Now, were all the steep land in the Mississippi valley kept covered with trees, as it should be, this enormous amount of sediment would not come down to raise the bottom and the dykes, and to do much other harm. In time it may break through its banks and make new channels for itself, leaving important towns high and dry, and, of course, destroying much property where its new route is cut. The vast mass of vegetable mould, ashes, etc., which will be washed down into the Hudson—if the forests of the Adirondacks are ever destroyed by fire, as there is danger that they may be—by the great floods which denudation of those mountain sides will often cause, will quite possibly ruin the navigation of that river and the harbor of New York, not to speak of the destruction of farms, factories, and towns lying where those floods can reach them.
Next in our list of evils prevented by forests come Droughts and Floods (_c_). The annual supply of water from rain and snow, if held back by woods on steep hillsides until it can soak down to the underground sources of springs, or if stored up as in a sponge by the mass of fine roots, dead leaves, decayed wood, moss, etc., which often accumulate on the surface under old forests to the depth of two or three feet—may be made to last through a whole summer. There falls from the clouds—one year with another—only a certain amount of water in any particular region. If there is nothing on the hillsides to hinder its rush down into the streams, it is lavished like the money of a spendthrift, where it does little or no good, and very likely much harm. The difference between the two is that between feverish, riotous waste and sober plenty. “Waste not, want not,” is as good a maxim for the management of water as for that of cash.
A torrent is a stream liable to extreme and sudden increase and decrease—usually very small or quite dry in a dry time, but liable to rise suddenly to a great height, and as quickly to shrink to its former size. By the loss of its once rich forests, the Ardêche, a tributary of the Rhone, became such a torrent, its principal branch often being entirely dry. It has been known to rise sixty feet and dwindle back to almost nothing within a few days. The upper Hudson has apparently all the conditions necessary for becoming such a torrent if once its forests are exterminated. It descends some 4,000 feet in a short and steep course, from a region where there falls a great deal of rain and snow.