374 



AMERICAN FORESTRY 



makes it easy to believe that this factor, 

 combined with the clearing away of the 

 forest for agricultural purposes, has been 

 the chief cause of the intensified flood 

 conditions which the records now disclose. 



"Considering the streams which drain 

 the mountain regions, the most vital parts 

 of the watersheds are the highest parts. 

 It is there that precipitation is the heav- 

 iest, slopes the steepest, and the whole 

 sum of conditions most influential. There- 

 fore, so far as stream flow is concerned, the 

 mountains are the portion which should be 

 given the most judicious care. It is on the 

 mountains that the best condition of soil 

 and ground cover should be maintained. 

 As the best ground cover, as has already 

 been explained, is the forest, it is to the 

 benefit of the streams that the mountains 

 should be kept densely forested, in order 

 that the conditions for the absorption and 

 retention of moisture may be as nearly as 

 possible ideal. Under such conditions the 

 course of the moisture will be one of con- 

 tinual interference from the time it 

 reaches the crowns of the trees till it is 

 in the stream. Every obstruction that can 

 be placed in the way of the water aids by 

 that much the even flow of the rivers. 

 Under perfect forest conditions it is well- 

 nigh impossible for any surface run-off 

 to take place. The rain first comes into 

 contact with the crowns of the trees, and 

 the drops are broken more or less into 

 spray. Some of the intercepted water runs 

 down the trunks or drips from the branches. 

 That which escapes interception by the 

 trees encounters the undergrowth and 

 brush immediately above the ground and is 

 intercepted and retarded. As the water 

 continues to make its way downward the 

 ground litter offers the next obstacle. The 

 water is caught up by the litter, and not 

 until this has become saturated can the 

 water pass on. If the rain continues for 

 bome time the litter becomes filled to its 

 capacity, and then the humus begins to 

 absorb the water which the litter can not 

 hold. After a time it, too, becomes satu- 

 rated. The water then gradually filters 

 into and through the underlying soil. If 

 this is deep it stores waters in large quan- 

 tity and then allows the surplus to pass to 

 still greater depths, where it finally pene- 

 trates the rock strata and replenishes the 

 great underground reservoir, from which it 

 t-raerges later as springs, some of which 

 may be far removed, even on the opposite 

 side of the mountain, from the place where 

 it has come in contact with the ground. If 

 the precipitation is in the form of snow 

 its course is substantially the same, but 

 with a still greater delay in passing 

 through the litter and entering the soil. 

 The period that may elapse between the 

 fall of rain and the appearance of water 

 in a river varies from minutes to months 

 or even years. 



"Even with the best regulated system of 



checking and storing the rainfall a con- 

 dition may arise where, to a large extent, 

 the effect of obstruction and storage in a 

 forest may temporarily be lost. Severe and 

 prolonged freezing may result in a sheet of 

 ice in and over the upper soil, which pre- 

 vents the entrance of water, and whatever 

 falls in the form of rain must at once run 

 off into the streams. Many rivers expe- 

 rience their worst floods when their drain- 

 age basins are frozen. On account of this 

 condition it is impossible to claim that ex- 

 treme floods will not come in a stream 

 with a forested watershed. Fortunately 

 the condition arises but seldom. 



"In such cases the conditions are similar 

 to what they would be were the covering of 

 the soil removed. The disastrous floods 

 which come once or twice in a generation 

 when heavy rains descend upon snow and 

 frozen ground would probably be duplicat- 

 ed or surpassed yearly, or several times a 

 year, were the soil stripped from the moun- 

 tain regions. The heavy rains of summer 

 which now fall upon a dense vegetation 

 and hardly affect the larger rivers would 

 then produce destructive floods. 



. The table on a preceding page gives data 

 regarding the flow of 10 important rivers 

 of the United States. It has not been 

 possible to obtain for each watershed a 

 record of the changes which are believed 

 to be responsible for the increased flood 

 conditions which the record shows. It has 

 been possible to make a detailed study of 

 parts of the watersheds of two of the 

 streams, the Cumberland and the Red. 



A Watershed Where Conditions Have 

 Grovm Worse 



"The Cumberland River is a good exam- 

 ple of how conditions have changed for the 

 worse on some of these watersheds. 



"The drainage area of the Cumberland 

 River above Burnside, Ky., comprises 3,739 

 square miles and lies in the heart of the 

 Cumberland Mountains. The figures given 

 in the table show that floods have in- 

 creased in the Cumberland River at Burn- 

 side, Ky., in number from 32 in the first 

 half to 43 in the second half, and in dura- 

 tion from 89 days in the first half to 102 

 days in the second half, while low waters 

 have increased in times from 61 in the first 

 half to 65 in the second half, and in days 

 from 1.261 in the first half to 1.576 in the 

 second half. 



"The removal of the forest on this water- 

 shed is progressing from three causes: (1) 

 Clearing for agriculture; (2) lumber 

 operations; (3) fires. 



"In 1890, when these measurements were 

 begun, 21 per cent of the watershed above 

 Burnside was cleared; in 1908 the cleared 

 area had grown to 32 per cent. During the 

 eight years 1900 to 1908 clearing of forest 

 for agricultural purposes went on on this 

 watershed at the average rate of five-eights 

 of 1 per cent per year. 



