292 C. \V. HAYES — GEOLOGY 01? NICARAGUA CANAL ROUTE 



deposit the alluvium, having opportunity to spread over a much larger 

 area, do not rise so high. 



The slope of the floodplains from the Boca San Carlos to the head of 

 the delta is about 11 inches per mile. This slope is dependent on the 

 volume of the river and the character and quantity of the sediment 

 which it carries. It is therefore much steeper below the mouth of the 

 San Carlos than above, for it is from this stream that the greater part of 

 the coarse sediment in the lower river is derived. 



As stated above, the floodplains are inconspicuous from the Boca San 

 Carlos to the head of the Toro rapids. The river flows in a compara- 

 tively narrow gorge and is generally bordered by rather steep hills, which 

 approach nearly to the river channel. At the head of the Toro rapids, 

 however, the valley widens, and from this point to the lake the river is 

 everywhere bordered on one or both sides by extensive floodplains. 

 Although their general relations to the river are similar to those border- 

 ing its lower course, they yet diff'er in some important particulars. They 

 have been formed by sediment borne, not by the river itself, but b}'- 

 tributaries coming into the valley on either side. They thus have the 

 form of coalescing deltas. The natural levee, which is a conspicuous 

 feature in the floodplains of tlie lower river, is absent, and the plains 

 generally show a gradual descent from their outer margins toward the 

 river. Hence there are no lagoons on the tributaries such as are found on 

 the tributaries of the lower river, and the floodplain becomes gradually 

 firmer and more heavily wooded with increasing distance from the river. 

 From the mode of formation of these plains it is manifest that the river 

 is in a stable position and does not show that tendency to seek a new 

 channel which is characteristic of delta streams. 



Most of the streams entering lake Nicaragua on its northeastern side 

 at one time entered the heads of estuaries. These estuaries have been 

 almost entirely filled with alluvial deposits, and in some cases somewhat 

 extensive deltas have been built out into the lake. The absence of a surf 

 in this portion of the lake, except on rare occasions, owing to the direc- 

 tion of prevailing winds, permits the building of deltas which carry the 

 distributaries of the streams a considerable distance out from the general 

 shore line. The most extensive alluvial deposits about the lake are at its 

 southern end. This portion of the lake basin appears to have been orig- 

 inall}^ rather shallow, and the sediment brought in by streams from the 

 south, notabl}'- by the E,io Frio, has considerably contracted its area. 

 The newl}^ added land forms about the margin of the lake an extensive 

 swamp, through which the streams meander in a network of interlacing 

 distributaries, all more or less obstructed by vegetation. The land be- 

 comes gradually firmer at increasing distances from the lake, and finall)'' 



