244 PHYSIOGRAPHY OF NICARAGUA CANAL ROUTE 



A few of these Pacific streams have not } T et completely filled their 

 old valleys with sediment, and the unfilled portion of one now 

 forms the harbor of San Juan del Sur. The San Juan river, flow- 

 ing eastward to the Caribbean, has not onty filled the estuary 

 which once occupied its valley, but has pushed the coast line 

 eastward b}^ a broad delta plain. 



The San Juan river and its valley bear such an intimate rela- 

 tion to any canal scheme that a somewhat more detailed account 

 of its peculiarities should be given. Considered from any point 

 of view, either wilh reference to the history of its development, 

 the present character of its channel and banks, or the possibility 

 of using it for a canal route, the San Juan valle}' is naturall} 7- 

 divided into three sections. Starting from the point where the 

 river leaves lake Nicaragua, the first extends to the head of the 

 Toro rapids ; the second from the head of tbe Toro rapids to the 

 mouth of the San Carlos river, and the third from the mouth 

 of the San Carlos to the sea. 



In the upper section the river has a moderate current and a 

 considerable depth. Its banks are low and swampy, except 

 where it swings against the foot of one of the numerous hills 

 rising above the alluvial plain through which it meanders. It 

 is evident that the lake formerly extended down to and beyond 

 this point, and that a large amount of territory has been reclaimed 

 from its waters. It is well recognized that lakes are ephemeral 

 features, and the ordinary ways in which they are obliterated 

 are by the filling from their upper ends and by the cutting down 

 of their outlets. In this case, however, the first of these pro- 

 cesses has been exactly reversed. The area of the lake is being 

 contracted chiefly hy filling at its lower end. The filling is be- 

 ing accomplished not by the water which comes from the lake, 

 since this is practically clear, but by the tributaries which en- 

 tered this lower portion, many of which have been converted 

 into tributaries of the San Juan. The present river channel 

 does not coincide with the position of the river which formerly 

 occupied this basin before it was drowned by the waters of the 

 lake. Its position is dependent upon the relative amounts of 

 sediment delivered by the tributaries on either side, and it has 

 been pushed toward the northern edge of the old basin by the 

 larger tributaries from the south — the Frio and Poco Sol. This 

 may best be described as a residual river channel ; that is, a broad 

 arm of the lake has been gradually constricted by the deposi- 

 tion of sediment on its margin, and all that remains is the nar- 



