FOEEST LANDS FOB THE PROTECTION OF WATERSHEDS. 69 



A striking example of the action of forests on snow melting may be seen in 

 the mountains of the Pacific coast. Here are the densest forests in the world, 

 the deepest beds of humus, and the most perfect reservoir effect so long as it 

 is in action. Yet in this very region, particularly around Puget Sound, are to 

 be found some of the most torrential streams in the country. This fact is 

 largely due to the distribution of snowfall caused by the forests. Conditions 

 like the following are constantly developing. Heavy snowstorms sweep over 

 the forest-covered mountains. The snow can not drift, for the dense woods 

 break the wind. A great deal of it does not reach the ground at all, but hangs 

 on the branches and undergrowth all the way from the highest tree tops down. 

 This covering is often so dense as to prevent cruising operations altogether, 

 because the cruisers can not see the timber through the impenetrable screen of 

 snow. Of an 18-inch fall, perhaps 12 inches is on the trees and the rest spread 

 evenly on the ground. To show what now happens, let an illustration be drawn 

 from the opposite process of drying clothes. When the housewife has finished 

 her washing and wishes to dry the clothes, she does not set them out in a 

 basket, where it would take weeks for them to dry, but spreads them upon the 

 ground or hangs them on a line so that the sun and air can reach them on all 

 sides. So these forests increase, by a thousandfold, the exposed area of the 

 snow over what it would be if heaped in nature's clothes baskets (the great 

 drifts), and give it the maximum possible exposure to the melting influences 

 whenever these shall arrive. As a general rule these snowstorms are followed 

 by warm southerly winds and rains the rains frequently heavy in themselves 

 and rain and snow join hands, two storms in one, and rush down to the ocean 

 in tremendous freshets and Hoods. The Skagit Iliver, the largest in Washington 

 except the Columbia, and a very considerable stream, has been known to rise 

 1 foot per hour for sixteen hours, and this where the stream has a fall of 4 feet 

 to the mile, and carries off its floods very rapidly. A photograph taken on 

 another stream with only 480 square miles of watershed above it, shows the 

 terrific power of these streams that come down from the most densely wooded 

 and perfectly protected watershed in existence. The great flood of 1906 in this 

 section was a perfect demonstration, not only of the vast intensifying effect of 

 forests upon floods due to snow melting, but of the utter helplessness of the 

 forest bed, when saturated with long rains, to restrain floods. 



The same effect was very manifest in the great flood of 1907 in the valley of the 

 Sacrameuto River, California. The tributaries on the east side come down from 

 the densely wooded slopes of the Sierras; those on the west side from the bare 

 or sparsely wooded slopes of the Coast Range. If the forest theory be true, 

 these smooth western slopes should send down a greater flow for the same pre- 

 cipitation than the eastern slope. Exactly the reverse seems to have been the 

 case. For the period, March 17-26, the precipitation on the Puta Creek water- 

 shed, on the west side (805 square miles), averaged 22.7 inches. The maximum 

 resulting run-off per second per square mile for one day was 39.1 cubic feet. 

 Directly across the valley on the Sierra slope the precipitation, on the American 

 River watershed (2,000 square miles), averaged 14.6 inches for the same period, 

 and the maximum daily discharge was 48.7 cubic feet per second per square 

 mile. Considering the fact that unit run-off for the same conditions is always 

 less the greater the watershed, this result is quite remarkable. It is undoubt- 

 edly due to the action of the Sierra forests on snow melting, and again illus- 

 trates the inability of forests to exercise any restraining influence upon great 

 floods. 



During the spring of 1908 occurred a record-breaking flood in western Mon- 

 tana, nearly all the streams on both sides of the Continental Divide going far 

 over their banks. As might have been predicted, this occurrence was promptly 

 cited as another example of the effect that a forest-barren country has upon 

 floods. Nevertheless it is as certain as anything of this kind can be, that if the 

 country affected by this extraordinary downpour (in some places breaking all 

 previous records) had been thickly forested, and the ground still covered, as it 

 would have been, with a solid layer of saturated snow, the flood would have 



a In the paper, The Flood of March, 1907, in the Sacramento and San Joaquin 

 River Basins, California, by Messrs. Clapp, Murphy, and Martin, published in x 

 Proceedings American Society of Civil Engineers for February, 1908, the author' 

 say : " In the Sierras the greater part of the precipitation is normally in the form 

 of snow, and the magnitude of floods depends largely on the rate of melting. A 

 heavy warm rain on deep, freshly fallen snow produces a maximum run-off." 



