132 Papers from the Marine Biological Laboratory at Tortngas. 



c and d) there is a growth of saw-grass and an accumulation of vegetable 

 muck of varying depth — in some places thin, merely a surface veneer; 

 in others 4 or 5 feet, or perhaps even more, in depth. The interior of 

 the keys is usually a jungle, while often, but not invariably, mangroves 

 fringe the water front (plates 9, 10, 11, and 12, fig. a). 



DRAINAGE AND RAINFALL OF SOUTHERN FLORIDA. 



Southeastern Florida is a very poorly drained country. There are 

 comparatively few streams leading from the interior swamp to the sea. 

 A small river at Miami, known as Miami River, leads from the Ever- 

 glades to Biscayne Bay. Another, Taylor River, empties into Florida 

 Bay in southern Dade County. However, a large proportion of the 

 waters of the interior works its way to the ocean, as there is a general 

 southward movement of the waters of the Everglades. There is in the 

 vicinity of Miami and also along the keys direct surface run-off from 

 the landmass into the ocean. 



The rainfall for this section of Florida, according to Gannett,* is 

 between 60 and 70 inches per annum. 



No accurate records have been kept of the surface run-off of the 

 streams and from the swamps of southern Florida. Therefore any figure 

 given must be derived by applying the results obtained in other areas 

 and making allowance for peculiar conditions prevailing in this region. 

 Mr. J. C. Hoyt, in his "Comparison between Rainfall and Run-off in the 

 Northeastern United States," makes the statement: 



The run-off is very consistent in the various groups, and decreases toward 

 the south, although the rainfall increases. It is about 60 per cent of the precipita- 

 tion in the northern areas, 55 per cent in the intermediate areas, and 40 per cent in 

 the southern areas. This decrease in run-off is due to the increase in evaporation 

 and the loss by vegetation, and shows that the climate and vegetation are probably 

 the principal regulating factors in the relation between rainfall and run-off. 



The climatic conditions are responsible for this change in the percentage of 

 run-off between northern and southern areas, rather than geologic conditions or 

 topography, as is shown in the "percentage" column for the summer months, where 

 the percentage of run-off s varies between 20 and 32, having a mean of 27 per cent 

 for all principal basins. This column shows no regular variations for the basins. 

 It also shows that the evaporation and loss through evaporation is very nearly 

 the same over the various areas considered, being about 9.5 inches. (American 

 Society of Civil Engineers, Transactions, vol. 49, p. 436.) 



As the temperature of southeastern Florida is never lowered to the 

 freezing-point and as there is a luxuriant vegetation, it is probable that 

 the lowest estimate of run-off given by Mr. Hoyt, 27 per cent, is slightly 

 too high. Therefore 25 per cent ^ is taken as a more probable figure; 

 and the annual run-off for this area is estimated as 25 per cent of 60 

 inches, or 15 inches. 



* U. S. Geol. Surv., Water Supply Paper 234. 



' Mr. J. O. Wright, Drainage Engineer, U. S. Department of Agriculture, by 

 a somewhat different process, arrived at the same estimate for the percentage of 

 surface run-off in southern Florida. (Report of the Special Joint Committee of the 

 Legislature of Florida on the Drainage of the Everglades of Florida, pp. 25-29, 

 Tallahassee, 1909.) 



