346 C. W. HAYES — GEOLOGY OP NICARAGUA CANAL ROUTE 



pies a region which had been comparatively well baseleveled, but its 

 waters also extended up the valleys, where extensive unreduced areas re- 

 mained U})on the divides. Many shallow estuaries were thus formed, 

 and these have subsequently been entirely filled with sediment b}' the 

 streams entering their heads. The most extensive filling was at the 

 southeast end of the lake, where the largest tributaries enter it. It is evi- 

 dent that the broad swampy plains bordering the Rio Frio and the upper 

 San Juan, down to and beyond the Toro rapids, were originally portions 

 of the lake which have subsequently been silted up. The ordinary 

 method by which lakes are obliterated is by the filling from their upper 

 ends and by the cutting down at their outlets. In this case, however, a 

 part of this process is exactly reversed. The lake is being filled most 

 rapidly from its lower end. This filling is manifestly accomplished not 

 by the water which comes from the lake, since this is practically clear, 

 but by the tributaries which enter this lower portion. 



The present river channel does not necessarily coincide with the posi- 

 tion of the river which formerly occupied this basin. Its present posi- 

 tion is dependent on the relative amounts of sediment brought down by 

 the tributaries on either side. If the Castillo and Toro rapids were 

 cut back and the channel of the river permitted to sink through the 

 alluvium forming the greater part of its banks and bed on the oldland 

 surface which the alluvium conceals, it would have the characteristics 

 of a superposed stream. At numerous points where its present channel 

 does not follow the old channel it would discover hard rocks in its down- 

 ward cutting. In its present condition this ma}^ be described as a 

 residual river channel — that is, a broad arm of the lake has been gradually 

 constricted by the addition of sediments on its margin — and all that re- 

 mains is the narrow river channel kept open by the current of the water 

 flowing from the lake. 



The Toro rapids, which retain the lake at its present level, are not 

 formed by a solid ledge of rocks crossing the valley, but by boulders, 

 sand, and clay. It is some distance below the Toro rapids that the rock 

 is first found crossing the valley. 



It appears that when this arm of the lake extended down to the Con- 

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

 Sabalos, near its head. The sediment carried by the Sabalos, consisting 

 of clay, sand and boulders, was deposited on reaching the quiet water 

 of the lake. A delta was thus formed which extended across this arm 

 of the lake, forming a shoal. 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 dam, which retained the surface of lake Nicaragua, 

 moved west from its original position on the divide to the present posi- 



