FOREST LANDS FOR THE PROTECTION OF WATERSHEDS. 87 



does this apply to mountain forests more than to any others? It is incontest- 

 ably true that whatever restraining effect forests have upon run-off is greater 

 upon the lowlands than upon steep mountain sides. This legal feature of the 

 question will be referred to further on. 



EESERVOIBS IN THEIR RELATION TO STREAM FLOW. 



Under this heading artificial reservoirs alone are included. Natural reser- 

 voirs of various kinds exist nearly everywhere and exert a profound influence 

 upon stream flow. The ground is the most important of these, absorbing on the 

 average probably one-third of the total rainfall. Natural lakes are great regu- 

 lators, the St. Lawrence system being the most perfect example. Forests are 

 effective reservoirs at certain seasons. Swamps and low-lying grounds along 

 river courses, like the great flood basins of the Sacramento and the Mississippi, 

 are, in their natural state, enormous reservoirs which greatly reduce the flood 

 flow of the river channels. Snowdrifts, particularly the great drifts of the 

 mountains, are splendid reservoirs. The streams themselves have immense 

 storage capacity ; for example, the Mississippi within levees stores at least 

 2,000,000,000,000 cubic feet of water from Cairo to the Gulf, between extreme 

 high and low water stages. All these reservoirs and many of less importance 

 are ever active in regulating the flow of streams. Without them precipitation 

 would flow off as fast as it arrives and our greatest floods would be magnified 

 many times. 



Here we are considering only those reservoirs constructed by man to sup- 

 plement and extend the regulating effect of nature's reservoirs. If the conclu- 

 sions reached in the first section of this paper are correct, forests can not be 

 relied upon in any degree to help solve the problems of high and low water. 

 Present conditions must be met by purely artificial means, since man has so far 

 discovered no way of controlling the climatic conditions which govern precipita- 

 tion. He can not " stay the bottles of heaven " in times of flood, nor open them 

 in seasons of drought. He must take the water after it reaches the earth and 

 deal with it the best he can. 



The artificial reservoir is intended to attack this problem at its source. It 

 catches and holds back the water in the near vicinity of its deposition, instead 

 of waiting until it gathers into the rivers and then building huge bulwarks to 

 contain it there in times of flood. It saves the stored-up supply and gives it out 

 in the low-water season, thereby helping navigation, instead of dredging and 

 otherwise treating the water courses to increase the low-water depth. It cor- 

 rects one of the greatest deficiencies of nature by abolishing inequalities of 

 stream flow and converting waste into utility. Theoretically, it is the perfect 

 plan. It has always appealed to the imagination of laymen and professional 

 alike. It has often been resorted to, and the number of reservoirs in the world 

 is very great and constantly increasing. Hitherto they have been mainly used 

 for power, municipal supply, irrigation, and for navigation in canals. In very 

 few instances have they been applied to improve the navigation of large natural 

 water courses, and in none, so far as the author is aware, for the exclusive pur- 

 pose of preventing floods. 



The question arises, AVhy are they not regularly applied to these last- 

 mentioned purposes? The answer may at once be given that in the general case 

 the cost is greater than the benefits to be received. This element of cost arises 

 mainly from the absence of good sites (including dam sites as well as holding 

 basins) and also, to considerable extent, from an interference with the purely 

 artificial conditions growing out of the settlement of the country. 



The best reservoir site is a natural lake. Such a site is already covered with 

 water, and original conditions are not materially changed. Evaporation is not 

 much increased by the necessary enlargement. Smaller and safer dams accom- 

 plish a given storage than for the average dry site. The question of public 

 health involved in uncovering large areas for reservoir beds in the heated por- 

 tion of the year is less serious. Everything makes these sites the most advan- 

 tageous that can be found, and it may be laid down as a rule that the public 

 good requires the utilization of every such site to the fullest possible extent. 



An interesting feature of these natural reservoirs may be noted. A natural 

 lake, wholly uncontrolled at its outlet, may have a more effective control of the 

 outflow than an artificial reservoir of equal superficial area when full, though 

 of far greater capacity between high and low water. The outflow from a lake 

 can be increased only by storing simultaneously a quantity of water measured 



