The Roosevelt Dam Case
Take a single case: that of one national forest in Arizona. At the upper end of this forest—which is a long, narrow tract covering a great mountain chain—rise two or three streams; on the eastern slope, the Rio Verde and the Salt River, on the western, the Agua Fria. A hundred miles below these heads the government is building, at a cost of more than $4,000,000, the great Roosevelt Dam which will furnish water to irrigate 250,000 acres of the richest of soils around the city of Phoenix in the Salt River valley. One of the most serious problems in the construction of the great dams in the West is the question of silt, which is washed down in the streams and will eventually fill up and render useless these expensive dams and reservoirs.
Careful studies of silt prove beyond doubt that its primal cause is the removal of the forest cover, such as underbrush, weeds, and grasses, along the streams, which allows the rainfall to run off rapidly. The grazing over these areas by sheep and goats not only exhausts this forest cover, but from the cutting up of the soil and the loosening effect of the thousands of tiny hoofs, the erosive action of the rain becomes disastrous. The wash of the hills and mountain-sides carries with it into the streams tons and tons of silt to fill up the dams and beds of the streams, as well as working irreparable injury to the comparatively thin soil covering the mountains.
On this national forest the watershed on the eastern side all runs into streams which eventually reach the Roosevelt Dam; on the western slope the water runs unused to the Gulf of California. So the National Reclamation Service, charged with the building and maintenance of these huge reservoirs, said to the Forest Service: "The watershed of the Roosevelt Dam must be protected from over-grazing, so that the forest cover may be preserved, and the deposit of silt reduced to the very lowest possible percentage."
