of the Cuzco Valley, Peru. 15 



falls. Where traversing the basin floor these streams are con- 

 fined within walls of masonry, which, however, have little 

 function to perform except during periods of heavy rainfall. 



The Huancaro is the chief southern tributary of the Huat- 

 anay within the Cuzco Basin. Though its drainage area is 

 small, its wide, gravel-filled floor, its moderate, grass-covered 

 slopes, its flat, longitudinal profile, and its position on a fault 

 zone result in a run-off whose uniformity is unusual for this 

 region. Measurements taken during September and again in 

 November, 1912, showed that the Huancaro at its junction with 

 the Huatanay carried more water than the master stream. 



The Huanacauri, which joins the Huatanay 1 mile below 

 San Geronimo, is a typical example of the streams entering 

 Cuzco Basin from the south. The lowest half mile of its 

 course is sunk in gravels, and terraces flank the streams for 

 about a mile from its mouth. At a distance of 2 miles the 

 gently sloping, relatively wide valley floor is broken up into a 

 number of sharply cut rock canyons, and at its headwaters the 

 tributary canyons are so numerous and so closely spaced that 

 no undissected land remains. Swamps are lacking; springs 

 are rare ; many canyon stretches are dry or occupied by a 

 broken chain of rock-rimmed pools. The surface run-off is 

 very rapid, and stream flow is the immediate and short-lived 

 consequence of showers. The valley form and conditions of 

 run-off along the Huanacauri fairly represent the relations 

 prevailing in the non-glaciated portion of the Cuzco Valley. 



Streams entering the Cuzco and Oropesa basins from the 

 lofty northern border of the Cuzco Valley possess certain dis- 

 tinctive features. The Chchiraura is a continuous perennial 

 stream from its source in the Pachatucsa cirque to its mouth 

 at Huasau. Its descent is broken by numerous falls, and at 

 the group of herdsmen's cottages after which the stream is 

 named, the floor drops 1,200 feet in the course of half a mile. 

 Rio Huaccoto, rising in the Atasccasa highlands, receives its 

 • water from a series of glacial deposits deeply mantling the bed- 

 rock over an area of about 3 square miles. In place of defined 

 drainage channels the headwaters of the Huaccoto include 

 lakes, swamps, rills trickling underneath peaty flats, and 

 masses of water-soaked gravels and bowlders. At the village 

 of Huaccoto three tributaries unite to form a permanent stream, 

 which at once drops into a canyon with sloping box head and 

 descends to its terminal fan, iy 2 miles distant, on a slope of 

 1,500 feet to the mile. Emerging from the mouth of its 



