Page:Popular Science Monthly Volume 59.djvu/137

Rh a large measure the reservations of these regions and the preservation of their forest cover that give such great value to the adjacent cultivated fields. It is the water and not the land that has value. It is the perennial supply, flowing from the reserved and unreserved forests of East and Central Arizona, that has in the past two decades rescued the Salt River Valley from its former barrenness, with its scattered growth of creosote brush and cacti, and transformed it into one of the most fertile and productive areas in America. It is the forest cover of the San Jacinto and San BarnardinoBernardino [sic] reservations in Southern California that gave Riverside and Redlands her splendid orange groves and made possible the development of a productive and thriving community.

When in our Western forests one is constantly impressed by the change in relative humidity wrought wherever the forest has been removed. Springs have disappeared and cañons and ravines are now dry, where there were formerly perennial streams. Under the leaf mold and other débris of the forest, the soil is always moist, while on denuded areas in the same locality it is parched and dry. Everywhere the deep mulch forming the floor of the forest grasps the descending rains and melting snows and guides them into the deeper recesses of the earth. Where the forests have been destroyed, or even the mulch and litter forming the forest floor, as it so often is by fire or the excessive grazing of sheep, the rains for the most part, instead of sinking into the soil, pass over the surface, carrying silt and other debris into the streams and reservoirs, causing vital injury to irrigation enterprises.

So also in the semi-arid regions, where there are no forests, or where they have been destroyed, the wind has a free sweep, resulting in an enormous increase in evaporation. In some instances the evaporation from a water surface exposed to the free sweep of the wind reaches a maximum of thirteen inches in a single month. In exposed situations, snows a foot in depth are frequently lapped up in a single day without even moistening the soil beneath. We do not appreciate how great the necessity for the preservation of the forests is to the irrigable West.

Reservoirs for the purpose of impounding water to be used in irrigation have been constructed by private enterprise in many parts of the West, and the possibility of governmental construction of such reservoirs is by no means improbable. Effective reservoirs are not possible in our irrigable regions without due regard for the forests that feed the streams which fill them. Forests everywhere are the great preventors of erosion, and nowhere is this more evident than in our Western mountains. The utility of reservoirs, and, to a lesser extent, of distributing canals and laterals, becomes destroyed as they fill with