Page:The National geographic magazine, volume 1.djvu/418

 There are very few islands in this section of the river, the banks are covered with huge trees matted with vines, and throughout the lower half of the division, from Toro rapids to the mouth of the San Carlos, the river is confined between steep hills and mountains.

As a result of the absence of considerable tributaries already noted, the fluctuations of this portion of the river conform closely to those of the Lake, and consequently take place gradually and are limited in range.

Below the Rio San Carlos the San Juan changes its character entirely. Its average width is twelve hundred and fifty feet, its bottom is sandy, there are numerous islands, and the slope of the river is almost uniformly one foot per mile.

The discharge into this section of two large tributaries, the San Carlos and the Sarapiqui, descending from the steep slopes of the Costa Rican volcanoes, causes much more sudden and considerable fluctuations of level than in the upper river.

While the lower portion of the river and especially the delta section presents very interesting features, yet the peculiar charm of the river is in the upper section, and the exceptional advantages it offers for obtaining miles of slack water navigation. This portion of the river with the lake and the narrow isthmus between it and the Pacific forms a trio of natural advantages for the construction of a canal, the importance of which it would be difficult to over estimate.

About three miles below the mouth of the San Carlos, the Caño Machado enters the San Juan on the north bank. This stream, about one hundred feet wide and from eight to ten feet deep, is the last of the mountain or torrential tributaries of the San Juan. It can scarcely be said to have a valley, but occupies the bed of a rugged ravine extending for several miles northerly and northwesterly up into the easterly flank of the cordillera. Every variety of igneous rock, from light porous pumice to dense metallic green-black hypersthene andesite, may be picked up in the bed of this stream. Agates also are common and there are occasional masses of jasper. Farther up, frequent outcrops of trap in situ occur, interspersed in some localities with numerous veins of agate.

Twelve miles below the Machado the San Francisco enters the San Juan. This stream, with its several tributaries, drains a large swampy valley sprinkled with irregular hummocks and hills. For