246 PHYSIOGRAPHY OF NICARAGUA CANAL ROUTE 



rapids has only a moderate current. Below the entrance of the 

 San Carlos it is usually muddy ; it is shallow, with a shifting, 

 sandy bed and has a uniformly strong current. Its slope is 

 nearly a foot to the mile in this section. The Sarapiqui is sim- 

 ilar to the San Carlos, although somewhat smaller. Both of 

 these streams have their sources on the slopes of the Costa Rican 

 mountains to the south. The recent volcanic eruptions of this 

 region have furnished an abundant supply of unconsolidated 

 sand to these streams and they are heavily loaded with this 

 material. Below the entrance of the San Carlos the fioodplain 

 immediately adjacent to the San Juan has been built up more 

 rapidly than the floodplains of its smaller tributaries; hence 

 the latter are ponded in their upper courses and many lagoons 

 are thus formed. From the mouth of the San Carlos eastward 

 the San Juan occupies the northern margin of its valley. This 

 is doubtless due to the more abundant supply of material fur- 

 nished by the southern tributaries and also to the northward 

 drift of the littoral current in the Caribbean sea. As the river 

 extended its course eastward by the filling of the estuary, and 

 later by the formation of the deltaplain, it was continually 

 crowded to the northward by the direction of the sand-drift 

 along the coast. This tendency became more pronounced the 

 farther out the delta was built, and the sharp northward bend 

 of the lower San Juan is its direct consequence. 



As the river channel was carried northward this northern por- 

 tion of the valley would be filled first and to a higher lev 7 el than 

 the southern portion. The river would thus at times find itself 

 in a position of unstable equilibrium and would seek a new 

 channel on the lower part of the deltaplain to the southward. 

 Thus it is probable that the river originally occupied the present 

 position of the San Juanillo. When this position became un- 

 stable it gradually deserted its northern channel for the present 

 position of the lower San Juan. Subsequentl} 7 the latter became 

 unstable, and a more favorable course to the sea was found still 

 farther south. The recent channel of the Colorado was then 

 developed at the expense of the lower San Juan. This process 

 is still going on, and the relative amounts of water carried by 

 the two channels has very materially changed within a genera- 

 tion. Unless artificially modified, the lower San Juan will con- 

 tinue to dwindle, and probably all the water will find its way to 

 the sea by the Colorado or by some more favorably located chan- 

 nel still farther south. 



