468 C. JR. Keyes — Aggraded Terraces of the Bio Grande. 



300 miles, the Rio Grande receives no lateral augmentation 

 from perennial tributaries in a distance of nearly one thousand 

 miles, or nearly to the mouth of the Rio Pecos in Texas. 

 Through most of this part of its course the river-bed is 1,000 

 to 2,500 feet below the level of the bolson plains lying on 

 either side of the great valley. 



Physiographically the Rio Grande has had a complex origin. 

 That portion of its course which lies in New Mexico is largely 

 antecedent in character. Other parts may be consequent. 

 Climatic conditions with which most of us are not very famil- 

 iar are chiefly responsible for many of the bold and unusual 

 relief effects that characterize the region through which the 

 Rio Grande flows. The valley mesas, or table-plains, are 

 among the most instructive of all of these topographic features. 



Being a stream of such high gradient, with an average fall, 

 as already stated, for its 300 miles in New Mexico of over five 

 feet to the mile, its corrading powers are little short of mar- 

 velous and its carrying capacity amazing. During the dry 

 season of the year the waters of this stream are greatly over- 

 burdened with sediments and the channels rapidly silt up. In 

 late years, since most of the water has been taken out for irri- 

 gation purposes, the silting-up process has become a very seri- 

 ous matter. Many localities show clearly that within the last 

 dozen years the deposition has filled up the valley to heights 

 of 20 to 30 feet. 



As already mentioned, no lateral drainage of a perennial na- 

 ture is received by the Rio Grande within the borders of New 

 Mexico. All increase of the grand stream from the sides takes 

 place only during brief periods of very heavy rainfall. The 

 side waters are then torrential. At other seasons of the year 

 these tributaries are true arroyos, or dry creeks, as their Span- 

 ish title signifies. These arroyos have very steep gradients, 

 often two to four and even five per cent. 



In the mountain ranges on either side of the valley of the 

 great stream the arroyos occupy deep canyons. In this part 

 of their courses the channels are being rapidly cut deeper and 

 deeper into the indurated bed-rocks. After emerging from 

 the mountains these lateral drainage-ways are as pronouncedly 

 constructive in character as they were destructive before. 



Into the broad Rio Grande valley the side arroyos pour vast 

 quantities of coarse mountain waste. The alluvial fans which 

 are formed become confluent. On either side of the river 

 broad plains several miles wide are built up ; and these are 

 inclined strongly towards the channel of the master stream. 

 "Were the river free from meandering, these plains on each 

 side would doubtless become continuous and even. 



In the course of its wide meandering the stream cuts rapidly 



