower and the air more humid 

 in the forest than in the open; finally, be- 

 cause the snow lies there much longer. To 

 these reasons must be added the mechani- 

 cal power of the roots to go deep into the 

 soil and break up the rocks, thus forming 

 channels for the ready entrance of water 

 into the earth. 



"On a level or slightly inclined surface a 

 well-tilled soil may be as effective in ab- 



sorbing and holding water as a forest soil. 

 Where the slope exceeds 10 per cent, culti- 

 vation does not long go on before erosion 

 sets in, and erosion if unchecked will 

 remove the soil and gully the surface until 

 all fertility has gone and all protective 

 power is lost. Agriculture under right 

 conditions may be an effective means of 

 stream protection; under wrong conditions 

 it may be the greatest menace to the even 

 flow and usefulness of the streams." 



From this point we give the language of 

 the circular in full. 



THE PBIXCIPAL CAUSES OF INCBEASED FLOODS 



"Undoubtedly it is the clearing away of 

 the forest on the mountainous watersheds 

 of the streams which have been described 

 that has caused the great increase in fre- 

 quency and duration of floods. 



"It is a known fact that the forests on 

 these several watersheds have been cut 

 away with great rapidity during the past 

 thirty years. European conditions proba- 

 bly offer no parallel to the rate at which 

 these watersheds have been cleared to 

 make way for the rapid advance of agri- 

 culture and to supply the great manufac- 

 turing industries with the wood which 

 they require. Forest lands, which offer the 

 best possible conditions for soil absorption 

 and underground storage, those great regu- 

 lators of stream flow, have been changed 

 to poorly tilled agricultural lands, which 

 are not so good. Then these agricultural 

 lands after a few years have been exhaust- 

 ed and their soil eroded into deep gullies. 

 Finally, many of them have been turned 

 into pasture or even entirely abandoned 

 because they reached a condition where 

 they could not support even a growth of 

 grass and weeds. The best condition has 

 been changed to the very worst condition. 



"Again, repeated burning of forest lands 

 has tended to reduce the thickness and 

 value of the ground cover and to lessen the 

 power of the soil to absorb and to store 

 water. The extent of damage from fire, 

 t-o far as water storage is concerned, is 

 generally vastly underestimated. A forest 

 fire strikes both above and below the sur- 

 face. It injures or kills the trees, destroys 

 the undergrowth and brush, and consumes 

 the great forest sponge the ground cover 

 and the humus. The extent of the injury 

 is, ot course, not always the same. It is 

 sometimes slight: sometimes very great. 

 Repeated fires tend to every kind of in- 

 jury that can possibly be inflicted upon a 

 forest soil, completely destroying the cover 

 down to the mineral substances, and thus 

 lendering it defenseless against the at- 

 tacks of erosion. When that stage is 

 reached, it may be depended upon that the 

 run-off of the watershed has been profound- 

 ly affected and the regimen of the stream 

 materially changed. The fact that forest 

 fires have repeatedly swept over the water- 

 sheds of the streams under consideration 



