RIVERS OF PATAGONIA. 237 



To the former class belong the Gallegos, Santa Cruz and the Rio Chico, 

 or north fork of the latter, while all the remaining rivers of the southern 

 Patagonian plains may be more properly referred to the second class. 

 The inability of such streams to maintain a constant and continued flow 

 of water is dependent primarily, of course, upon the nature of the source 

 from which they are supplied. In addition to this, however, the character 

 of their beds exerts a very certain and definite influence in the same direc- 

 tion. Rivers, like the three mentioned above, which have their sources 

 in the midst of the Andes, where there is an abundant precipitation and 

 where, in addition to the almost daily rains and snows, they are fed by 

 the melting snow and ice of numerous glaciers, have a constant, abundant, 

 and never-failing water supply, undiminished throughout the entire year, 

 and hence their uninterrupted flow. On the other hand, such streams as 

 the Sheuen, Aubone, Coy and Chico, or south fork of the Gallegos, which 

 reach only to the foothills of the Andes, have a less abundant and less 

 constant water supply. Owing to the more diminished volume of their 

 waters, they have been unable to clear their channels of the deposits of 

 shingle, with which they have been obstructed during the slow emergence 

 which the country is undergoing. This is especially true of the middle 

 and lower courses of these streams, where such deposits of shingle act as 

 a sponge, with the ultimate effect that, during the summer, autumn and 

 early winter, the volume of water in such streams is entirely absorbed 

 long before it reaches the sea, and that, for a considerable portion of "each 

 year, these streams are actually engaged in silting up portions of their 

 channels, rather than in deepening them. Thus it is, that, while toward 

 their sources, they appear as considerable streams throughout the entire 

 year, in their middle and lower courses, they flow only during and imme- 

 diately after the melting of the winter snows in late winter, spring and 

 early summer. 



While these conditions prevail with the rivers I have just mentioned, 

 quite a different set of conditions conspire to produce a much more com- 

 plete and well-nigh perfect desiccation of the streams north of the Rio 

 Chico of the Santa Cruz and within the limits of the plains of southern 

 Patagonia. Here the continental watershed, instead of lying within or 

 even along the foothills of the Andes, is situated some distance to the 

 eastward, so that these rivers are deprived of even the limited rainfall of 

 that region. Moreover, throughout their upper courses the channels of the 



