National Geographic : 1899 Jul
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 floodplain 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 level 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. Subsequently 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.