ment of water power: (1) rugged relief, (2) lakes or other reser voirs, and (3) an abundant rainfall well distributed throughout the year.
(1) How Rugged Relief Favors the Use of Water Power.—In a rugged country the streams descend rapidly, and thus furnish a proper head of water. How important this is may be illustrated by comparing the Mississippi River in its upper and lower portions. The available power from the main stream of the river during its course of nearly a thousand miles in the great central plain, where it descends only five inches per mile, is only 147,000 horse-power. A smaller amount of water flowing a similar distance in the upper trib utaries in regions of rugged relief where it descends rapidly, is cap able of furnishing 6,430,000 horse-power, or about forty-three times as much as in the plain.
(2) How Lakes Favor the Use of Water Power.—Lakes are also a great help in the development of water power. They serve as reservoirs so that the volume of the rivers which flow from them varies relatively little from season to season. For example, the Niagara River, corning from the huge reservoirs of the Great Lakes, carries only one-third more water at its highest than at its lowest level. The Potomac, with no lakes whatever, is sometimes 250 times as large in flood as at low water. In July, 1911, a drought caused the lakeless Catawba River in the Carolinas to become so low that 152 cotton mills shut down for lack of power, and 70,000 operatives were thrown out of work. Such variations do so much harm that power companies have spent millions of dollars in creating artificial lakes by means of dams. This has been done on many small rivers, the Connecticut and its tributaries being notable examples.
The presence of abundant vegetation has somewhat the same effect as lakes in steadying the volume of rivers. Where the slopes are well covered with vegetation, the rain does not run off all at once, but is caught in the rootlets and soil and seeps out slowly in springs. This is one of the chief arguments for forest conservation.
(3) How Abundant Rainfall Favors the Use of Water Power.—
The value of abundant and regular rainfall in promoting the use of water power may be judged from a comparison of Wisconsin and Nevada. Although Wisconsin is only half as large as Nevada and is much less rugged, its water power possibilities are se veral hundred times as great because of its heavier rainfall. In the northern Pacific drainage area of Washington, Oregon, and Idaho the abundant rains combine with favorable relief to cause that region to be capable of furnishing two-fifths of the water power of the United States. Al though the water power in that region is not yet greatly developed, it may some day furnish nearly 15,000,000 horse power, that is, more than is obtained from the 150,000,000 tons of coal burned each year by the railroads. This, however, would require a dam every few miles, that is, at every place where a head of water could be secured. The presence of cheap power is sure some day to cause that region to become prominent in manufacturing.
How Seasonal Variations Hinder the Use of Water Power.—The chief disadvantage of the northwestern water power is that, although the rains fall heavily part of the year, they diminish greatly in sum mer. In the United States the irregularity of the rains reaches a maximum in the Southwest. The winter rains on many of the mountain ranges of Utah, Arizona, and southern California would furnish abundant water power, but it does not pay to build power plants because they would have to be idle during the long dry sum mer. Moreover, they might be ruined by the floods which are char acteristic of such regions, where the bare slopes of the mountains have little vegetation to hold back the water in winter. Some of these mountains, however, are so high that much of their precipitation takes the form of snow. If this melts slowly it acts like a reservoir, and holds back the water until the warm dry season when it is needed. Sometimes it melts rapidly and forms bad floods. Some of the worst floods in regions like New York and Pennyslvania are due to the melt .