[Illustration: FIG. 2.--LOOKING DOWN THE COLORADO RIVER FROM ABOVE THE NEEDLES]
Over a large portion of the watershed of the Colorado the rainfall is light. This fact might lead one to think that upon its slopes the work of erosion would go on more slowly than where the rainfall is heavy. This would, however, be a wrong conclusion, for in places where there is a great deal of rain the ground becomes covered with a thick growth of vegetation which holds the soil and broken rock fragments and keeps them from being carried away.
The surface of the plateaus and lower mountain slopes in the basin of the Colorado are but little protected by vegetation. When the rain does fall in this arid region, it often comes with great violence. The barren mountain sides are quickly covered with trickling streams, which unite in muddy torrents in the gulches, carrying along mud, sand, and even boulders in their rapid course; the torrents in turn deliver a large part of their loads to the river. As the rain passes, the gulches become dry and remain so until another storm visits the region. It is storming somewhere within the basin of the Colorado much of the time, for the river drains two hundred and twenty-three thousand square miles. So it comes about that whether one visits the river in winter or summer one always finds it loaded with mud.
But what becomes of all this mud? The river cannot drop it in the narrow cañons. It is not until the river has carried its load of mud down to the region about its mouth, where the current becomes sluggish, that the heavy brown burden can be discharged. Dip up a glassful of the water near the mouth of the river, and let it settle, then carefully remove the clear water and allow the sediment in the bottom to dry. If the water in the glass was six inches deep, there will finally remain in the bottom a mass of hardened mud, which will vary in amount with the time of the year in which the experiment is performed, but will average about one-fiftieth of an inch in thickness. Each cubic foot of the water, then, must contain nearly six cubic inches of solid sediment or silt.
It has been estimated that the average flow of the Colorado River at Yuma throughout the year is eighteen thousand cubic feet of water per second. From this fact we can calculate that there would be deposited at the mouth of the river every year, enough sediment to lie one foot deep over sixty-six square miles of territory. Nearly one three-hundredth part of the Colorado River water is silt, while in the case of the Mississippi the silt forms only one part in twenty-nine hundred.