THE CLIMATIC THEORY OF TERRACES. 



33 



feet, at C, while at D there will be none whatever. Therefore the curve will assume a new 

 form. It will be concave as far as C , but beyond that it will become convex. In other 

 words, beyond C" there will be a steeper slope than above it, or than formerly existed 

 beyond C. Wlien the climate becomes more moist, and the revived stream flows in full 

 force past C and D, and on to the sea, its velocity will naturally be accelerated between 

 C and D. As it is not loaded to its full capacity, it will inevitably begin to erode the gravel 

 and silt of its own previous deposits. A gully will soon be formed, and will rapidly work 



Fig. G. — Profile of Climatic Terraces. 



backward toward B. In course of time the stream will once more make its bed concave 

 upward, perhaps at the level C'. Then it will widen its channel as well as deepen it, and 

 we shall have a flood-plain bordered on either side by a terrace. 



TERRACE-MAKING AT THE PRESENT TIME. 



The process described above seems to be taking place on a small scale at the present 

 time, although man interposes in a way which makes the matter confusing. To-day, as 

 has been said, the Santa Cruz River flows in a channel from 100 to 300 feet wide and from 

 10 to 20 feet deep. The channel is cut in a smooth alluvial plain from half a mile to a mile 

 wide. Thirty years ago the alluvial plain was a genuine flood-plain, and there was no inner 

 channel whatever. In times of flood the river wandered over all parts of the plain, flowing 

 very slowly and not eroding at all. At Tucson, as we have seen, the mail was sometimes 

 rowed across half a mile of water, although now the river passes under a bridge scarcely 

 100 feet long. The water must have carried sediment when it left the mountains, but 

 by the time it reached Tucson it had spread out so much and fallen to such a low 

 velocity that it had deposited most of it. A few miles farther downstream it came entirely 

 to an end as a surface stream, although some underground water seems to have come to the 

 surface and made a new stream at a point farther down toward the Gila. In the last two 

 decades of the nineteenth century man interfered with nature's plans. For one thing 

 he introduced cattle, which ate the grass, and which also made paths. He also dug ditches 

 for irrigation in various places near Tucson, along the portions of the river corresponding 

 to C'-D in figure 6. Then, at the end of the eighties, there came some unusually heavy 

 rains. The paths trodden by the cattle, and still more the ditches dug by man, served as 

 gathering-places for the water which had previously flowed in a slow sheet among grasses 

 and bushes. Being confined in a channel the water gained enormously in erosive power 

 and quickly deepened and broadened its bed. One of the first places where this happened 

 was along an irrigation ditch at the San Xavier Indian Reservation, 9 miles south of Tucson. 

 Later it occurred in still worse form at the ditch of Mr. Samuel Hughes, about 2 miles north 

 of Tucson. Subsequent floods enlarged the various channels, which finally coalesced into 

 a single broad channel extending tens of miles above and below Tucson. A record of the 

 whole matter can be seen in Plate 1, figure a, from a photograph taken at the "Point" of 



