LEVEL OF LAKE NICARAGUA 159 



(3) A third wa}' in which the level of the lake might have been 

 lowered is by the cutting down of its outlet. As fully explained in 

 the report of the Canal Commission, 1897-'98, it appears probable 

 that the level of the lake was earl}' in its history determined by a 

 rock sill over which the Rio San Juan flowed at Castillo. This sill 

 has since been cut down somewhat, and the lake level is now held 

 by the delta of the Rio Sabalos which forms the Toro Rai)ids. From 

 the point Avhere it issues from the lake to the Toro Rapids the Rio 

 San Juan meanders through an alluvial plain, which represents a 

 ibrmer extension of the lake silted up by tributary streams except 

 ibr the channel kept open by the outflow from the lake. The sur- 

 face of this plain stands at such a level that it is just covered by the 

 streams when in flood. In other words, it has the character of a 

 growing floodplain and proves conclusively that present relations 

 have held for a considerable time. Any lowering of the lake level 

 by cutting down the outlet would at once leave this alluvial plain 

 above the reach of floods and completely change its character. As 

 has alread}'^ been pointed out, the sill which holds the lake at its 

 present level is a delta deposit, and it will not long resist corrasion of 

 the waters which cross it ; so that in a relatively short time, as geo- 

 logical changes go, the river may be expected to begin the rapid 

 trenching of its upper channel and eventuall}^ unless artificially 

 checked, lower the lake level. 



The evidence that the lake level has not been lowered b}- this third 

 method is, of course, confirmed by the absence of raised beaches 

 about the lake, where they would certainly be a conspicuous feature 

 if the change had taken place as suggested. 



Changes in the conditions of the upper end of Lake Nicaragua 

 liave been cited by Professor Heilprin as evidence of recent loAver- 

 ing of the lake's level. This doubtless arises from ignorance of 

 the peculiar physical conditions which prevail there. As stated 

 above, the constant trade winds which sweep across the lake produce 

 a heavy surf along its southwestern margin throughout the greater 

 ])art of the year. The oblique direction at which the waves strike 

 the shore sets up a strong littoral current, l)y which the sand is trans- 

 porte<l toward the nortliwest and deposited at the end of the lake. 

 A sand spit 10 miles in length has been built across the point of the 

 lake, cutting off a broad, shallow lagoon and crowding the Tipitapa 

 River to the extreme margin of the valley. From the rate at which 

 the shore in the vicinity of Granada is being cut away and at which 



