GENERAL REPORT ON INVESTIGATIONS IN PORTO RICO. 
11 
HYDROGRAPHY. 
The rivers of Porto Rico are remarkable for their great number, their shortness, 
the restricted areas of their catchment basins, and the rapid descent which they make 
from their headwaters in the central mountains to the sea. The precipitation is in 
most parts of the island very great and, with the multitude of streams, the island 
is unusually well watered. Moreover, 1 because of the steepness of the slopes, 
especially on the northern coast, and the impervious character of the clay soil which 
covers them, the proportion of precipitation which runs off makes these rivers of even 
larger volume than would otherwise be expected under corresponding conditions. 
Into the northern ocean ffow 12 streams of considerable magnitude; toward the west 
coast How 1 of relatively equal size; into the eastern sea ffow 5 of less magnitude, and 
into the southern sea ffow 17 of considerable size but comparatively small perennial 
volume. There are between 1,200 and 1,300 streams and branches of less volume, 
but yet of sufficient size to have received separate names. 
Since the average width of the island is but 35 miles and its extreme length 
but 95 miles, while its commanding summits range in altitude from 2,800 to 3,300 
feet, it is evident that the slopes are steep, the fall of the rivers great, and the 
velocity of their waters high. Moreover, as the main summits of the dividing 
mountain ranges have been shown to be one-third nearer the southern and eastern 
coasts than the northern and western, it is also evident that such streams as flow north 
and west are three or four times as long and drain ten to fifteen times as great areas 
as those flowing to the south and east. The former have average lengths of 25 to 40 
miles, measured along their stream beds; the latter have lengths of but 5 to 15 miles. 
For these and reasons already given the streams flowing north and west necessarily 
have less abrupt slopes than do those which drain eastward and southward, which 
plunge from an altitude of 3,000 feet to sea-level within a comparatively few miles. 
It is thus seen that, as the island is divided climatologically into two distinct 
portions, it is similarly divided hydrographically, largely as a result of the same 
causes — the trade winds and the topographic configuration of the surface. 
The twenty-eight larger rivers have their sources high among the summits of 
the Cordillera Central. Those flowing to the north and west are characterized by 
precipitate descents of 1,000 to 2,000 feet in the first 5 miles of their headwaters. 
Thereafter they flow more leisurely and with consequent increased size to within 5 
miles of the coast. There they emerge practically at sea-level in long meandering 
curves through the alluvial playas about their mouths. Because of the lowness of 
their grades near the coast and their resulting low velocity all are of considerable 
width and moderate depth in the playa levels. A few miles inland, where they flow 
over steep, rocky beds, their channels are narrow and often confined by precipitous 
rocky walls, their width is of but comparatively few feet, their depth often less than 
a foot, and their velocities so high as to render them veritable mountain torrents. 
On the southern coast the larger rivers have bed-widths as great as those which 
enter the northern and western coasts. Their lengths, however, are so short for the 
1 Mr. H. M. Wilson, of the U. S. Geological Survey, made a study of the water resources of Porto 
Rico in January, 1899, the results of which have been published as “Water-Supply and Irrigation 
Paper of the U. S. Geological Survey No. 32.” We have made free use of this excellent report in our 
account of the hydrography of the island, sometimes copying literally, without quotation marks. 
