PHYSIOGRAPHY OF NICARAGUA CANAL ROUTE 2-15 



row river channel, kept open by the current of water flowing, 

 from the lake. 



The second section of the San Juan extends from the head of 

 the Toro rapids to the mouth of the San Carlos. Its essential 

 characteristics are the rapid fall of the river and the narrow val- 

 ley in which it flows. The Toro rapids which retain the lake 

 at its present level are not formed by a solid ledge of rocks cross- 

 ing the valley, but by boulders, sand, and clay. It appears that 

 when this arm of the lake extended down to the continental 

 divide it received a rather large and swift tributary, the Rio Sa- 

 balos, near its head. The sediment carried by the Sabalos, con- 

 sisting of clay, sand, and boulders, was deposited on reaching 

 the quiet water. A delta was thus formed extending as a shoal 

 across the arm of the lake at this point. As the river channel 

 sank in the gap across the divide the latter became lower than 

 the surface of the Sabalos delta, and the crest of the barrier 

 which maintains the level of Lake Nicaragua moved westward 

 from its original position at the former divide to the present po- 

 sition of the Toro rapids. 



From the head of the Toro rapids to Machuca the river chan- 

 nel consists of rather long, quiet reaches separated by rapids. 

 The total fall in this section is about 40 feet, or an average of 

 two feet to the mile. Of this fall, however, all but about six feet 

 is accomplished by the numerous rapids. These appear to be 

 due to the unequal hardness of the underlying rocks. The in- 

 tervening quiet stretches are located upon softer rocks, which are 

 worn down by the moderate current more rapidly than the 

 harder rocks by the swift current of the rapids. Between Ma- 

 chuca and the mouth of the San Carlos the river is deep and 

 narrow and the current is generally moderate. In some places 

 at low stages of the river it is almost imperceptible, and when 

 the San Carlos is in flood the current may even set upstream 

 for a time. The water has a depth varying between 15 and 60 

 feet, the bottom of the channel being at some points below sea- 

 level. It is evident that the present river is here flowing in a 

 channel which was cut when the land stood higher than now, 

 and which has not yet been filled by sediment. This portion is 

 called the Agua Muertas, or dead water. 



The third section of the river extends from the mouth of the 

 Sao Carlos to the Caribbean sea. With the entrance of the San 

 I ;i rlos the character of the San Juan is entirely changed. Above 

 the junction it is a comparatively clear stream, and except at the 



