T

CHAPTER IV

PHYSICAL CONSIDERATIONS

HE portion of Nicaragua whose topography bears directly upon the problem

of canal construction consists of a broad depression bounded on the south by the high volcanic range of northern Costa Rica, and on the north by the mountainous portion of Nicaragua. Within this depression lie Lake Nicaragua, Lake Managua, and the Rio San Juan, a stream which carries nearly the entire drainage of this region to the Caribbean Sea. In former days the continental divide, now west of the lake, ran north and south about where the town of Castillo stands, and the present valley of the San Juan was occupied by two streams, one flowing eastward to the Caribbean Sea, the other westward to a bay of the Pacific Ocean since inclosed by a barrier of volcanic ejecta and now forming part of Lake Nicaragua.

Traversing the Nicaraguan depression from north to south are ranges of hills whose summits, decreasing in height from the axis of the isthmus to the coasts, mark the former level of an undulating plain formed by the action of

streams flowing east and west from the old continental divide. This plain was subsequently destroyed by the erosive action of streams, chiefly during a period when the land was much higher than it now is. A comparatively recent subsidence of the earth's crust plunged portions of the river valleys below sea level, causing them to silt up and form the broad alluvial plains characteristic of their lower reaches.

Residual hills, surviving the general process of degradation because of inherent hardness or fortunate positions upon divides, rise at places above the level of the old plain, particularly along the axis of the isthmus. If the plain were restored, it would extend across the isthmus through gaps in these residual hills, and at an altitude varying from one hundred to two hundred feet above the sea.

In order to obtain a clear idea of the Nicaraguan depression, it will be well to trace the method in which it was presumably formed. It is probable that prior to the formation of the plain already mentioned, a plateau, increasing in altitude to the north and south, extended from the Caribbean Sea to the east side of that portion of Lake Nicaragua which was then a part of the Pacific Ocean. The continental divide was about where Castillo now is, and from it streams flowed east and west along the

present location of the San Juan valley, gradually reducing large portions of their drainage areas to base level, and cutting back toward the divide. After a long period of degradation, the land was slowly elevated to a height some two hundred feet greater than that which it now occupies, greatly increasing the erosive action of its streams, particularly near the coast, where the plain already formed was soon deeply dissected by the same agents which had made it. A subsequent subsidence of the country gradually submerged the lower portions of these eroded valleys, checking the current and allowing a precipitation of alluvium which resulted in the formation of silted estuaries. In the lower valley of the Rio San Juan the deposit of alluvium was such that the present river follows very nearly the bed of the old stream, although in places it has left its former channel, and cut a new one for itself. In the upper San Juan valley, however, the existing channel has been determined largely by the amount of sediment brought down by tributaries, the main stream having been pushed from side to side of the valley by the deltas of its affluents.

The delta formation which characterizes the mouth of the Rio San Juan is of recent and constantly increasing growth. Sediment, most of it from the Sarapiqui and San Carlos tributaries, is delivered somewhat faster than it can