CHARACTERISTICS OF THE PLOODPLAINS 291 



The latter are therefore dammed and form lagoons in then' upper basins. 

 The Florida lagoon is a typical example of this class. 



The lagoons of the deltaplain formed in these various ways are at first 

 open lakes, but they gradually become choked by vegetation and filled 

 with fine silt, so that they are converted into grassy marshes, and finally, 

 when the silt becomes sufficiently consolidated to form a stable support, 

 the forest trees encroach on the marsh and all trace of the lagoon is lost. 

 Numerous examples occur on the deltaplains illustrating every step in 

 the process : first the open lagoon, then the floating grass mat, then the 

 silico swamp, and finally fhe heavy forest. 



As already indicated, the deltaplain at its inner margin merges with 

 the broad fioodplain of the San Juan river, and any line separating the 

 two would be purely arbitrary. For convenience, however, the head of 

 the delta may be placed at the*point where the first distributary, the 

 San Juanillo, leaves the main stream. 



Most floodplains are farmed by the lateral cutting of streams as they 

 swing from side to side in their valleys. A plain thus cut in the under- 

 lying rocks is usually covered with a thin sheet of alluvial material. 

 The floodplains of this region, however, belong to a totally different 

 class. They include no level plains cut in the underlying rock or resid- 

 ual material which covers the rock. On the other hand, the alluvium 

 has very considerable depth, and, instead of forming a layer of uniform 

 thickness, fills a series of old stream channels. It is evident that these 

 channels were formed when the land stood higher than now, for many 

 of them extend below sealevel. There is thus an old land surface con- 

 cealed beneath the alluvial deposits, and a consideration of its topography 

 becomes a matter of prime importance to the engineer. This buried 

 topography will be considered more fully in connection with the un- 

 buried portion of the same surface — that is, the surface of the hills rising 

 above the margins of the alluvial plains (see plate 31). 

 ' Extensive floodplains extend up the San Juan river to the mouth of 

 the San Carlos. Above this to the head of the Toro rapids the river 

 flows in a comparatively narrow gorge, and its floodplains are narrow 

 and inconspicuous. From the Boca San Carlos downward to the head 

 of the delta, floodplains are always present on one or both sides of the 

 river, though they are most extensively developed on the south side. 

 The surface is slightly higher near the river, forming the natural levee 

 which characterizes most floodplains. The outer margins are depressed 

 and occupied by swamps or lagoons. The surface of the floodplains in 

 the vicinity of the Boca San Carlos varies from 15 to 20 feet above the 

 river at ordinary low stages. As the plains become more extensive 

 downstream their surface is slightly less elevated, since the floods which 



