16 II. E. Gregory — A Geologic Reconnaissance 



rock-walled canyon the stream, with diminished flow, enters a 

 gravel trench 20 to 110 feet in depth and after a further 

 descent of 500 feet reaches the Tluatanay. As regards per- 

 manency of flow and the number of tributaries the Huaccoto, 

 draining a glaciated highland, is strongly contrasted with 

 streams from the unglaciated southern rim of the Cuzco Valley. 



During the dry winter season — March to November — many 

 of the large streams of the Cuzco Valley are reduced to brooks, 

 some of them become intermittent, and the minor tributaries, 

 except in areas coated with glacial debris, cease to flow alto- 

 gether. Deficient rainfall combined with the demands of 

 irrigation deplete the water to such an extent that during my 

 stay at Cuzco all the streams in the valley could be crossed 

 dry-shod by taking advantage of friendly bowlders of moderate 

 size. Even in the Huatanay at San Geronimo stepping stones 

 6 inches in height were well above water. During the rainy 

 season — December to March — especially in January and 

 February, an enormous amount of water is crowded into the 

 narrow channels to depths of 6 to 10 feet. Where not arti- 

 ficially protected the streams on the valley floor and on the 

 larger fans overflow their banks. Even during the dry sea- 

 son abrupt increase in discharge follows the infrequent but 

 heavy downpours. In spite of the large annual rainfall and 

 the presence of grass on many slopes the conditions of run-off 

 for the Cuzco Valley approximate those characterististic of 

 arid regions. On the flat-lying fertile lands of the Cuzco 

 Basin disastrous flooding is always to be guarded against. 

 That success in restraining flood waters has not been attained 

 is indicated by stretches of gravel-strewn flats, by the abandoned 

 lands above the Angostura Narrows, and by the zones of bare 

 gravel extending across the San Geronimo fan in the midst 

 of terraced fields that are in a high state of cultivation. The 

 fertile, low-lying lands south of San Sebastian, below Saylla 

 and in the Lucre Basin, devoted to pasturage instead of to 

 agriculture, indicate the control exerted by flood waters. 



Lakes. — Lakes in the Cuzco Valley are small and few in 

 number. On the floor of the Lucre Basin, which is practically 

 without relief, two permanent water bodies, Laguna de Huaton 

 and Laguna de Lucre, occupy shallow depressions. I am 

 informed that during the rainy season these lakes unite with 

 their bordering swamps to form a body of water nearly 2 miles 

 in length. 



On the highland rim of the Cuzco Valley are lakes sunk 

 in glacial debris. With a few unimportant exceptions, they 



