32 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



fail to reach the main rivers except in occasional floods. The death of the vegetation will 

 lead to the denudation of the mountains, as may be seen on a small scale in certain places 

 in Oregon or the Southern States where forests have been cut and afterward the ground 

 has been burned over in such a Avay as to kill off the roots and new shoots, or where land 

 has been carelessly plowed in such fasliion that the rains have had a chance to wash away 

 the soil. In a country where the climate has become so dry that plants will no longer 

 grow in abundance, there is nothing to check the process of denudation, and ultimately 

 the slopes will become almost absolutely naked, as they are in Persia. Such a condition 

 of extreme denudation, as has been shown in one of the earlier sections of this report, is 

 also characteristic of the lower mountains in Arizona. 



The rapid removal of soil from the slopes of the mountains will inevitably increase the 

 load of the streams, and in many cases will overload them. Accorchngly, wherever the 

 grade is less steep than on the slopes or in the minor tributaries, the advent of aridity will 

 cause deposition to begin at once, either at the base of the mountains or in the larger 

 valleys. The streams which fail to reach the main rivers, as the majority of those of 

 Arizona now do, must inevitably deposit all the rock waste which they carry. Formerly 

 they bore part of it to the sea, but now none whatever gets there in many cases, and only 

 a small fraction of the former amount in the case of streams which occasionally run through 

 to the main rivers in times of flood. This process of deposition tends to build up deep 

 accumulations of gravel in the valley bottoms and vast fans or allu\dal aprons (bahadas) 

 at the base of the mountains. As these deposits increase in size the streams are less and 

 less able to reach the sea. They are obliged to flow for long distances over porous deposits 

 of gravel and silt which are rarely saturated with water and which accordingly act as great 

 sponges. The grade is continually diminished, also, which tends to make the streams 

 spread more widely and therefore evaporate or sink into the ground more rapidly. Thus, 

 so long as aridity continues, the main mountain valleys and the piedmont regions tend to 

 retain all the material which comes down from the mountains. Wliere the mountams are 

 high and the slopes steep the process of bringing material from them is naturally rapid. 



To complete the process of terracing the only requisite is a return to moist conditions. 

 Vegetation will increase in amount, the streams will become more uniform in size from 

 season to season, the gravel deposits will become saturated with moisture, the water of 

 the streams vfill be less subject to loss by sinking into the ground and by evaporation, 

 and the streams will become longer. Many streams which formerly came to an end at 

 the foot of the mountains will now flow through to the sea. In theh- upper portions they 

 will be supplied with waste less abundantly than hitherto, because the greater abundance 

 of vegetation will tend to hold in place whatever new soil may be formed. Hence the 

 streams will not be so heavily loaded with waste as previously. They will possess the 

 relatively clear character of rivers in rainy regions, such as the Connecticut or the Illinois, 

 rather than the muddy character of the Missouri or Colorado. Being clear, the rivers 

 and streams will also be ready to become erosive agents at the first opportunity. They 

 will find their opportunity when they leave the mountains and flow out beyond the limits 

 ordinarily reached in the prececUng dry epoch. Figure 6 illustrates the matter. Suppose 

 that originally a stream flowed from the mountains at A down into the low country at C, 

 and to the ocean at E. During an ensuing time of aridity, suppose that the extreme limit 

 of floods was D, and that usually the stream entirely disappeared by the time it reached C ; 

 under such ckcumstances deposits w^ould accumulate as shown in the figure between 

 the lower Une passing through C and the upper line passing through C. The lower Une 

 represents the normal profile of a stream. It is concave upward because, after a stream 

 has once attained the thoroughly graded condition characteristic of maturity, the slope 

 steadily decreases from head to mouth. A diy epoch will manifestly destroy the perfect 

 concave curve, for there will be abundant deposition, amounting perhaps to hundreds of 



