576 C. R. KEYES EBLATIVE EFFICIENCIES OF EROSIONAL PROCESSES 



the small amounts of water which they contain during the greater part of 

 the year. Their gradients are high, enabling them to carry prodigious 

 quantities of water when the snows are melting in the Eocky Mountains. 

 Their effective erosive powers are well shown in the turbid character of 

 their waters and the vast amounts of silt which they at all times transport. 



The immediate valleys of these streams are among the most remarkable 

 known. They extend from mountain range to mountain range on either 

 side, a distance often of a score of miles. Their bottoms lie many hun- 

 dreds of feet below the level of the general plains surface of the region. 

 The Eio Pecos flows in a wide valley the river level of which is 1,500 feet 

 beneath the general plains surface. The Eio Grande, which is probably 

 the most characteristic of all of the through-flowing rivers, follows a line 

 of old bolsons below the level of which the present bed of the stream is 

 about 2,000 feet. The canyon of the Colorado Eiver is a mile deep. 



The Eio Grande, being typical of the large rivers crossing the desert, 

 has received more detailed consideration than any of the others. This 

 stream is really one of the great rivers of the American continent. It is 

 as long as the Mississippi. Unlike the latter waterways it has, for a large 

 river, a very high gradient. For the first 1,000 miles from its head- 

 waters the average fall is over 5 feet to the mile. In times of flood the 

 waters are almost of torrential nature. At certain other times of the 

 year, as in the months of July and August, the stream is very nearly dry, 

 although there is always a strong underflow beneath the sandy bottom. 



Physiographically the origin of the Eio Grande is complex. That part 

 of the river's course which lies in New Mexico is in the main antecedent 

 in character. As already noted, no lateral drainage of perennial nature 

 is received by the great stream above the mouth of the Eio Pecos. All 

 increase in the waters of the grand stream from the sides takes place only 

 during very brief and infrequent periods of heavy rainfall. The side 

 waters are then torrential. At other seasons of the year these tributaries 

 are true dry creeks, as their Spanish title appropriately signifies. These 

 arroyos have very steep gradients, often 2 to 4 and even more feet in a 

 hundred. 



The exact role that the arroyos play in the general erosion of the coun- 

 try is not always clear. In the mountain ranges on either side of the 

 valley of the grand stream the arroyos occupy deep canyons. In this part 

 of their courses their channels are being rapidly cut deeper and deeper 

 into the indurated bedrocks. After emerging from the mountains these 

 lateral drainageways may become as pronouncedly constructive in charac- 

 ter as they were destructive before. Into the Eio Grande channel the 

 side arroyos no doubt pour in the aggregate large volumes of coarse 



