Ariel E. Lugo

Olga M. Ramos González

Carlos Rodríguez Pedraza

U.S. Department 

of Agriculture

Forest Service

FS-980

June 2011

The Río Piedras Watershed

and Its Surrounding Environment
Acknowledgments

The land cover section of this study was part of an 

agreement (AC-800140; IITF-98-RCRA-001) between 

the Puerto Rico Highway and Transportation Authority 

and the U.S. Department of Agriculture (USDA), 

Forest Service, International Institute of Tropical 

Forestry, under the provisions of Public Law 95-307 of 

the Forest and Renewable Resources Research Act of 

1978, as amended, 16 U.S.C. 1641–1646. We thank 

Brook Edwards, Iván Santiago, and Maya Quiñones for 

their collaboration in the image editing and accuracy 

assessment of the land cover study, Gisel Reyes for 

library support, and Mildred Alayón for editing the 

manuscript. The following colleagues reviewed the 

manuscript: F.N. Scatena, K. McGinley, G. Reyes, 

J.R. Ortiz Zayas, and J. Hernández. The work was done 

in collaboration with the University of Puerto Rico. 

We thank Professor Aníbal Sepúlveda Rivera for 

making available to us historical maps of San Juan.

This publication is a contribution of the USDA Forest 

Service, International Institute of Tropical Forestry 

to the San Juan ULTRA-Ex program of the National 

Science Foundation (grant BSS-0948507), the Puerto 

Rico Conservation Foundation, and the USDA Forest 

Service National Office in Washington, DC.

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employer.
The Río Piedras Watershed

and Its Surrounding Environment 

Introduction 1

Geography 2

Climate 6

Geology 8

Hydrology 10

Ground Water 10 

Surface Water 10

Floods and Flood Control 13

San Juan Bay and Coastal Lagoons 14

Water Quality 17

In Relation to Rainfall 17

In Relation to Surface Water 17

In Relation to Ground Water 20

Aquatic Organisms and Other Wildlife 20

Vegetation 22

Forests 22

 Natural matrix of forests 22

 Natural corridors 26

 Green oases 29

 Artificial corridors 31

Wetlands 32

Human Activity 33

Land Cover for the Metropolitan Area 35

Overview of the Effects of Human Activity in the San Juan Bay Area 39

Discussion 40

Literature Cited 42

Appendix 45

Contents

The Río Piedras Watershed and Its Surrounding Environment 1

Introduction

Today, more people live in cities than in rural environments. In 

Puerto Rico, the proportion of the population that lives in cities 

passed the 50-percent threshold in the late 1960s and was 94 percent in 2000 (López Marrero and Villanueva Colón 2006). 

Such a high concentration of human activity raises numerous 

conservation challenges. On the one hand, concentrating people 

in cities has positive outcomes to rural landscapes, which benefit 

from reduced human presence (Lugo 1991). On the other hand, 

natural and artificial ecological systems of cities are exposed to 

greater human influence, and it is not clear whether their resilience mechanisms will cope with anthropogenic effects or if the 

level of disturbance causes them to change states. Regardless of 

how particular urban ecosystems respond to human population 

activity, it is clear that quality of life in the city will benefit from 

healthy environments within and around the city.

Box 1. Identification of overlapping geographic locations used in this publication (fig. 1).

Figure B1-1. Map of the barrios within the Río Piedras 

Watershed.

The need to understand the functioning of urban environments has led to a new science focus on social ecological 

systems (SES) (Redman et al. 2004). Studying, understanding, 

and managing SES require access and synthesis of information 

from both natural and social sciences. New interdisciplinary 

and transdisciplinary alliances between these fields, in collaboration with urban communities, government agencies, and 

nongovernment organizations, will benefit from the compilation of information that is usually dispersed and possessed 

according to discipline among fragmented technical groups 

that seldom interact. This publication is such a compilation 

of biophysical information for the Río Piedras Watershed and 

its environment, developed in support of the establishment 

of a new scientific alliance intended to study and understand 

the SES of the San Juan Metropolitan Area (metropolitan 

area) (see box 1 for a description of the terminology of geographic locations used in this document and the use of units 

of measure).

The San Juan Metropolitan Area (metropolitan area) includes 

the municipalities of Carolina, San Juan, Trujillo Alto, Guaynabo, 

Cataño, and Bayamón (fig. 1). Some consider Caguas (to the 

south) and other municipalities to the east and west as part of 

the metropolitan area, but we do not. We use only the name 

of the municipality when referring to the municipalities in the 

metropolitan area. The Río Piedras Watershed is part of the 

metropolitan area and includes parts of the municipalities 

of San Juan, Guaynabo, and Trujillo Alto. The Río Piedras 

mainstream flows through the following sectors of the 

municipality of San Juan: Caimito, Cupey, Monacillo, El Cinco, 

Puerto Nuevo, and Hato Rey. The watershed includes sectors of 

the municipality of Guaynabo: San Patricio (officially known as 

Barrio Pueblo Viejo) and Frailes. It also includes sectors from 

Trujillo Alto: Sabana Llana Sur, Cuevas, and Carraízo. Fifteen 

traditional barrios or neighborhoods occur within the Río 

Piedras Watershed (fig. B1-1). Río Piedras was an independent 

municipality until it was incorporated into the municipality of 

San Juan and became a sector of the city. We use Río Piedras 

when referring to the original municipality or current barrio, 

Río Piedras River when referring to the river, and Río Piedras 

Watershed when referring to the river’s watershed. There 

is also a Río Piedras U.S. Geological Survey gaging station 

(50049000). Old San Juan is used to identify the 500-year-old 

urban sector of the municipality north of San Juan Bay. San 

Juan Bay Estuary Watershed is a Federal designation that is 

mostly located within the metropolitan area. Hydrologically, this 

watershed extends beyond the metropolitan area. We address 

only those aspects of the San Juan Bay Estuary Watershed of 

relevance to the Río Piedras Watershed.

We report units of measurement in the metric system, 

except when the original reference used other systems of 

measurement. In these situations we use the original unit 

and convert the value to metric in parentheses or provide a 

conversion factor.
2 The Río Piedras Watershed and Its Surrounding Environment

Geography

The Río Piedras Watershed is an urban watershed fully contained within the metropolitan area of San Juan (fig. 1). The watershed has an area of 49 km2 (Haire 1971) and is located within 

two geographic regions of Puerto Rico (Picó 1950): the Northern 

Coastal Lowlands Humid Alluvial Section and the Humid 

Northern Foot Hills Northeastern Cretaceous Section (fig. 2). 

The Río Piedras is the only river in San Juan, and it originates 

at about a 150 m elevation and flows north for 16 km (fig. 3). Its 

headwaters are located in the Caimito district (or barrio) of San 

Juan, and as it flows to the Martín Peña Canal (where it enters 

Bahía de San Juan), the river passes through the Río Piedras, 

Hato Rey, and Puerto Nuevo districts of the metropolitan area. 

Two other rivers delimit the metropolitan area (fig. 1): the Río1

Bayamón to the west, and the Río Grande de Loíza to the east. 

An additional river, the Río Puerto Nuevo, originates between 

the Río Piedras and Río Bayamón. Río Puerto Nuevo joins the 

Río Piedras (box 2) and, with the Río Bayamón, all are heavily 

influenced by urban development and have had their channels 

significantly modified (box 2).

The lowlands of the Río Piedras Watershed contain a coastal 

plain that is about 8 km wide. Anderson (1976, p. 3) described 

this coastal plain thusly: “...[it] slopes gently from the sea to 

the foothills. The plain has been built up by surficial deposits 

consisting of sand, silt, clay, and muck overlying a dissected 

older surface, the remnants of which stand above the plain as 

isolated mogotes or hills. Many of the mogotes rise more than 

90 m above sea level and 60 m or more above the coastal plain.” 

Marshes, mangroves, and lagoons lie inland offshore. The foot1 The use of either lowercase or uppercase for río (river) in Spanish is 

determined by whether the name describes the river—for example, Río 

Grande, which describes its size, as opposed to río Mameyes, which 

is a nondescriptive name. To avoid confusion, this document will use 

uppercase at all times.

Figure 1. Map of the San Juan Metropolitan Area showing its 

municipalities (outlined in black), the Río Piedras Watershed 

(outlined in blue), many of the geographic locations mentioned in 

the text, and environmental monitoring stations in the region. The 

appendix contains more information on each monitoring station. The 

map also shows the division of the city into three socioecological 

regions: the coastal zone, as defined by the National Oceanographic 

and Atmospheric Administration; the builtup matrix; and the rural 

areas to the south and above the 100-m elevation contour. 

Figure 2. Delimitation of the geographic regions of the San Juan 

Metropolitan Area, according to Picó (1950). Numbers 1B and 7A 

represent the following categories: 1. northern coastal lowlands (BHumid alluvial section); 7. humid northern foot hills (A- Northeastern 

Cretaceous section).

Figure 3. Elevation above mean sea level of the Río Piedras River as 

a function of distance from the ocean. Data were obtained from the 

Sand Juan Topographic Quadrangle of the U.S. Geological Survey.
The Río Piedras Watershed and Its Surrounding Environment 3

Box 2. Historical maps show different modifications of the channels and different perceptions and 

cartographic delineations for the location of the main channels of the Río Piedras and Río Puerto Nuevo Rivers 

(based in part on Sepúlveda Rivera 2004a and 2004c).

The 1950 San Juan Quadrangle (fig. B2-1) shows the Río 

Piedras River merging as the same streambed into the Río 

Puerto Nuevo River about 1.6 km after its confluence with 

the Quebrada Doña Ana in Barrio Puerto Nuevo northwest 

of the Roosevelt Avenue midsection. The Río Puerto Nuevo 

then flows into San Juan Bay. At present, the two rivers come 

together to discharge into San Juan Bay but do so at their 

engineered confluence with the Caño Martín Peña, east of the 

historical point of discharge to the bay (fig. B2-2). Río Puerto 

Nuevo River is identified in some maps as the river reach 

between the confluence of the Río Piedras River with Canal 

Margarita and the confluence with the Caño Martín Peña. 

Over decades and centuries, many alterations have been 

made to the channels of these two rivers, initially because 

the channels passed through extensive mangrove forests 

before discharging into the bay and, more recently, because 

of dramatic urban development on filled wetlands and 

outright burial of stream channels. The dramatic alteration of 

river channels is illustrated in fig. B2-3, which compares the 

surface channels in 1949 with those in 1998. Figure B2-4 

shows the network of potable water pipes and sewer lines 

that now constitute an artificial hydrologic network below the 

surface of the city. This network carries more water annually 

than does the natural hydrologic network (table 1). 

More changes are in progress for the Río Piedras River 

channel and tributaries in the watershed (fig. 17) as part 

of the channelization works of the Río Puerto Nuevo River, 

which is the designation of the U.S. Army Corps of Engineers 

Table 1. Annual water budget for the Río Piedras Watershed 

(Departamento de Recursos Naturales y Ambientales 2004). 

Item

Acre-feet 

of water

Inputs

Rainfall 106,076

Transfer from other watersheds 174,892

Fluxes

Evapotranspiration 66,963

Storage in aquifer 8,000

Aquifer extraction 448

Discharge of used water to the ocean 64,726

Net flow to the ocean 61,920

Aquifer discharge to the ocean 1,000

Unaccounted water 88,410

One acre-foot is 325,872 gallons or 1,233.4 cubic meter.

Figure B2-1. Detail of San Juan U.S. Geological Survey 7.5 min 

Topographic Quadrangle (1950) [1:25,000 scale]. 

Figure B2-2. Detail of San Juan Bay, orthophoto 2007. The Río 

Piedras-Río Puerto Nuevo stream channel is highlighted in light 

blue.

(USACE) that includes both the Río Puerto Nuevo and Río 

Piedras Rivers. The USACE reported an area of 62.8 km2

for the basin of the Río Puerto Nuevo, with a population of 

250,000 people in the 1980s (Colón 1984).
4 The Río Piedras Watershed and Its Surrounding Environment

Box 2. Historical maps, continued

Figure B2-3. Stream and river channels in the Río Piedras 

Watershed as they appeared in the 1949 US. Geological Survey 

Topographic Map and in the 1998 Centro de Recaudaciones de 

Ingresos Municipales hydrology delineation.

Figure B2-4. Network of pipes (potable water in white and 

sewer lines in red; stormwater not shown) in the Río Piedras 

River Watershed. The watershed is delineated with a black line; 

the surface river channels are in blue. Data are for 2006. The 

infrastructure is managed by the Puerto Rico Authority of Sewers 

and Acueducts. The Geographic Information System layer data 

set was obtained from the Puerto Rico Highway Authority. 

Figure B2-5. Detail of a 1660 map of the San Juan Bay, depicting 

the Río Piedras River, the settlement of Río Piedras, and the 

Caño Martín Peña (Sepúlveda Rivera 2004a).

Figure B2-6. Detail of the 1776 map of San Juan Bay and its 

environment by Thomas O’Daly (Sepúlveda Rivera 2004a).
The Río Piedras Watershed and Its Surrounding Environment 5

Box 2. Historical maps, continued

The natural hydrologic connection between the Río Piedras 

and Río Puerto Nuevo Rivers, the historical changes to their 

channels, and the historical uses of these rivers for commerce 

and other purposes cause confusion, because different 

maps over the past 300 years show different alignments and 

connections between them. Here, we present a comparison of 

historical maps to illustrate how the perception of the location 

of these rivers has changed.

The Río Piedras River, depicted in a 1660 map of the San 

Juan Bay, is shown discharging directly into the bay (fig. B2-5). 

The map also shows the Caño Martín Peña; unlike today, these 

two channels were not connected to each other. The 1776 map 

of San Juan Bay and its environment (today’s metropolitan 

Figure B2-7. 1887 Spanish Army Corps of Engineers map for San 

Juan Bay area (Sepúlveda Rivera 2004a).

Figure B2-8. 1884 Spanish Army Corps of Engineers map for 

San Juan Bay area (Sepúlveda Rivera 2004a).

area) by Thomas O’Daly (reproduced in Sepúlveda Rivera 

2004a) shows the natural features of this region before any 

development took place (Old San Juan being the exception). 

In that map (fig. B2-6), which includes lagoons and extensive 

mangrove and wetland areas, the mouth of the Río Puerto 

Nuevo River is labeled and shown to be discharging directly 

into the San Juan Bay. Although the Río Piedras River is not 

named, a continuous channel from the mouth is shown with 

a similar alignment to the 1950 river location, leading us 

to believe it is the same Río Piedras riverbed. Large areas 

of mangroves and wetlands fringe the lowland reaches of 

the river. A Spanish Army Corps of Engineers map for 1887 

(fig. B2-7) shows these tributaries and canals as being part 

of the drainage of a fringe mangrove forest, including several 

tidal channels. All canals drained independently into the bay, 

however. Yet, an 1884 map by the same Spanish Army Corps 

of Engineers (fig. B2-8) identifies the Río Puerto Nuevo River 

as merging with Quebrada Las Margaritas and then flowing 

into the bay. That map does not show the Río Piedras River.

Both rivers remain as separate canals draining into 

San Juan Bay in a regional map of San Juan, dated 1898 

(fig. B2-9). This map shows a small creek to the east of the 

Río Piedras River as a tributary to the Río Piedras River and 

its apparent connection at its inland origin with another canal 

that drained into the bay between the Río Piedras River and 

the Caño Martín Peña.

Figure B2-9. 1898 U.S. Military map for San Juan Bay area 

(Sepúlveda Rivera 2004a).
6 The Río Piedras Watershed and Its Surrounding Environment

hills are rounded with elevations of 300 m and up to 600 m. 

They are composed chiefly of sandstone and siltstone, associated 

volcanic and intrusive rocks, and some limestone (Anderson 

1976). Streams and rivers, such as the Río Piedras, pass through 

these foothills through narrow valleys whose flood plains are 

flanked by alluvial terraces.

Climate2

San Juan is in Holdridge’s subtropical moist forest life zone 

(1967). Mean annual rainfall (fig. 4) in the basin increases from 

the coast (1,509 mm) to the uplands (1,755 mm at 8 km inland), 

and the ratio of pan evaporation to rainfall changes in the same 

distance from 1.37 to 0.98. Rainfall is seasonal (fig. 4), with the 

rainy season coinciding with the hurricane season (July through 

October) and a dry season (January to April). May has a secondary wet period. Severe droughts are recurrent in San Juan and 

have been recorded in the 1920s, 1930s, 1960s, 1970s, and 1990s 

(fig. 5a). Anderson (1976) reported 209 days with measurable 

rainfall during an average year in Old San Juan. The long-term 

rainfall record for Old San Juan (fig. 5a) reflects a decreasing 

trend over a 107-year period. Picó (1969) reports six hurricanes 

and storms with significant effects on Puerto Rico between 1893 

and 1956. Since then, several other hurricane events have affected 

the island, with Hurricanes Hugo and Georges being the most 

severe in terms of wind effects. Two hurricanes, San Nicolás, in 

1931, and San Ciprián, in 1932, passed directly over the metropolitan area. Hurricane San Felipe crossed the island diagonally and 

San Juan was protected by the Cordillera Central; however, the 

strongest winds recorded in San Juan were those of Hurricane 

San Felipe in 1928, with 150 mi/h (240 km/h or 66.7 m/s) winds. 

2 A list of official climatic, hydrologic, and air quality stations in the 

metropolitan area is shown in the appendix.

Figure 4. Walter (1971) climate diagram for Old San Juan (a) and 

Río Piedras (b), Puerto Rico. Solid black represents wet conditions 

with > 100 mm per month; vertical lines represent moist conditions 

where the rainfall exceeds potential evapotranspiration. The 

information on top of each graph shows the location, length of 

record, and elevation of the station and the annual means for 

temperature and precipitation.

Figure 5. Long-term annual precipitation (a) and air temperature (b) for Old San Juan and Río Piedras, Puerto Rico. Lines represent a 5-year 

running average. Data are from the National Oceanographic and Atmospheric Administration.

5a.

5b.

4a.

4b.
The Río Piedras Watershed and Its Surrounding Environment 7

Maximum winds in San Juan for San Ciprián reached 120 mi/h 

(192 km/h or 53.3 m/s).

Mean annual temperature for Old San Juan and Río Piedras 

is 25.9 °C and 25.7 °C, respectively (fig. 4). The average temperature ranges from 27.2 °C in August to 23.9 °C in January. 

Old San Juan Climate Station (2002399) was in service from 

1898 until 1977. Temperature data are available until 1973. A 

new station (2002398) was established in 1955 at the San Juan 

International Airport (now Luis Muñoz Marín International 

Airport). Data from this station are available from 1955 to the 

present. Only 16 years of coincidental temperature data exist between these two stations in San Juan. Both stations show similar 

temperature records until 1964. The new station shows an 

increase of temperature from 1965 and a rapid increase after the 

1970s (fig. 5b). If the data are plotted monthly for representative 

years, every month shows temperature increases between 1952 

and 1981 (fig. 6). After 1981, the monthly means are higher than 

before 1981 but tend not to exceed 1981 values. These temperature increases are not reflected in the National Oceanographic 

and Atmospheric Administration (NOAA) Río Piedras Climate 

Station (20023985) nearby. This difference suggests a heat island 

effect in Old San Juan rather than global warming. González 

et al. (2005) and Velázquez Lozada et al. (2006) reported heat 

island effects for the metropolitan area, and Velázquez Lozada 

et al. (2006) found that air temperature has been increasing at 

a rate of 0.06 °C/yr over the past 40 years. Murphy et al. (in 

press) found a 1.54 °C heat island effect in the metropolitan 

area and related it to land cover. This effect may be reflected in 

urban climate stations, such as the one in Old San Juan in its 

last 8 years of service, where the surroundings have been urbanized. The sharp increase of temperature after 1964 in the newest 

San Juan station might be explained similarly, given that the 

surroundings have been urbanized over the past 100 years.

The prevailing daytime winds in the city are from the northeast at 4.3 to 6.1 meters per second (m/s) and they prevail from 

the southeast at 2.2 to 4.8 m/s during nighttime (fig. 7). For the 

region, the prevailing winds are from the east and northeast, 

but, from September to November, wind currents alternate 

from the south and southwest. City climate is moderated by 

diurnal sea breezes that result from differential land/sea heating. 

These offshore breezes reverse direction at night.

At the onset of the 20th century, a group of scientists 

sponsored by The United States Fish Commission Steamer Fish 

Hawk (U.S. Fish Commission 1900) recorded climate parameters in San Juan for a complete year between May 1899 and 

April 1900. Their observations in figures 8 to 10 are presented 

in the original units of measurement as a historical curiosity. 

The long-term rainfall pattern was visible that year (fig. 8), with 

dry months in February to March and peak monthly rainfall 

in September. A 5-inch (125 mm), 24-hour rainfall event took 

place in August. Minimum temperatures dipped in December 

and maximum temperatures were high in April (fig. 9). Clear 

days prevailed during the dry season, and June had the greatest 

number of rainy days (fig. 10).

Figure 6. Mean monthly air temperature for a variety of years using 

data from the Old San Juan Climate Station. The original analysis 

from Seguinot Barbosa (1983) was updated.

Figure 7. Wind speed and direction for daytime, nighttime, and all 

hours in San Juan (Webb and Gómez Gómez 1998). Wind speed is in 

meter per second and is indicated at the end of each ray. The length 

of the ray is the percentage of time the wind blows in the indicated 

direction.

Figure 8. Total rainfall and maximum 24-hour rainfall in Old San Juan 

in 1900 (U.S. Fish Commission 1900). To convert to millimeters, 

multiply inches by 2.54.
8 The Río Piedras Watershed and Its Surrounding Environment

Figure 9. Maximum (squares), minimum (triangles), and mean 

(circles) air temperature in Old San Juan in 1900 (U.S. Fish 

Commission 1900). To convert to °C, subtract 32 from °F and 

multiply by 5/9.

Figure 10. Sky condition in Old San Juan during 1900 (U.S. Fish 

Commission 1900).

Geology

The Río Piedras River flows over surficial deposits (alluvial, 

terrace, blanket sand, beach, and dune deposits) ranging in age 

from Miocene (about 23.5 million years [Ma] ago) to Holocene 

(about .01 Ma) (Anderson 1976). These surficial deposits mantle 

the bedrock over the length of the channel (fig. 11). They are less 

than 15 m deep above the contact between the volcanic and sedimentary rocks and 15 to 30 m deep below the contact point in 

the lowlands (Anderson 1976). The sequence of underlying rocks 

from the uplands to the coastal zone described by Anderson 

includes (in order): volcanic, San Sebastián, Cibao, Aguada, and 

Aymamón (the last three are sedimentary rocks).

The volcanic rocks occur in the northern foothills south of 

the metropolitan area and include volcanic tuff, breccia, and 

lava (Anderson 1976). These rocks are of Cretaceous (about 89 

Ma) and Tertiary (about 65 Ma) age and generally lack permeability, except in a weathered and fractured surficial zone 

that follows the general topography. Pease and Monroe (1977) 

mapped to a depth of 400 m the geology of the San Juan 

quadrangle, which includes the Río Piedras Watershed. They 

mapped 22 stratified rock types and one intrusive rock type. 

The oldest types are the Guaynabo and Monacillos formations. 

The Río Piedras formation is from the Eocene Epoch and is 

about 1,000 m thick.

Pease and Monroe described the stratigraphy of the quadrangle as follows: “...consists of Cretaceous and lower Tertiary 

clastic and volcaniclastic stratified rocks and thin lavas that are 

unconformably onlapped by unconsolidated to poorly consolidated sand, silt, and gravel of middle Tertiary to Holocene age 

along the southern border of the quadrangle [inland towards 

the origin of the Río Piedras]. Reef limestone, also of middle 

Tertiary age, occurs in sporadic outcrops, mostly forming 

prominent hills and ridges that have been deeply excavated by 

quarrying. Quaternary [about 1.64 Ma] and possibly upper 

Tertiary [about 5.2 Ma] beech, eolian, swamp, and alluvial 

deposits form most of the land area in the northern half of the 

quadrangle [towards the coastal zone], except that large parts 

of Bahía de San Juan and Laguna San José have been artificially 

filled.” (Pease and Monroe 1977)

The following descriptions of the geologic formations 

that constitute the sedimentary rocks within the Río Piedras 

Watershed are from Anderson (1976, pp. 9–11); consult Lugo et 

al. (2001) for more detail on these formations.

✦ San Sebastián Formation. The San Sebastián Formation of 

Oligocene age is composed of crossbedded to massive beds 

of sand, sand and gravel, and sandy clay, with some thin 

beds of sandstone and sandy limestone. The formation can 

be considered consolidated but is essentially uncemented. 

The formation attains its maximum thickness of about 500 

ft (150 m) in the western part of the metropolitan area, 

but Pease (1968) attributes 150 ft (45 m) of this thickness 

to the Mucarabones Sand, a formation overlying the San 

Sebastián and grading into the Cibao. Work in the Bayamón quadrangle by Monroe (1973) indicates that the 

Mucarabones Sand intertongues eastward with the entire 

lower part of the overlying Cibao Formation, so that the age 

of the Mucarabones Sand is Oligocene and Miocene. The 

San Sebastián Formation commonly is 165 to 330 ft (50 to 

100 m) thick, becoming progressively thinner to the east.

✦ Cibao Formation. The Cibao Formation is of Oligocene and 

Miocene age and conformably overlies the San Sebastián 

Formation. The Cibao is composed of interbedded marl, 

limestone, clay, sand, and gravel. It attains its maximum 

thickness of 380 ft (115 m) in the westernmost part of the 

metropolitan area, thinning eastward to about 130 ft (40 m) 

thick in the vicinity of Carolina. In the westernmost part of 

the metropolitan area, the Cibao has been divided into five 

Members (Monroe and Pease 1962): (1) Upper Member—

chalk, sandy limestone, and sandy marl; 100 to 160 ft (30 to 

50 m) thick. (2) Miranda Sand Member—sand and gravel, 
The Río Piedras Watershed and Its Surrounding Environment 9

filling channels cut into the underlying limestone; 0 to 60 ft 

(0 to 20 m) thick. (3) Quebrada Arenas Limestone Member—

consists of alternating beds less than 3 ft (1 m) thick, of 

dense crystalline limestone and soft marly limestone; 0 to 

70 ft (0 to 20 m) thick. (4) Río Indio Limestone Member—a 

yellowish-orange earthly limestone that grades into a sandy 

marl toward the east; reported to be 230 ft (70 m) maximum 

thickness. (5) Lowermost member (unnamed)—consists of 

sandy limestone and marl facies transitional with the San 

Sebastián; 100 to 230 ft (30 to 70 m) thick.

 The three middle members apparently crop out only in 

the Bucarabones Valley. The upper and lowermost members 

are identifiable in the vicinity of the Río de Bayamón.

 In the Carolina area, the Cibao overlaps the San Sebastián 

and is believed to rest directly on the bedrock formations of 

tuff and volcanic rocks. Here the formation is about 130 ft 

(40 m) thick.

 In places, the Cibao forms topographic ridges but more 

commonly makes up the lower slopes of hills and mogotes 

capped by the more resistant Aguada and Aymamón Limestones. In the San Juan area, exposures of the Cibao are evident at the base of Montes San Patricio, Montes de Canejas, 

and the smaller hills south of the San José Lagoon.

Figure 11. Generalized surficial geology for the San Juan Bay Estuary Watershed (from Webb and Gómez Gómez 1998). The Río Piedras 

Watershed is delineated with a blue line.

✦ Aguada Limestone. The Aguada Limestone of Miocene 

age is a transitional formation between the Cibao marl and 

the relatively pure crystalline limestone of the overlying 

Aymamón Limestone. Briggs and Akers (1965) described the 

Aguada on the north coast as “hard, thick-bedded to massive 

calcarenite and dense limestone interbedded with chalky 

limestone and marl; commonly containing some quartz 

grains and locally thin-bedded near top.” It ranges in thickness from 100 to 250 ft (30 to 75 m) in outcrop in the study 

area. South of the Bahía de San Juan, the hard crystalline 

beds occur in the upper part while the softer and marly beds 

predominate in the lower part (Gelabert 1964).

 The Aguada is a ridge former and, together with the 

Aymamón Limestone, makes up most of the mogotes in the 

San Juan area. Except for these last remaining vestiges of the 

great karst region to the west, the limestones in San Juan 

have been largely eroded away and covered with alluvial clay, 

sand, and beach deposits.

✦ Aymamón Limestone. The uppermost limestone, the 

Aymamón of Miocene age, is exposed as case-hardened caps 

on a few hilltops in Montes de Canejas and in the small hills 

around the juncture of Caño Martín Peña and the Laguna 
10 The Río Piedras Watershed and Its Surrounding Environment

San José. Elsewhere it lies beneath a mantel of alluvium and 

other surficial deposits. The following description of the formation is given by Zapp, Berquist, and Thomas (1948): “It 

consists almost entirely of very dense conchoidally fracturing limestone of white, light gray, buff and rose colors; unit 

is quite uniform over its entire outcrop and throughout its 

entire thickness.” The map by Monroe and Pease (1962) of 

the Bayamón quadrangle shows the limestone outcrops, both 

of the Aguada and Aymamón to be quite extensive, forming the tops of many of the haystack hills. In the Bayamón 

area, the Aymamón is either pink or white, well indurated, 

usually dense limestone; 215 ft (65 m) thick were exposed. 

Kaye (1959) estimates the thickness of the Aymamón to be as 

much as 2,000 ft (600 m) at the coastline. Geophysical studies at Palo Seco indicate an aggregate thickness of middle 

Tertiary limestones (Aguada and Aymamón) to be about 

3,000 ft (900 m).

Pease and Monroe (1977) mapped four faults in the San Juan 

quadrangle, including one on the southern part of the quadrangle, which is part of the Leprocomio fault, one of the principal 

faults of the great northern Puerto Rico fault zone. This fault is 

the eastern of two wrench faults, with the western fault moving 

left lateral and the Leprocomio fault moving right lateral. The 

entire block between the two faults appears to have moved 

several kilometers to the west relative to rocks on the other side.

Hydrology

Ground Water

Ground water is mostly stored in the Tertiary sand and limestone aquifers. These aquifers recharge mostly by rainfall in their 

outcrop areas, from infiltration of streamflow, and from infiltration in the volcanic zone. Leaky water and sewer lines in the 

urban area are a secondary source of aquifer recharge (Anderson 

1976). The aquifers discharge to the large wetlands and lagoons 

along the coastline.

In the volcanic zone of the watershed, most drainage is 

converted to streamflow. In some areas, volcanic rocks can 

discharge into sedimentary rocks. Most ground water discharge 

is to streams (Anderson 1976). Ground water recharge is cyclic 

and occurs during the rainy season and also during large storms 

that can occur at anytime. Urbanization and paving of surfaces 

is reducing the recharge areas in the metropolitan area, but leakage from sewer and water mains makes up part of the loss. At 

the time of Anderson’s report (1976), that leakage was 1.2 m3

/s.

The Aymamón Limestone contains saltwater throughout most 

of the metropolitan area, yielding as much as 130 L/s of saltwater (Anderson 1976). The artesian aquifer has not encountered 

seawater intrusion, however. The upper part of the Cibao Formation and the Aguada and Aymamón Limestones form a prolific 

unconfined aquifer with the freshwater part, ranging from 3 to 

1 km in width between Bayamón and Carolina. The yields of 

this water-table aquifer range from 2 to 160 L/s and an average 

of 32 L/s. This aquifer is a high-conductivity aquifer, with an 

average transmissivity of 1,200 m2

/day (range from 650 to 12,500 

m2/day). Ground water is mostly a calcium bicarbonate type, and 

some water is a sodium sulfate type (Anderson 1976).

Surface Water

Most of the runoff in the Río Piedras Watershed originates over 

the volcanic rocks and the impervious urban surfaces. Infiltration occurs largely through the weathered zone from which it 

flows into stream valleys. The Río Piedras River has a steep gradient within its first 8 km before it reaches the lowlands (fig. 3). 

This topography causes rapid stage increases after intense rainfall 

events and flashy streamflows (Osterkamp 2000). For example, 

a 26-year record at the U.S. Geological Survey (USGS) Río 

Piedras River at Hato Rey Gaging Station (50049100; see appendix for location and other data) shows that peak discharge is 

170 times the mean discharge. In addition to the steep gradient 

in the uplands, other factors contribute to high-velocity flows 

and flash foods. One factor is the channelization of most of the 

tributaries nearly to their headwaters, including concrete-lined 

channels. A second factor is the large fraction of impermeable 

surfaces in the city. Osterkamp (2000) estimated that the mean 

discharge of the Río Piedras River at the Hato Rey Gaging Station was 1.7 m3

/s before urbanization and now averages 2.3 m3/s 

after urbanization.

A third factor altering the urban hydrology is the paving 

over ephemeral and intermittent streams that form the drainage 

network of this watershed. In some cases, permanent streams 

are buried under thick layers of fill accompanied by considerable land movements involving reshaping of the local topography (box 2). An example is Quebrada Chiclana (photo 1a–d), 

where a gigantic French drain was constructed over the stream 

with rocks and soil fill. Concerns for erosion and safety led to 

litigation and the stream channel was ordered restored in 2003 

at a cost of millions of dollars.

USGS Gaging Station at Río Piedras 50049000, which 

drains 39.4 km2, is located at a 5 m elevation, averages about 26 

Mm3/yr (million cubic meters per year) (15-year record), equivalent to 810 mm of runoff. With an average rainfall of 2,000 

mm, this runoff results in a runoff-to-rainfall ratio of 0.40 for 

the less urbanized area of the basin (Webb and Gómez Gómez 

1998). A ratio of 0.60 was used for the urbanized areas of the 

basin to raise the input to San Juan Bay from the Río Piedras 

River to 185 Mm3/yr. Ortiz Zayas et al. (2006) used a 29-year 

record for the same station (updated to 2002) to estimate an 

annual runoff of 1,200 mm and reported a 63-percent urban 

cover in the watershed.

The discharge record shows wide fluctuation in two stations 

along the river (fig. 12). The USGS Río Piedras River at El 

Señorial Gaging Station (50048770) (fig. 12a) shows a decrease 

discharge during the dry season months of February and the 

drought of 1994. The trend line for the USGS gaging station 

at Río Piedras (fig. 12b) reflects annual discharge variation and 
The Río Piedras Watershed and Its Surrounding Environment 11

Photo 1. The Quebrada Chiclana (a) is a small rocky stream in the uplands of the Río Piedras Watershed that flows through deep valleys covered 

with vegetation (b). As is customary in Puerto Rico, a developer filled the steep valleys (c) to level the terrain and construct expensive housing 

over Quebrada Chiclana (d). Photos by A.E. Lugo.

Figure 12. Water discharge for two U.S. Geological Survey gaging stations (station numbers in parentheses) along the Río Piedras River. 

Monthly values for the station at El Señorial (a) are connected, while a 5-year running average is shown for the monthly data from the Río 

Piedras station (b). To convert to cubic meter per second, multiply cubic feet per second by 0.02832.

12a. 12b.

1c. 1d.

1a. 1b
12 The Río Piedras Watershed and Its Surrounding Environment

Figure 15. Mean annual discharge (open circles) and suspended sediment concentration (solid circles) in U.S. Geological Survey (USGS) gaging 

stations at (a) Río Piedras at El Señorial (station number in parentheses), and mean annual discharge, suspended sediment concentration, and 

suspended sediment export (solid triangles) at (b) Río Piedras at Río Piedras. Data are from the USGS. To convert discharge to cubic meter per 

second, multiply cubic feet per second by 0.02832, and to convert export to megagram per day, multiply tons per day by 0.9072.

15a.

15b.

apparent longer trends with periods of decline 

(1971 to 1974), followed by a period with variable but increasing annual discharge values 

(1974 to 1988) and another period of decline 

(1988 to 1993). Unfortunately, data collection 

at these stations has stopped. The USGS gaging station at Hato Rey drains an area of 15.2 

mi2 (39 km2) and has a mean flow of 51.21 ft3/s 

(1.45 m3/s), a Q99 of 1.5 ft3/s (0.04 m3/s), and 

minimum and maximum flows of 28.7 (0.81 

m3/s) and 10,500 ft3/s (297.4 m3/s), respectively.

Periods of high discharge are common in the 

available record (fig. 13) with peak discharges at 

about or higher than 10,000 ft3/s (283.2 m3/s) 

during hurricanes and events associated with 

stationary fronts. The peak flows are considered 

part of the mean daily flow computation and 

are reflected in the mean daily flows used to 

generate the flow duration curve as in the one 

for the period 1971 to 1982 (fig. 14). In contrast 

with mean annual discharges of about 55 ft3

/s 

(1.56 m3/s) during years of high discharge, the 

mean annual discharge during the drought 

years of 1974 and 1994 was about 10 ft3/s (0.28 

m3/s) and less than 10 ft3/s (0.28 m3/s) (fig.15). 

These droughts resulted in water rationing in 

the metropolitan area.

The relative flows of tidal and freshwater 

runoff to the coastal lagoons and to the San 

Juan Bay favor the marine fluxes, but, during dry weather, the Río Piedras River is a 

main source of freshwater to the San Juan Bay 

(fig. 16). During intense rainfall events, river 

and stormwater discharges dominate the tidal 

flow patterns of the lagoons. For example, a 

70 mm rainfall over a 12-hour period (a common event) generated sufficient storm runoff 

Figure 13. Annual peak discharge rates for the U.S. Geological Survey (USGS) Gaging 

Station at Río Piedras (station number in parentheses). The two data points before 

1973 are from Haire. Data are from the USGS. To convert to cubic meter per second, 

multiply cubic feet per second by 0.02832.

Figure 14. Flow duration curve for U.S. Geological Survey (USGS) Gaging Station at 

Río Piedras (station number in parentheses). Data are from the USGS. To convert to 

cubic meter per second, multiply cubic feet per second by 0.02832.
The Río Piedras Watershed and Its Surrounding Environment 13

(3.6 Mm3 or 20 percent of the total volume of Laguna San José, 

Laguna La Torrecilla, and Laguna de Piñones) to change tidal 

flows within these lagoons (Webb and Gómez Gómez 1998).

The Departamento de Recursos Naturales y Ambientales 

(2004) delineated the Río Piedras Watershed and developed a 

water budget for the watershed (table 1). The budget shows a 

larger transfer of water to the watershed (for human consumption) than the annual rainfall on the watershed. The water 

budget is not balanced because a large proportion of the water 

is unaccounted for. This unaccountability is probably the result 

of leaky water pipes, which is estimated at about 50 percent at 

the present (Departamento de Recursos Naturales y 

Ambientales 2004).

Floods and Flood Control

Haire (1971) reported floods in the Río Piedras watershed for 

May 23, 1958; November 12, 1961; October 12, 1963; September 

16, 1966; June 15 through 17, 1970; and October 5 through 7, 

1970. He reported flooding depths of up to 2 m in the last 

kilometer reach of the Río Piedras. Peak discharges at the Jesús 

T. Piñero Avenue (4.55 mi [7.3 km] from the mouth) during 

the flood of June 17, 1970, were 10,000 ft3/s (283.2 m3/s) or 650 

ft3/mi2 (7.11 m3/km2). The corresponding discharges for the 

October 1970 flood were 7,800 ft3/s (221 m3/s) and 506 ft3/mi2

(6.63 m3/km2). These peak discharges had a recurrence interval 

of 8 and 5 years, respectively, based on data available in the 

1970s. Figure 13 substantiates the assumption. A recent flood 

occurred in November 2009 when the discharge at the USGS 

Río Piedras Gaging Station (50049000 exceeded 11,000 ft3

/s 

(>312 m3/s). 

Floods in the Río Piedras Watershed affected some 5,700 

families in the 1980s, 325,000 m2 of commercial space, large 

land and sea-oriented transportation facilities, and numerous public buildings and facilities (Colón 1984). The value of 

property in the flood plain was estimated at $3 billion, with 

quantifiable average annual damages of $20 million and $38.9 

million when future conditions are considered (Colón 1984). 

This increase was expected to occur because of higher stages in 

flooding and the affluency factor rather than new construction 

in the flood plain. These calculations were used to justify a 

proposal for the channelization of the river (fig. 17). The channelization involves 10.4 km of the Río Puerto Nuevo River 

(lower reach of the Río Piedras, see box 2), 6 km of two of its 

principal tributaries, and diverting 1.3 km of a third tributary to protect against the 100-year flood. The plan requires 

eliminating 13.5 ha of mangroves and replacing 22 bridges 

(15 of them are major highways and avenues). In 1984, the 

estimated cost of the project was $235.5 million (Colón 1984). 

The channelization activities began by channelizing the lower 

reaches of the river, and plans continue to channelize upstream as initially planned. The validity, however, of assumptions made 30 to 40 years ago to justify the channelization of 

the river have been questioned by the University of Puerto 

Rico and the Conservation Trust of Puerto Rico, which have 

developed alternative concepts of water management for the 

watershed (see Discussion).

Figure 16. Average daily net flows in dry weather for canals, streams, rivers, pumping stations, tidal creeks, and aquifer discharge that 

interconnect San Juan Bay with the lagoons of the San Juan Metropolitan Area (Webb and Gómez Gómez 1998).
14 The Río Piedras Watershed and Its Surrounding Environment

Table 2. Area of San Juan Bay geological formations and 

subsystems.

Formation or subsystem Area (km2)

Land in the basin 215

Water in the basin 25

Volcaniclastic and intrusive rocks 55

Limestone 13

Quaternary deposits (marine and riverine, 

alluvium, beach and swamp)

147

Mangrove and swamp deposits 27

Beach deposits 10

Quartz-sand deposits underlie Santurce, and 

San José and Torrecilla lagoons

11

Urban fill 29*

*12 percent of the watershed.

Source: Webb and Gómez Gómez (1998).

San Juan Bay and 

Coastal Lagoons

The San Juan Bay has a 240-km2 drainage basin (table 2) that 

includes the Río Piedras Watershed, five coastal lagoons, the 

Torrecillas-Piñones-Vacía Talega mangroves, and, during flood 

periods, even the Río Grande de Loíza River. Most of the drainage area of the bay is in the metropolitan area, with the exception of the drainage from the Río Grande de Loíza River, which 

originates as far as the Luquillo Mountains.

The hydrology of San Juan’s five coastal lagoons, all 

eurihaline and estuarine, is summarized by Ellis and Gómez 

Gómez (1976). These coastal lagoons are shallow and lined 

with mangrove forests (table 3). The lagoons are interconnected 

with channels and export sediments and nutrients to the ocean 

(table 4). Laguna San José is the coastal lagoon most closely 

connected to the Río Piedras Watershed as it drains into Caño 

Martín Peña, as does the Río Piedras River. This lagoon is fed 

by urban runoff and upwelling of ground water from an aquifer 

that is recharged in part within the Río Piedras Watershed 

(fig. 16).

Figure 17. U.S. Army Corps of Engineers channelization plan for the Río Piedras River. The proposed channelization (blue line) continues inland 

(south) past the Botanical Gardens of the University of Puerto Rico and past highway PR-177 toward Caimito.
The Río Piedras Watershed and Its Surrounding Environment 15

Table 3. General characteristics of the San Juan Metropolitan Area’s coastal lagoons.

Parameter La Torrecilla Piñones San José Condado

Area (ha) 26 105 547 31

Average volume (106 m3) 5.91 0.87 13.2 1.42

Maximum depth (m) 18 1.3 11 11

Average depth (m) 2.4 0.8 2.4 4.5

Maximum length (km) 2.5 1.58 5.52 1.2

Width (km) 1.65 0.82 2.2 0.31

Shoreline (km) 19.1 4.91 13.5 2.7

Mangrove cover (km) 14.8 4.66 6.35 0

Source: Ellis and Gómez Gómez (1976). 

Table 4. Sediment and nutrient fluxes through two channels from San José Lagoon in San Juan. 

Parameter

Inflow Outflow Net load

Canal Suárez

Caño 

Martín Peña Canal Suárez

Caño 

Martín Peña Canal Suárez

Caño 

Martín Peña

TOC 4.77 2.52 8.70 3.08 – 3.93 – 0.56

TOP as P 0.19 0.13 0.31 0.13 – 0.12 0

TP as P 0.26 0.21 0.39 0.20 – 0.13 0.01

Suspended 

Sediment 4.32 4.22 0.10

Note: All data are in Mg based on one tidal cycle (25 hours) from January 24 through January 25, 1974. Negative numbers indicate a net export. Canal Suárez 

data were taken at the Highway 26 bridge and the Caño Martín Peña data were taken at the Barbosa bridge (Ellis and Gómez Gómez 1976). Total organic 

phosphorus (P) is TOP, total P is TP, and total organic carbon is TOC. Empty cells mean no data are available.

Dredging activities in the coastal lagoons of San Juan have 

modified their volume and affected their hydrology, tidal oscillations, and water turnover (Ellis 1976). The increase in water 

volume in the lagoons (fig. 18) has been the result of an increase 

in water depth due to dredging, despite a decrease in area due 

to filling and construction (fig. 19). In their original state, these 

lagoons were shallow, flushed by tides, and had a well-mixed 

Figure 18. Historical change in the water volume of three lagoons of 

the San Juan Metropolitan Area (Ellis 1976).

water column. When the lagoons were under tidal control, 

dissolved oxygen levels in the water column were usually high. 

After heavy rains, the lagoons received pulses of organic matter 

from mangroves and their bottom sediments were resuspended. 

Both effects caused sudden reductions in dissolved oxygen and 

occasionally massive fish mortalities. Among the coastal lagoons 

in the metropolitan area, only the Piñones Lagoon continues 

Figure 19. Historical change in the area of three lagoons of the San 

Juan Metropolitan Area (Ellis 1976).
16 The Río Piedras Watershed and Its Surrounding Environment

Table 5. Rainfall quality (mg/L) in Río Piedras at 15.2 m elevation. 

Parameter Mean

Standard 

deviation Maximum Minimum

Calcium 1.8 1.4 6.5 0.3

Magnesium 0.8 0.6 4.6 0.1

Bicarbonate 11 4.5 27 2

Sulfate 1.5 1.3 5.2 0

Chloride 4.9 4.6 9.8 0.2

Nitrate 0.2 0.8 1.2 0

Dissolved solids 21 12 81 7

Hardness as CaCO3 (calcium, magnesium) 8.5 5.4 29 2

Hardness as CaCO3 (noncarbonate) 1 2.2 13 0

pH 6.8 0.4 7.6 5.7

Note: Analyses are based on a monthly collection between 1963 and 1967.

Source: Quiñones Márquez (1975).

to function as it did before the anthropogenic transformation 

of the past 500 years. All others have been modified by changes 

in area and depth as a result of human activities. Ellis (1976) 

reports depths of up to 18 m in La Torrecilla Lagoon and up to 

11 m in Condado and San José Lagoons.

A consequence of the increase lagoon volume is the reduction in water turnover, because the same tidal flux encounters 

a greater amount of water in the lagoons. Thus, water quality 

deteriorates due to the accumulation of pollutants and the development of anaerobic conditions in the water column, which 

becomes stratified. The anaerobic layer forms at depths that 

exceed the original average depth of 2 m for the lagoons; in fact, 

all dredged areas below 2.5 m depth turn anaerobic (Ellis 1976).

Figure 20. Trace of the expected inland penetration of a tsunami in the San Juan Metropolitan Area. The maximum runup wave level was 

estimated at 8.99 m. This information was provided by a Federal Emergency Management Agency/University of Puerto Rico Mayagüez Campus 

study sponsored through Sea Grant.

The largest storm surge (still-water elevation above the 

predicted astronomical tide) in Bahía de San Juan was 73 cm 

in November 1982 (Webb and Gómez Gómez 1998). This 

surge was caused by a storm in the North Atlantic. Hurricane Hugo in 1989 generated a surge of 70 cm. In the past, 

sand dunes from El Boquerón (by Laguna del Condado) 

to Punta Vacía Talega absorbed the energy of such surges. 

In the Piñones area, however, 10-meter-high dunes were 

removed to construct the Luis Muñoz Marín International 

Airport, and now these areas are overtopped frequently by 

moderate surge events. A large portion of the coastal zone 

of the metropolitan area is subject to tsunami inundation 

(fig. 20).
The Río Piedras Watershed and Its Surrounding Environment 17

Water Quality

In Relation to Rainfall

Quiñones Márquez (1975) found a low correlation (r = 0.2) 

between rainfall intensity and its specific conductance in Río 

Piedras. The concentrations of calcium (Ca), magnesium (Mg), 

and dissolved solids in Río Piedras rainfall (table 5) tended to 

be higher, however, than in other locations in the island. Dissolved solids in rainfall varied seasonally. During dry months 

(December through March), concentrations increased and then 

decreased as the rainy season progressed.

In Relation to Surface Water

Kwak et al. (2007) analyzed the waters of the Río Piedras River 

at the bridge of PR 176 and found that, in comparison with the 

mean of 46 insular rivers, the waters of the Río Piedras River 

had higher dissolved solids, conductivity, ammonia (NH4), 

alkalinity, and hardness and lower phosphorus (P), nitrate 

(NO3), and nitrite (NO2) (table 6). The Environmental Quality 

Board and the U.S. Environmental Protection Agency classify 

the waters of the Río Piedras River as highly polluted. Figure 21 

shows that the fecal bacteria counts in USGS Río Piedras River 

at Hato Rey Gaging Station (50049100) are normally above the 

Puerto Rico sanitary quality goals for class SD waters, which, 

by definition, are waters with unacceptable quality (http://www.

ct.gov/dep/cwp/view.asp?a=2719&q=325620).

W. McDowell, University of New Hampshire (McDowell 

2010), found that the level of urbanization in the Río Piedras 

Watershed was linearly related to the concentration of NH4

and phosphate (PO4) but not with the concentration of NO3

in riverine waters. The following mean, maximum (max.), 

minimum (min.), and standard deviation (SD) concentration values represent data that McDowell collected between 

January 15 and July 30, 2009, at the Río Piedras Puente Parque 

Station. All data are in mg/L, except for PO4 and NH4, which 

are in μ/L. NH4 and PO4 are generally high and variable. In 

the table, DOC, TDN, and DON are dissolved organic carbon, total dissolved nitrogen, and dissolved organic nitrogen, 

respectively.

DOC TDN Cl NO3 SO4 PO4 Na K Mg Ca NH4 DON

Mean 4.6 1.40 29.77 0.88 4.56 63 23.20 3.26 10.25 24.43 288 0.23

Max. 45.51 1.84 34.91 1.24 5.87 284 27.75 4.54 11.94 32.90 686 0.39

Min. 1.70 0.92 21.19 0.60 3.57 19 17.96 2.48 7.19 15.89 75 0.09

SD 8.97 0.28 4.23 0.18 0.56 58 2.79 0.46 1.31 4.54 197 0.08

Cl = chlorine. K = potassium. Na = sodium. SO4 = sulfate. 

Table 6. Water quality measures of Río Piedras River water at the bridge off highway PR-176. 

Parameter Río Piedras Mean of 46 rivers

Water temperature (°C) 23.85 24.32

Total dissolved solids (g/L) 0.294 0.209

Conductivity (μS/cm) 452 322

Salinity (ppt) 0.22 0.15

Nitrate (mg/L as NO3

-

) 0.2 3.7

Nitrite (mg/L as NO2

-

) 0.062 0.076

Ammonia (mg/L as NH3) 0.12 0.08

Phosphorus (mg/L as PO4

-

) 0.49 0.65

Alkalinity (mg/L as CaCO3) 169 130

Hardness (mg/L as CaCO3) 168 135

Turbidity (FAU) 6 6.6

pH 7.50 8.29

Dissolved oxygen (mg/L) 7.49 8.19

μS/cm = microsiemens per centimeter, ppt = parts per thousand, FAU = Formazin Attenuation Units

Note: Data correspond to the spring of 2007 by Kwak et al. (2007).
18 The Río Piedras Watershed and Its Surrounding Environment

discharge events can transport thousands of tons of sediment 

per day to San Juan Bay. These high-sediment discharge events 

are due to both increases in water discharge and suspended 

sediment concentrations. For example, during events associated with Hurricane Hugo in September 1989 and Hurricane 

Georges in September 1998, the USGS gaging station at El 

Señorial (50048778) exhibited suspended sediment discharges 

of 4,477 and 11,110 tons (short) per day, respectively (or 4,064 

and 10,079 Mg/day [megagrams per day]). The concentration 

of suspended sediments in this station during Hurricane Hugo 

was 2,357 mg/L. The suspended sediment discharge at the Río 

Piedras USGS Gaging Station (50049000) was 6,390 tons 

(5,800 Mg) per day. It is clear that events such as hurricanes 

dominate the hydrological year for the Río Piedras River. These 

events are shown for 1989, a hurricane year for which we have 

annual discharge data (fig. 15). Table 7 shows that the annual 

export of suspended sediments from the Río Piedras River is 

generally high, even in nonhurricane years, in comparison with 

other island rivers for which we have data.

Figure 21. Long-term fecal bacteria concentrations in waters of the U.S. Geological Survey Río Piedras Gaging Station. Notice the logarithmic 

Y-scale. The horizontal red line is the sanitary quality goal for class SD waters (Larsen and Webb 2009). Waters classified as SD are waters with 

unacceptable conditions.

Figure 22. Monthly suspended sediment concentration (circles) and suspended sediment export (triangles) in two U.S. Geological Survey gaging 

stations in the Río Piedras River (number of stations in parentheses). The stations are Río Piedras at El Señorial (a) and Río Piedras at Río 

Piedras (b). Monthly data begin on April of each year. To convert export to megagram per day, multiply tons per day by 0.9072.

Ortiz Zayas et al. (2006) found that nitrogen (N) export by 

the Río Piedras River was 32.9 kg/ha.yr (SE = 3.0), the highest among urban and agricultural watersheds they studied in 

Puerto Rico. Annual runoff explained most of the variation in 

fluvial N yield. Nitrogen concentration in the Río Piedras River 

was not sensitive to streamflow and averaged 2.7 mgN/L (SD = 

1.97). Larsen and Webb (2009) plotted the available temporal 

record of N and P concentration data for USGS Río Piedras at 

Hato Rey Gaging Station (50049100) and found that these Nrich discharges reach export values of about 140 kg/ha.yr. They 

surmised that urban centers generate nitrogen oxide NOx and 

nitrous oxide (N2O) that contribute to greater N deposition on 

natural forests on the island.

The Río Piedras River also carries large sediment loads, as 

illustrated in fig. 22 and photo 2. At the upstream USGS Río 

Piedras at El Señorial Gaging Station (50048770) (fig. 22a), 

suspended sediment concentrations and daily sediment discharge are lower than at the Río Piedras USGS Gaging Station 

(50049000) (fig. 22b). In both stations, however, peak water 

22a. 22b.

10,000,000

1,000,000

100,000

10,000

1,000

100

0

Fecal concentration, in colonies per 100mL

Station 50049100 - Río Piedras

8/29/19898/29/19908/29/19918/28/19928/29/19938/29/19948/29/19958/28/19968/29/19978/29/19988/29/19998/28/20008/29/2001

Geometric Mean of Five Sequential Fecal Streptococcus Samples

Measured Concentrations of Individual Fecal Streptococcus Samples

Geometric Mean of Five Sequential Fecal Colliform Samples

Measured Concentrations of Individual Fecal Colliform Samples

Puerto Rico Sanitary Quality Goal for Class SD Waters 

(Junta de Calidad Ambiental de Puerto Rico, 1990)
The Río Piedras Watershed and Its Surrounding Environment 19

Guevara González (1996) related the production of sediments in the Río Piedras Watershed with the construction 

activity in the basin. He noted that rapid construction of 

housing units between 1970 and 1990, coupled with poor 

control of soil erosion and sediment retention at construction 

sites, contributed to high rates of soil losses. For example, the 

number of housing units in 1970, 1980, and 1990 were 134,000, 

156,000, and 167,000 units, respectively. He noted that, during 

the years from 1987 to 1993, soil losses in the watershed were 

as extreme as 12,254 tons/mi2.yr (4,295 Mg/km2.yr) in 1988 and 

27,406 tons/mi2.yr (9,606 Mg/km2.yr) in 1989. Hall and Taylor 

(1997) developed a nonpoint model to estimate the sediment 

and nutrient discharges of the Río Piedras River. Their report, 

which covers the years 1989 to 1994, used sediment yields to estimate the average N and P yields for the river at the Hato Rey, 

Río Piedras, and El Señorial USGS gaging stations. The range 

of values for the Hato Rey Gaging Station was 427 to 760 lbs/

day (194 to 345 kg/day), and 851 to 1,518 lbs/day (386 to 689 kg/

day) for N and P, respectively.

Urban-influenced discharges to the San Juan Bay and associated estuaries and lagoons have an effect on the surface water and 

sediment quality (Webb and Gómez Gómez 1998). For example, 

ammonia-nitrogen (N-NH3) concentration at the Caño Martín Peña averaged 2.3 mg/L and fecal-coliform bacterial counts 

ranged from 270,000 to 610,000 colonies/100 ml between 1994 

Photo 2. The Río Piedras River at flood stage with its typical chocolate-colored waters due to the sediment load it carries. Photo by A.E. Lugo.

Table 7. Suspended sediment export for the Río Piedras River 

at Río Piedras. 

Water year Mg/km2.yr Mg/yr

1988 5,486 268,814

1989 9,600 470,400

1990 1,669 81,781

1991 1,705 83,545

1992 2,398 117,502

1993 2,425 118,825

Notes: Data are from the USGS Gaging Station 50049000 at Río Piedras. 

Seven rivers reported by Lugo et al. (1980) averaged 2,158 with a 

maximum and minimum of 5,150 and 511 Mg/km2.yr, respectively.

and 1995. This station was the most polluted station in the San 

Juan Bay estuary. Sedimentation rates ranged from 0.2 cm/yr in 

the Piñones Lagoon to 0.61 in San Juan Bay and 3.9 cm/yr in the 

Caño Martín Peña. During storm events, these rates potentially 

reach 10 cm/yr in shallow San Juan Bay waters.

Webb and Gómez Gómez (1998) also described temporal 

trends in surface water quality in the Río Piedras River and 

San Juan Bay estuarine waters. Overall, the water quality in the 
20 The Río Piedras Watershed and Its Surrounding Environment

In Relation to Ground Water

Ground water becomes progressively mineralized from the 

mountain region to the coast where both limestone and sea water 

contribute to the mineralization (Anderson 1976). The Aguada 

Limestone waters have good quality, an alkaline pH of 7.7 to 8.3, 

moderate dissolved solids concentrations of 293 to 4,550 mg/L, 

and are very hard (110 to 1,100 mg/L). Ground water from the 

Cibao and San Sebastián Formations are less mineralized. In 

San Juan, ground water is used for public consumption (table 1) 

mostly during emergencies or extreme drought.

Aquatic Organisms 

and Other Wildlife

Despite the high level of water pollution and human activity, the 

Río Piedras River and its riparian zone support diverse levels of 

biological richness. For example, the richness of aquatic organisms living in the Río Piedras River is surprisingly high (table 8). 

The river recently became front-page news in Puerto Rico when 

Omar Pérez Reyes reported a new record for a shrimp species for 

this watershed and for Puerto Rico (fig. 23). The small shrimp 

Palaemon pandaliformis was found to be common in the river, 

but despite its abundance it had escaped scientific discovery for 

almost a century. He also found a freshwater sponge in the Río 

Piedras River, Spongilla alba, the first time a freshwater sponge is 

reported for Puerto Rico. Further study revealed two species of 

freshwater sponges in the samples.

Kwak et al. (2007) found 10 freshwater fish species in one 

reach of the Río Piedras, 6 of which were native (table 9). 

Native fish species are all migratory (diadromous). Table 8 has 

12 fish species reported for the river. Although the native fish 

species were more abundant than ones introduced in the study 

by Kwak et al. (2007), the introduced species accumulated as 

much biomass as did the native species. Despite the differences, 

native fish in the Río Piedras River were found to be lacking 

in external abnormalities such as deformities, lesions, ulcers, or 

tumors, while introduced fish did exhibit these deformities including tumors in anal fins (N. Martínez Rivera et al. 2010). N. 

Martínez Rivera and A. Ramírez also found large numbers of 

macro consumers in the Río Piedras River, but they could not 

find a significant role for these organisms (relative to controls) 

in the breakdown of leaf litter. Apparently abiotic controls in 

lowland urban rivers play more important roles in litter breakdown then they do in montane streams. Their study showed 

that water temperature in the Río Piedras River ranged between 

24 and 29 °C, while in nonurban streams it ranged from 22 to 

25 °C. Such temperature differences influence microbial respiration and could accelerate processes in stream waters.

Marine fish are not included in the species list in tables 8 and 

9. They enter the estuary at the point of contact between fresh 

and seawater. There, in places like the Caño Martín Peña where 

Figure 23. Cartoon of the shrimp Palaemon pandaliformis, which 

appeared in the editorial page (p. 58) of El Vocero on November 26, 

2008, when the species was rediscovered in the Río Piedras River. 

Cartoonist is Yépez. 2008. 

region has improved since the 1970s in most stations, but some 

parameters reflected a reduction in water quality. For example, 

the total P and Lead concentrations and dissolved oxygen 

saturation in the upper reaches of the Río Piedras River (USGS 

Hato Rey Gaging Station 50049100) decreased, while pH and 

N concentrations showed no trend. The sediments of Caño 

Martín Peña had the following trends, suggesting a reduction 

in DDT (dichloro-diphenyl-trichloroethane) and accumulation of heavy metals and other chemicals used heavily in the 

modern city:

Parameter

Pre 

1950

1950 

to 1974

1975 

to 1995

PCB 38 μg/kg 12 μg/kg

DDT 62 μg/kg 27 μg/kg

Lead 0 μg/g 750 μg/g

Hg 0.16 μg/g 4.7 μg/g

Bis (2-ethylhext) phthalate, a common plasticizing agent, 

exceeded concentrations of 20,000 μ/kg and, together with the 

mercury (Hg) and Polychlorinated biphenyl (PBC) concentrations exceeded values considered hazardous to humans (Webb 

and Gómez Gómez 1998).
The Río Piedras Watershed and Its Surrounding Environment 21

Table 8. Species list for aquatic organisms found in the Río Piedras River. 

Native Fish

Anguilla rostrata, Anguillidae (American eel, Anguila)

Agonostomus monticola, Mugilidae (Mountain mullet, Lisa de río, Dajao)

Awaous banana, Gobiidae (River goby, Saga, Olivo)

Eleotris perniger, Eleotridae (Smallscale spinycheek sleeper, Morón)

Gobiomorus dormitor, Eleotridae (Big mouth sleeper, Guavina)

Introduced Fish

Tilapia sp. Cichlidae (Tilapias)

Oreochromis mossambicus, Chichlidae (Mozambique tilapia, Tilapia mosambica)

Pterygoplicthys pardalis, Loricariidae (Sailfin catfish, Amazon or Armored catfish, Chupapiedras, Pleco, 

 Corroncho de América del Sur)

Gambusia affinis, Poeciliidae (Mosquito fish)

Poecilia reticulata, Poeciliidae (Guppy)

Xiphophorus hellerii, Poeciliidae (Green swordtail, Pez de cola de espada)

Sicydium plumieri, Gobiidae (Sirajo goby, Olivo, Chupapiedra)

Aquatic Insects

Order: Ephemeroptera Families: Leptophlebidae, Baetidae, Caenidae

Order: Odonata Families: Libellulidae, Coenagrionidae

Order: Hemiptera Family: Veliidae

Order: Trichoptera Families: Hydroptilidae, Philopotamidae

Order: Coleoptera Family: Elmidae

Order: Diptera Families: Chironomidae, Simuliidae, Tipulidae

Arthropoda-Crustacea Shrimp (Decapoda)

Atya lanipes

Xyphocaris elongata (Salpiches or Piquines)

Macrobrachium faustinum (Camarón zurdo)

M. carcinus1 (Camarón bocu)

M. crenulatum

Palaemon pandaliformis

Atya innocuous1 (Gata, Guabara, Chagara)

A. scabra1 (Gata, Guabara, Chagara)

Epilobocera sinuatifrons (Buruquena)

Other Aquatic Invertebrates

Mollusca-Gastropoda Marisa cornuarietis

 Tarebia granifera

Platyhelminthes-Turbellaria Dugesia antillarum (Planaria de Puerto Rico)

Acari, Hydracarina, water mites (Acaros)

Annelida, Hirudinea, leeches (Sanguijuelas)

Clitellata, Tubificida and others, aquatic worms (Lombrices)

Hexapoda, Collembola, springtails (Colémbolos)

Crustacea, Peracarida, amphipods (Anfípodos)

Porifera, Spongilla alba, freshwater sponge

Reptilia

 Trachemys stejnegeri stejnegeri (Jicotea)

1 Based on biological collections.

Notes: This list is based on information provided by Alonso Ramírez (Tropical Limnology Laboratory of the University of Puerto Rico, Río Piedras) and Omar Pérez 

Reyes (Utah State University). Kwak et al. (2007) was used to supplement the fish list.
22 The Río Piedras Watershed and Its Surrounding Environment

the Río Piedras River discharges, sport and artisan fisherman 

harvest large tarpon and other marine fish species.

The riparian vegetation is also used by a large number of 

organisms. Over a few weeks of casual observations, S. Lugo et 

al. (2001) recorded 22 bird species, 3 species of mammal, 2 species of fish, 1 species of amphibian, 4 species of reptile, 1 species 

of snail, and 1 species of decapods (table 10). The section on 

Forests contains more information on birds.

Vegetation

The vegetation of the metropolitan area consists of forests (including salt and freshwater forested wetlands), pastures, freshwater herbaceous wetlands, riparian vegetation, and ephemeral 

and/or dispersed vegetation that emerges in abandoned lots and 

throughout the city. The urban forest was described as consisting of four types based on their presence on the landscape (Lugo 

2002). These types are: (1) a natural matrix of forests composed 

of closed mature forest stands that either escaped deforestation or were allowed to mature naturally after abandonment 

of agricultural or urban land cover; (2) natural corridors such 

as those along riparian zones; (3) green oases, which are small 

planted or naturally regenerating forest fragments scattered over 

the built-up urban landscape; and (4) artificial corridors such 

as those along streets and avenues. Examples of these stands 

exhibit different structural and compositional characteristics as 

will be illustrated below. The section on the land cover of the 

metropolitan area contains information on the area and types of 

vegetation in the metropolitan area.

Forests

Natural matrix of forests. Natural forest stands within the 

metropolitan area include the mangrove forests lining San 

Juan Bay and Lagoons, the karst forests on the tops and slopes 

of mogotes, and the tall evergreen forest within the Botanical 

Gardens and in the vicinity of the University of Puerto Rico, 

in the Commonwealth Forest of the New Millennium in Río 

Piedras. A large area of mangrove forests occurs in the Torrecilla, 

Piñones, and Vacía Talega sectors of the metropolitan area. The 

mangroves of this region are described elsewhere (Lugo and 

Cintrón 1975) and will not be addressed in this overview.

The USDA Forest Service inventoried the forests of the San 

Juan Bay Estuary Watershed using 108 systematically located 

points, where 6 plots covering 0.42 ha were sampled (Lugo 

and Brandeis 2005). They found high species dominance and a 

high proportion of introduced species in the urban forest. The 

three top-ranked species of urban forests had higher importance values than the corresponding species in dry, moist, and 

wet forests elsewhere in the island (fig. 24). Introduced species 

attain their highest importance and abundance in urban forests 

(fig. 24). For example, Albizia procera, an introduced species, 

was the second most important urban tree species that invades 

vacant barren and institutional lands with highly compacted 

soils (Chinea 2002). In contrast, endemic species attained their 

lower presence in urban forests compared to other forest types 

inventoried (table 11).

Table 9. Species richness, diversity (H’), density, and biomass of fishes in the Río Piedras River at the bridge off highway PR-177

Parameter Native fish Introduced fish All species

Richness 6 4 10

Diversity 1.27 1.69

Density (fish/ha, SE) 1,638 (106) 270 (15) 1,908 (107)

Biomass (kg/ha, SE) 56 (6) 57 (12) 113 (13)

Note: Data correspond to the spring of 2007 and SE is the standard error of the mean (n is unknown). Empty cell means data are not available.

Source: Kwak et al. (2007).

Figure 24. Importance Value curve for tree species from the San 

Juan urban forest and a combined data set for islandwide forests. 

The Importance Value is the sum of relative basal area and relative 

density of a species expressed in percent. Species are ranked from 

the highest to the lowest in Importance Value. The islandwide forest 

species list is truncated at the number of species in the urban 

forest. Data are from the USDA Forest Service Forest Inventory and 

Analysis program.
The Río Piedras Watershed and Its Surrounding Environment 23

Site Species Common name

RP1 Ardea herodias Great egret

Egretta thula Snowy egret

Butorides virescens Green heron

Buteo jamaicensis Red-tailed hawk

Rattus rattus Rat

Iguana iguana Green iguana

Trachemys stejnegeri Hicotea (water turtle)

Lepidodactylus albilabris White-lipped coqui (frog)

Anolis sp. Lizard

RP2 Zenaida aurita Zenaida dove

Z. asiatica White-winged dove

Butorides virescens Green heron

Margarops fuscatus Pearly-eyed thrasher

Sicydium plumieri Sirajo goby

Rattus rattus Rat

Tilapia mossambica Tilapia

Anolis sp. Lizard

Gastropoda Turret snail

RP3 Ardea herodias Great blue heron

Nycticorax nycticorax Black-crowned night heron

A. herodias Great egret

Tyrannus savana Grey kingbird

Tiaris bicolor Black-faced grassquit

Columbina passerina Ground dove

Tilapia mossambica Tilapia

Canis sp. Stray dogs

Anolis sp. Lizard

Site Species Common name

RP4 Ardea herodias Great egret

Egretta thula Snowy egret

Charadrius vociferus Kildeer

Tringa melanoleuca Greater yellowlegs

Lonchura punctulata Nutmeg mannikin

Tiaris bicolor Black-faced grassquit

Buteo jamaicensis Red-tailed hawk

A. herodias Tilapia

Actitis macularia Spotted sandpiper

Anolis sp. Lizard

Tilapia mossambica Tilapia

Trachemys stejnegeri Hicotea (water turtle)

Herpestes javanicus Mongoose

RP5 Ardea herodias Great egret

Egretta thula Snowy egret

E. tricolor Tricolored heron

Nycticorax nycticorax Black-crowned night heron

Crotophaga ani Smooth-billed ani

Tyrannus savana Grey kingbird

Zenaida aurita Zenaida dove

Rattus rattus Rat

Iguana iguana Iguana

Decapoda Crab

Source: S. Lugo et al. (2001).

Table 10. Common diurnal animal species along the Río Piedras River.

Table 11. Introduced and endemic tree species found in the inventory of the San Juan Bay Estuary Watershed. 

Parameter Urban forest Mature moist forest

Endemic species (% of total species) 0 10

Introduced species (% of total species) 47.9 6.3

IV of introduced species 76.5 41.4

Number of sapling species 20 28

Number of tree species 19 (76) 38

Total number of species 33 80*

IV = Importance Value.

* Includes seedlings (58 species)

Notes: Data for mature moist forest are included for comparison. The area sampled for urban and mature forests was 0.42 and 0.49 ha, respectively.

Source: Lugo and Brandeis (2005).
24 The Río Piedras Watershed and Its Surrounding Environment

of the Río Piedras Watershed. The establishment and growth of 

introduced species in novel forests is a natural process of forest 

regeneration that is also common in the rest of the island on 

abandoned agricultural lands (Lugo and Helmer 2004).

Albizia and Spathodea dominated the San Patricio stand. 

Other introduced species were present in these stands; however, 

some like Delonix regia were preexisting during the Naval Station’s operation (table 12). Physiognomically and structurally, 

little difference exists between the novel forest stand and the 

natural matrix that survived the deforestation of the mogote at 

San Patricio (table 13). The species density (richness) is higher 

at the top of the mogote, however, where only native species 

occur, than in the novel forest dominated by introduced species 

(tables 12 and 13). Similarly, Súarez et al. (2005) found that most 

of the regeneration in the San Patricio forest was attributed to 

native species, with very few introduced species showing regeneration in the most disturbed stands.

At San Patricio, the top of the mogote that constitutes most 

of the area of this public forest contains a full complement of 

native tree species forming a closed forest (elevations 56 and 92 

in tables 12 and 13) with no regeneration of introduced species (Súarez et al. 2005). The dominant species in the mogote 

top was Coccoloba diversifolia followed by Bucida buceras. The 

dominance of Coccoloba was high (Importance Value was 52 

percent), as is typical of other mogotes to the west of San Patricio (Aukema et al. 2007).

Mangroves and karst forests represent forest types on soils 

with severe limitations for plant growth (salinity and flooding in mangroves and shallow nutrient-poor soils in karst). 

Those on volcanic substrates, however, develop on deeper and 

nutrient-rich soils, with the consequence that forests are taller 

with a larger basal area. The Commonwealth Forest of the New 

Millennium in Río Piedras, a public forest, is an example. This 

forest developed naturally after the abandonment of agricultural activity in the rural zone of Río Piedras (compare images 

in photos 3 and 4). The changes in land use that led to the 

development of this forest are discussed below in the section 

on land cover. Despiau Batista (1997) and Lugo et al. (2005) 

studied the forest when it was 60 and 68 years old, respectively. 

Because the forest developed on agricultural soils and a moist 

climate, its growth was rapid and it is one of our best examples 

of what forests on the lowlands looked like before these lands 

were deforested for agricultural use.

The species with the highest Importance Value in this forest was Spathodea campanulata. This species was dominant in 

most topographic positions and in two instances was second 

to the natives Tabebuia heterophylla and Hymenaea courbaril

(on slopes). Other species with high Importance Values in this 

forest were Hura crepitans, Terminalia catappa, and Guarea guidonia. Overall the forest had 53 to 55 species per hectare, a value 

comparable to most mature forests in Puerto Rico (Lugo 2005). 

The species density changed with topographic position, with 

the highest values on slopes (table 14). The highest basal areas 

were in river valleys, where the Importance Value of the most 

dominant species was high and tree density was low.

Table 12. Distribution of tree species along an elevation 

gradient in the San Patricio mogote in Guaynabo. 

Species

Elevation (m)

32 26 31 56 92

Tabebuia heterophylla 9

Roystonea borinquena 1

Calophyllum calaba  1

Terminalia catappa* 12

Delonix regia* 10 59 25

Albizia procera* 26 10 41

Spathodea campalulata* 30 18 1

Citharexylum fructicosum 9 5 4 9

Eugenia monticola 1 2 2

Casearia guianensi 1

Malpighia infestissima 1

Bourreria succulenta  4 1 2

Carapa guianensis* 1 1 1

Coccoloba diversifolia 1 32 52

Leucaena leucocephala 28

E. rondicollia 8  3 

Eucalyptus robusta* 2

E. lancea 3

Exostema caribaeum 5

Guapira fragans 3

Guatterria caribaea 4

Krugiodendron ferreum 5

Maitenus eliptica 1

Ficus laevigata 2 14

Bursera simaruba 6 9

Bucida buceras 16 12

Sideroxylon foetidissimu 1

S. salicifolium 2

Adenanthera pavonina* 4

* Identifies introduced species.

Note: Numbers correspond to the Importance Value of the species.

Source: Suárez et al. (2005).

The prevalence of introduced species in the urban forest 

is the result of two processes: tree planting (discussed below 

under green oases and artificial corridors) and natural regeneration. The natural regeneration of species like Albizia procera, 

Spathodea campanulata, Melaleuca quinquenervia, and others, is 

resulting in the formation of novel forest types within the city. 

Notable examples of novel natural forests dominated by introduced species is a stand in the San Patricio forest growing after 

the abandonment and demolition of houses of a Naval Station, 

the Melaleuca stand in the San Juan Bay Estuary, and the 

Commonwealth Forest of the New Millennium in the center 
The Río Piedras Watershed and Its Surrounding Environment 25

Table 13. Structural indices for individual trees and plots along an elevation gradient in the San Patricio mogote in Guaynabo. 

Elevation (m)

Parameter 32 26 31 56 92

Trees 

> 5 cm dbh

Average diameter (cm) 11.2 13.6 10.1 12.7 12.8

Average height (m) 8.1 8.6 8.5 9.6 9.0

Density (trees/ha) 1,422 1,395 1,763 1,763 1,790

Basal area (m2/ha) 18.4 38.6 20.8 29.1 30.8

Number of species 10 9 6 16 10

Trees 

> 2.5 < 5 cm 

dbh

Average diameter (cm) 3.2 4.0 4.1 0.9 3.5

Average height (m) 3.1 2.3 4.8 1.1 5.8

Density (trees/ha) 3,567 510 3,568 3,057 1,529

Basal area (m2/ha) 3.0 0.6 0.7 2.5 0.5

Number of species 3 1 2 4 2

dbh = diameter at breast height.

Note: The sampling area was 380 y 19.63 m2, respectively, for tree with a dbh of > 5 cm and > 2.5 < 5.0 cm.

Source: Súarez et al. (2005).

Table 14. Stem density, basal area, number of species, and Importance Value of the most dominant tree species in 1997 and 2005 at the 

Commonwealth Forest of the New Millennium in Río Piedras. 

Topographic 

position

Density

(stems/ha)

Basal area

(m2/ha) Species/plot Number of species

Importance 

Value (%)

Year 1997 2005 1997 2005 1997 2005 1997 2005 1997 2005

Riverine valley 1,316 1,246 38.1 38.1 6.8 7.1 26 26 44.4 38.3

Slope 2,208 1,819 33.4 26.8 10.6 10.9 34 28 28.6 14.2

Ridge 2,330 1,961 25.8 26.8 8.5 9.3 32 33 25.9 21.8

Draw 1,393 1,362 27.4 25.1 7.8 7.4 29 25 37.2 38.9

All forest 1,808 1,562 30.8 28.6 — — 56 53 32.9 26.7

Notes: Data are for trees with a diameter at breast height > 4 cm. The sampling area in ha was 0.2545, 0.2290, 0.2545, 0.2799 y 1.0 for riverine valley, slope, 

ridge, draw, and all forest, respectively. Each plot had an area of 254 m2. 

Source: Lugo et al. (2005).

Photo 3. Aerial photograph of the Río Piedras region in 1931. Photo 

by Philadelphia Aerial Surveying. Sponsored by the U.S. Department 

of Defense.

Photo 4. Aerial photograph of the Río Piedras region in 2006. 

Photo by Puerto Rico Department of Natural Resources and the 

Environment.
26 The Río Piedras Watershed and Its Surrounding Environment

Figure 25. Tree basal area (a) and height (b) as a function of canopy position in the Commonwealth Forest of the New Millennium in Río Piedras. 

Crown positions are dominant (D), codominant (CD), intermediate (I), and suppressed (S), which receive full sunlight on all sides, from the top 

only, through gaps, or not at all, respectively. Mean and 95 percent confidence level are shown (Lugo et al. 2005).

25a. 25b.

Tree dimensions varied with the position of the crown 

(fig. 25). The largest trees were those with their canopy exposed 

to full sunlight on all sides, identified as dominant crowns in 

fig. 25. These large trees were followed by codominant canopies 

with full sun exposure only from the top, intermediate canopies 

shaded by dominant and codominant crowns, and suppressed 

canopies, which were exposed to shade all the time. Trees with 

suppressed canopies had the smallest dimensions. Tree growth 

was faster in the dominant canopy class, particularly in the 

riverine valley and least in suppressed canopy trees, regardless of 

topographic position (table 15).

Hurricane Georges affected the forest in 1998 between 

the study of Despiau Batista (1997) and Lugo et al. (2005). 

The effects of the hurricane were slight given the distance to 

the center of the hurricane (Lugo et al. 2005), but they were 

most significant on ridges, which had trees exposed to winds 

(fig. 26). There, tree mortality exceeded tree ingrowth, but in 

other topographic positions the two processes were balanced. 

Another measure of hurricane effects was tree growth, particularly height growth (tables 15 and 16). The hurricane reduced 

the height of the forest, and trees exhibited negative height 

growth. Consult Lugo et al. (2005) for a detailed analysis of 

hurricane effects. A critical finding was that the species composition, including the dominance of the introduced species S. 

campanulata, did not change significantly after the hurricane. 

This finding suggests that the introduced species can survive 

moderate hurricane winds and maintain dominance in these 

novel forests.

Natural corridors. S. Lugo et al. (2001, p. 74) discussed the 

riparian vegetations of the Río Piedras River thusly:

The vegetation along the Río Piedras tends to be patchy. 

Within an urban environment, land use can change in 

very short distances. With these changes, the riparian 

vegetation also changes, which may contribute to the high 

species diversity along the river. Such diversity of habitats 

provides a wide range of niches, which allow for a large 

number of species. When undisturbed, the riparian zone 

may become a forested habitat. Although a few patches 

of forest habitat were found at most sites, this was not 

possible because the urban environment influenced and 

disturbed the riparian zone. The average distance to the 

first urban structure (fences, yards, roads, sidewalks, etc.) 

was only 20 m from the edge of the channel. The diversity 

of landscapes along this urban river is due to the different 

stages of regrowth after constant disturbance, different 

land uses, and topography. In an urban habitat, change is 

inevitable, always altering this dynamic community. The 

species found in this habitat are well adapted to, and thrive 

in this ever-changing environment.

Figure 26. Tree mortality and ingrowth according to topographic 

position in the Commonwealth Forest of the New Millennium in Río 

Piedras 7 years after Hurricane Georges passed south of the forest. 

Mean and standard errors are shown (n = 10, except for draw = 9) 

(Lugo et al. 2005).
The Río Piedras Watershed and Its Surrounding Environment 27

Table 15. Tree growth rate in the Commonwealth Forest of the New Millennium in Río Piedras. 

Annual growth

Analysis criteria Diameter (cm) Basal area (cm2) Height (m)

Topographic 

position

River valley 0.41 (0.04, 232) 14.3 (2.8, 232) – 0.26 (0.03, 230)

Slope 0.20 (0.02, 318) 5.0 (0.8, 318) – 0.05 (0.02, 308)

Ridge 0.24 (0.02, 352) 5.4 (0.6, 352) – 0.12 (0.02, 351)

Draw 0.25 (0.02, 237) 6.3 (0.8, 237) – 0.21 (0.03, 236)

Crown 

position

Dominant 0.48 (0.12, 60) 29.0 (9.1, 60) – 0.43 (0.08, 58)

Codominant 0.36 (0.05, 292) 16.5 (2.9, 67) – 0.35 (0.08, 64)

Intermediate 0.30 (0.02, 712) 8.3 (1.0, 292) – 0.21 (0.03, 289)

Suppressed 0.22 (0.01, 712) 3.9 (0.4, 712) – 0.08 (0.01, 712)

By 

plot

Plots 0.30 (0.02, 39) 8.4 (1.0, 39) – 0.15 (0.03, 39)

All 

trees

All trees 0.27 (0.01, 1,139) 7.3 (0.7, 1,139) – 0.15 (0.01, 1,125)

Notes: Results are given for different analysis criteria. The standard error followed by the number of samples is given in parentheses.

Source: Lugo et al. (2005).

Table 16. Mean, median, and mode of tree dbh, BA, height, and their respective annual rates of change and descriptive statistics. 

Statistical parameter

Tree dimensions Growth

dbh

(cm)

BA

(cm2)

Height

(m) dbh

(cm)

BA

(cm2)

Height

1997 2005 1997 2005 1997 2005 (m)

Mean 11.4 13.6 176.5 234.8 9.2 8.1 0.27 7.3 – 0.15

Standard error 0.3 0.3 11.4 13.8 0.1 0.1 0.01 0.7 0.01

Median 7.7 9.7 46.5 73.9 8 7 0.14 1.8 – 0.12

Mode 4.9 6.3 13.8 31.2 7 7 0 0 – 0.13

Range 78 81 5,305 5,659 29 20 8.2 590 4

Sample size (n) 1,139 1,139 1,139 1,139 1,131 1,130 1,139 1,139 1,125

Highest value 82.3 85 5,317 5,672 30.7 22 6.6 527 2.5

Lowest value 4 4 12.6 12.6 1.7 2 – 1.6 – 63.0 – 1.9

Coefficient of variation (%) 84 79 218 198 47 40 165 306 257

Confidence level (95.0%) 0.56 0.62 22.3 27.0 0.3 0.19 0.03 1.3 0.02

BA = basal area. dbh = diameter at breast height.

Notes: Data are for live trees in the Commonwealth Forest of the New Millennium in Río Piedras. Some values were rounded. 

Source: Lugo et al. (2005).

Their study led them to six generalizations about these 

plant communities. (1) The dominance of grasses in the 

riparian zone of the river is associated with frequent flooding. 

(2) Most riparian areas had little to no tree cover due to flooding and human activities. (3) Community structure includes 

high species dominance in both tree and nontree communities. (4) Geometric rank-abundance curves were found for 

tree species, while nontree species followed a lognormal curve 

(fig. 27). (5) Species richness (table 17, fig. 28) and species 

diversity of the nontree component were high and exceeded 

those of the tree component. (6) Most species found in the 

riparian forest are nonnative and/or invasive. The introduced 

species all had some connection with human use such as 

food, ornamental, or timber species. Tree height was variable among sites, but average between 6 and 7 m at the best 

locations (fig. 29).
28 The Río Piedras Watershed and Its Surrounding Environment

Figure 27. Log-transformed Importance Value curves for riparian 

vegetation in the Río Piedras River (S. Lugo et al. 2001). The tree 

curve is logarithmic, while the grass curve is lognormal.

Figure 28. Species/Area curve for tree and nontree riparian 

vegetation in the Río Piedras River (S. Lugo et al. 2001). A wet 

karst forest known for its high tree species richness is shown for 

comparison.

Figure 29. Mean tree height in four riparian-forest stands in the Río 

Piedras River (new plot based on the study of S. Lugo et al. 2001). 

Mean and standard errors are shown; n = 42, 69, 18, and 10, 

respectively, for sites RP 1 to RP 4.

Table 17. Number of tree and nontree species in five sites along the Río Piedras River.

Site Area (m2) Tree species Nontree species Land use

RP1 750 5 35 Undeveloped, one riverbank flood plain

RP2 648 10 48 Undeveloped, some construction near

RP3 900 8 34 Neighborhood, groomed vegetation

RP4 900 1 28 City park

RP5 450 1 28 Roadside

All sites 3,648 14 92

Note: The sampling area and land use at the site are also shown.

Source: S. Lugo et al. (2001). 

The grassy riparian vegetation was not only diverse in species but also tall, with average heights of 2 to 3 m. The most 

abundant grass was Paspalum paniculatum, a native grass. In 

the forest stands, the most abundant tree was Albizia procera, a 

nitrogen-fixing tree capable of growing on compacted soils and 

competing with the grasses. Few tree species grew at 0 to 5 m 

from the riverbank (table 17). Only three species in this region 

were found close to the riverbank (S. Lugo et al. 2001): Musa 

sp., Terminalia catappa, and Ricinus communis. All are fastgrowing, flexible species and one (Musa) is an herb with a tree 

habit. Most of the trees (49 percent) grew between 10 and 20 m 

from the water, while 22 percent of the trees grew between 

20 and 30 m from the water’s edge (table 18).

Flooding frequency appears to be the main driver of vegetation distribution in the Río Piedras riparian zone (S. Lugo et 

al. 2001). The grass-dominated zone is frequently flooded, and 

most trees grow in higher topographic positions away from 

these flood-risk zones. Because of the steepness of the slopes 

next to the river, this reduces the flooding frequency and trees 

can grow closer to the water at higher elevations in the watershed. In the Caimito region, with steep topography, a forest 

of Cecropia schreberiana, a native pioneer species, grows on the 
The Río Piedras Watershed and Its Surrounding Environment 29

Photo 5. Development of forest fragments in abandoned lots of 

Río Piedras, Puerto Rico. Construction in the lot was halted due to 

violations of zoning regulations in 2006 (a). Since then, vegetation 

removal has taken place periodically, but, in 2010, Albizia procera

trees can be seen growing among the weeds (b and c). If left 

unchecked, they will form a tall forest despite site modification. 

Photos by A.E. Lugo. 

riparian zone, and few grasses are present. The domination of 

grasses occurs downstream over low-elevation flood plains. This 

habitat was at one time dominated by Pterocarpus officinale, 

which formed floodplain forests, but this species has almost 

been completely extirpated from the flood plain of the Río 

Piedras River. The largest tree measured by Lugo et al. (2001) 

was a 19 m tall Pterocarpus macrocarpus, an introduced species 

not known to grow well in saturated soils (Francis 2000).

Green oases. Forest fragments, or patches, are abundant in the 

metropolitan area, many times in abandoned lots (photo 5a–c). 

These urban forests, which are usually embedded in the urban 

built-up land, moderate air temperature, provide substantial 

recreational opportunities, attract significant numbers and 

variety of wildlife, and influence the hydrological cycle through 

evapotranspiration and reduction of urban runoff.

An example of this type of forest is a 1.1-ha forest planted 

in 1988 by a neighborhood association at Urbanización El 

Paraíso in Río Piedras (Román Nunci et al. 2005). By 2007, the 

forest had 37 tree species (9 native and 28 introduced) including two endemic species, species representative of primary 

forests from dry, moist, and wet forests (table 19). The species 

combination of these urban forests reflects the preferences of 

those who planted the trees, and this explains the novel aspect 

of the species list. One species, Cecropia schreberiana, regenerated naturally after the flowering of the individual that was 

planted at the site. Also, because of the heavy use by residents 

and safety concerns, the understory of this forest is poorly 

developed and mostly absent, however, the forest has a closed 

canopy. Litterfall accumulation on the forest floor is also low, 

as the government periodically removes leaves and dead wood. 

The respective mean diameter at breast height (dbh) and height 

of trees are 26.8 cm and 9.8 m. The forest as a whole had a basal 

area of 17.3 m2

/ha and a tree density of 127 trees/ha (215 stems/

ha due to multiple stems in some trees). Román Nunci et al. 

(2005) found that about one-half of the trees were growing very 

slowly but the remaining trees grew at better than a 20 cm2/yr 

clip, including 15 percent that grew at better than 100 cm2/yr 

(fig. 30). Annual tree mortality was 1.4 to 1.5 percent.

Table 18. Characteristics of tree population in relation to 

distance from the river at five sites along the Río Piedras River.

Distance

(m)

Basal area 

(m2/ha)

Species

(no)

 Mean height 

(m)

Density 

(trees/ha)

0 to 5 1.37 3 3.2 246

5 to 10 3.03 7 4.9 315

10 to 15 6.00 6 4.8 578

15 to 20 8.00 6 5.6 458

20 to 25 0.77 3 5.5 104

25 to 30 16.00 6 8.1 383

Source: S. Lugo et al. (2001).

5a.

5b.

5c.
30 The Río Piedras Watershed and Its Surrounding Environment

Table 19. Tree species composition, structural attributes, and Importance Value in Parque Central of the El Paraíso urbanization 

in Río Piedras. 

Species

dbh Height Density BA IV

(cm) (m) (stems/ha) (trees/ha) (m2/ha) (%)

Swietenia macrophylla x mahagoni 46.1 11.8 27 24 5.1 24.3

Peltophorum pterocarpum 31.6 12.6 19 6 1.9 7.7

Ficus benjamina 91.6 10.3 3 2 2.3 7.4

Melaleuca quinquenervia 17.2 8.7 24 12 0.7 6.7

Tabebuia heterophylla* 21.1 9.2 19 11 0.7 6.4

Lagerstroemia speciosa 25.0 8.4 19 6 1.1 5.3

Cordia sebestens 16.2 5.8 9 9 0.2 4.2

Terminalia catappa 23.1 8.4 7 7 0.3 3.9

Swietenia mahagoni 37.0 12.2 6 5 0.6 3.6

Tabebuia rosea 35.3 13.0 6 5 0.6 3.6

Roystonea borinquena* 4.5 10.3 4 4 0.5 3.0

Callistemon citrinus 17.0 7.5 11 5 0.3 2.7

Swietenia macrophylla 31.1 11.9 5 4 0.4 2.6

Delonix regia 36.1 10.5 6 2 0.6 2.5

Eucalyptus robusta 28.5 14.1 5 3 0.3 2.1

Pterocarpus indicus 70.0 12.0 1 1 0.4 1.4

Cassia fistula 18.5 12.7 6 2 0.2 1.2

Thespesia grandiflora* 18.5 11.6 5 2 0.1 1.1

Ceiba pentandra* 48.0 10.0 1 1 0.2 0.9

Cecropia schreberiana* 13.5 8.0 2 2 0.0 0.8

Schefflera actinophylla 12.4 6.5 2 2 0.0 0.8

Tamarindus indica 16.8 8.5 5 1 0.1 0.7

Cassia javanica 18.2 11.6 4 1 0.1 0.6

Cananga odorata 29.2 10.5 1 1 0.1 0.5

Chrysophyllum cainito 12.9 7.0 4 1 0.1 0.5

Tabebuia capitata 11.6 7.5 5 1 0.1 0.5

Syzygium malaccense 25.5 11.0 1 1 0.0 0.5

Mangifera indica 25.2 8.0 1 1 0.0 0.5

Melicoccus bijugatus 23.8 8.0 1 1 0.0 0.5

Tabebuia aurea 23.2 12.0 1 1 0.0 0.5

Petitia domingensis* 22.7 11.0 1 1 0.0 0.5

Grevillea robusta 19.8 7.5 1 1 0.0 0.4

Pimenta racemosa* 9.8 8.0 3 1 0.0 0.4

Cocos nucifera 16.5 5.0 1 1 0.0 0.4

Crescentia cujete 12.1 6.0 1 1 0.0 0.4

Guaiacum officinale* 7.3 3.0 2 1 0.0 0.4

Manilkara bidentata* 9.6 4.4 1 1 0.0 0.4

* Indicates native species; the rest are introduced.

BA = basal area. dbh = diameter at breast height for all trees with dbh > 4 cm. IV = Importance Value. 

Note: Species are arranged in the order of IV.

Source: Román Nunci et al. (2005). 
The Río Piedras Watershed and Its Surrounding Environment 31

Another example of a green oasis is the urban forest in the 

campus of the University of Puerto Rico at Río Piedras. This 

campus was established in 1903 and contains an approximate 

area of 279 cuerdas (110 ha) and a large diversity of planted trees 

and lawns. Otero Vázquez (1995) established a route through 

campus and systematically observed birds twice a week between 

6:30 and 7:30 a.m. between 1987 and the 1990s. His paper contains a list of 47 bird species, including 8 endemic species. In 

addition to the endemic species, his list includes resident species known to reproduce or not known to reproduce in Puerto 

Rico, introduced species, naturalized species, migratory species, 

migratory species that reproduce in Puerto Rico, and resident 

and migratory species. This list is a conservative estimate of 

the number of avian species in this urban forest, because he 

included only those species that he observed more than once 

and ignored reports of other species present in campus.

Artificial corridors. The streets and avenues of San Juan 

are lined with trees of many species, including native and 

introduced species. These artificial corridors are known to 

attract wildlife and they are valued in the city for the shade 

they provide (photo 6). The management of these corridors 

Figure 30. Histogram of basal area tree growth rates for trees in 

the Parque Central at the El Paraíso urbanization, Río Piedras. The 

2001-to-2005 values are based on 155 individual rates; the 2005-

to-2007 values are based on 160 individual rates (Román Nunci et 

al. 2005).

Photo 6. A forested urban corridor at San Patricio, Río Piedras, Puerto Rico. The pavement in the photo is an abandoned street of a former 

Naval Station, now an urban Commonwealth forest. Photo by A.E. Lugo.
32 The Río Piedras Watershed and Its Surrounding Environment

is problematic because little attention is paid to the species to 

be planted and less so to the space needed for the tree to grow 

its root system and canopy. The result involves many types of 

conflict, such as the following:

✦ Trees overtopping power lines and then interrupting service 

during periods of high winds.

✦ Tree roots breaking sidewalks, cracking building walls, and 

clogging drainage systems and water lines.

✦ Trees growing to excessive heights and then causing hazards 

to vehicles and pedestrians due to falling limbs.

In the absence of professional management, these problems 

are addressed without proper plans or technical knowledge, 

which explains the decapitation and injury of trees (photo 7a–

c), with the resulting loss in city aesthetics and public conflict 

by those who disapprove of any cutting of trees in the city. The 

artificial corridors present the greatest vegetation management 

challenge to municipal and State governments.

Wetlands

Wetlands, other than forested wetlands discussed above, are 

common in the urban setting of Río Piedras. Some wetlands 

emerge as a result of broken pipes or accumulation of water 

due to changes in drainage patterns throughout the city (photo 

8a-c). One of the largest remaining natural wetland areas in the 

metropolitan area is Ciénaga Las Cucharillas, a large swamp 

dominated by freshwater herbaceous vegetation with eurihaline 

transition to mangroves and saline vegetation (fig. 1). This large 

swampy area is a remnant of what would have been a more 

extensive wetland region behind the mangroves in the lowlands 

of the Río Piedras River at its confluence with San Juan Bay.

Photo 7. Examples of excessive pruning of trees along urban corridors in Río Piedras, Puerto Rico. Pruning is often done to protect power lines 

(a and b) but sometimes to convert large trees into shrubs (c). Photos by A.E. Lugo.

7a.

7b.

7c.
The Río Piedras Watershed and Its Surrounding Environment 33

Photo 8. Closeup of an herbaceous wetland in Río Piedras, Puerto Rico (a). The plants are reproducing. A wider view of the wetland uncovers its 

location along a street (b). The water supply for this wetland is from broken water pipes due to construction (c). Photos by A.E. Lugo.

Human Activity

Río Piedras appeared depicted for the first time as a settlement 

in a map of Puerto Rico prepared by Miguel de Muesas in 1775 

(Sepúlveda Rivera 2004a). In 1777, Río Piedras had three houses 

and a population of 15 people. The town had been founded in 

1714. In 1828, Pedro Tomás de Córdova reported that Río Piedras 

had 41 artisans, 77 naturalized aliens, 16 pulperias and ventorrillos, and more than 100 houses (Sepúlveda Rivera 2004a). The 

parroquia San Pilar was built in 1714. This church was the main 

structure in the first urban map of Río Piedras reproduced in 

Sepúlveda Rivera (2004 b). A more elaborate expansion plan for 

the town, including streets, appeared in 1880 (fig. 31).

By 1878, Manuel Úbeda y Delgado reported the following: 

an agricultural richness of 40,677 pesos and 20 cents, 12,092.20 

in urban, and 6,823.20 in livestock; 7 schools; 9,612 people 

belonging to 1,660 families; 373 houses, and 1,275 ranchos

and bohíos. In the town itself were 140 houses, 50 bohíos, 190 

families, 3 general stores, 2 fondas, a café, a botica, and 14 pulperías. The main transportation venues were to San Juan (11 km) 

and Caguas (24 km), secondary roads to Carolina (11 km), and 

one in construction to Trujillo Alto (7 km). The Río Piedras 

aqueduct was built by 1890, as was the bridge over the river 

on the Caguas road (Sepúlveda Rivera 2004c). That bridge 

8a.

8b. 8c.
34 The Río Piedras Watershed and Its Surrounding Environment

was completed in 1853 (Pumarada O’Neill 1991) and spanned 

21.5 m. Today, it is the only surviving bridge from the original central road between San Juan and Caguas. When it was 

recently restored, the span of the bridge increased due to erosion of the channel as a result of urban growth and high river 

discharge. The construction of the University of Puerto Rico 

campus’ buildings and facilities began between 1900 and 1910.

Figure 31. The 1880 expansion plan for the urban core of Río 

Piedras (Sepúlveda Rivera 2004b).

Figure 32. Urban growth in San Juan and Río Piedras between 1940 

and 1949 (Picó 1950). This map depicts the early stages of urban 

sprawl in the San Juan Metropolitan Area.

Picó (1950, p. 52) described the development of Río Piedras (at the heart of the Río Piedras Watershed) thusly: “...Río 

Piedras, a small rural village which had been selected in 1903 as 

the site for the main branch of the University of Puerto Rico, 

was also beginning to attract the attention of the metropolitan 

dwellers and its population increased from 2,249 in 1899 to 

5,820 in 1920.” By 1940, Río Piedras had a population of 19,935 

and some 20,000 people were living in new urban developments, mostly in the Hato Rey sector, that marked the beginning of urban sprawl in Puerto Rico (fig. 32). The metropolitan 

area comprising Old San Juan, Santurce, Hato Rey, and Río 

Piedras coalesced as a continuous urbanized zone by 1948 (Picó 

1950). In 1951, the municipality of Río Piedras was incorporated into the municipality of San Juan (Picó 1969). In 1950, 

the population of Río Piedras was 143,989 or 39 percent that of 

San Juan. Afterwards, the population of Río Piedras declined 

sharply (fig. 33) as a result of urban sprawl. By the 1990 census, 

urban population density in the San Juan Bay basin ranges 

from 94/km2 in Piñones to 8,300/km2 in the business district of 

Río Piedras (Hato Rey; Webb and Gómez Gómez 1998).

The initial rapid increase in population of Río Piedras was 

attributed to public transportation and the connection of the 

city to San Juan via a train (Sepúlveda Rivera 2004b). The decline was the result of sprawl caused by increasing dependence 

on private cars. The dramatic flow of vehicles in and out of 

the metropolitan area during the 1960s is illustrated in fig. 34. 

Today, this high level of connectivity is more pronounced and 

much higher than it was in the 1960s. A large proportion of the 

approximately 2.5 million vehicles in Puerto Rico enters and 

exits the metropolitan area daily.

Saez (1988) gives a vivid and detailed account of the development of Río Piedras and the activities of its habitants. His 

accounts cover between 1898 and 1945, and he provides streetby-street information and covers all aspects of the daily lives of 

people. Picó (1950) reported that the hills of the Río Piedras region, including Trujillo Alto, Carolina, and Gurabo, are prime 

locations for vegetable production, where in 1935 one-half of 

the island production was harvested in this region.

The history of water supply for San Juan includes a period 

of time before 1953 when the city obtained water from the Río 

Piedras aqueduct (fed by the Río Piedras River) and the Río 

Bayamón River, supplemented with ground water from several 

high-yield wells (Anderson 1976). Before that, people used 

rainwater, carried water from streams, or depended on public 

faucets to satisfy their water needs (Saez 1988). The Río Piedras 

River was used for swimming, washing clothes, water supply, 

and to deposit wastes. Seepage from latrines and agriculture 

runoff reached the river.

In 1953, the Loíza Reservoir was built, and surface water replaced ground water as the major water supply for the 

metropolitan area. Ground water pumpage declined from 

0.22 m3

/s in 1945 to 0.13 m3/s in 1960 (Anderson 1976). In 1971, 

the now historic water purification plant in the University of 

Puerto Rico Botanical Gardens had a capacity of 0.11 m3/s and a 

production of 0.10 m3/s. The plant got its water from the Lago 
The Río Piedras Watershed and Its Surrounding Environment 35

Table 20. Water sources for the San Juan Metropolitan Area in 1971 and their capacity and production in cubic meter per second and 

million gallons per day in parentheses.

Plant Capacity Production Source Distribution area

Sergio Cuevas 2.6 (60) 3.0 (67.5) Lago Loíza Carolina-San Juan

Guaynabo 1.1 (26) 1.3 (29.7) Lago de Cidra, Río de Bayamón, 

Río Guaynabo

Guaynabo-Bayamón

Botanical Gardens 0.11 (2.5) 0.10 (2.2) Lago Las Curias Río Piedras-San Juan

San Juan 0.47 (10.7) 0.01 (0.2) 18 wells San Juan

Campanilla-Sabana Seca 0.68 (15.6) 0.39 (8.88) 16 wells Levittown

Source: Modified from Anderson (1976). 

Las Curias, also in the Río Piedras Watershed. This reservoir 

stored 1.39 hm3 of water (367 million gallons). Anderson (1976) 

estimated that the 1971 output of municipal water supply in the 

metropolitan area averaged 4.7 m3/s, double the 1959 output 

of 2.4 m3/s. This water consumption originated mostly outside 

the Río Piedras Watershed (table 20), however. In 1971, the loss 

of water through leaky mains was 25 percent. By the 2000s, 

the leakage was more than 50 percent, and a water balance 

developed by the Department of Natural and Environmental 

Resources could not be balanced (table 1). A severe drought 

in the mid-1960s uncovered the vulnerability of the reservoir 

system, and this led to increased drilling for ground water as a 

supplement for the city water supply.

Land Cover for the 

Metropolitan Area

The metropolitan area has rapidly increased its urban land cover 

since 1900 (fig. 35). Ramos González et al. (2005) updated this 

cover using satellite images corresponding to November 27, 

1999 (box 3). The most extensive individual land cover class in 

the metropolitan area (fig. 36, table 21) was high-density urban 

or built-up (35 percent) followed by forested vegetation (26 

percent). Nonforest vegetation also comprised about 26 percent 

of the metropolitan area and barren land was 2 percent. They 

found that 53 percent of the study area was classified as vegetation. The largest coverage of vegetation and forests was on the 

southern limit or the uplands of the various municipalities 

(fig. 36). Bayamón had the highest forest area, while Carolina 

had the largest area of agricultural systems, wetlands, and barren land. San Juan had the largest area of high-density urban or 

built-up land in the region (table 21).

In terms of the percent coverage of each municipality, 

San Juan had 56 percent of its area as urban built-up and 17 

percent forest cover (fig. 37). In contrast, Trujillo Alto was the 

least urban and most rural municipality with 35 percent forest 

cover and 80 percent vegetation. Guaynabo was the only other 

municipality with a larger percentage of forest than urban 

Figure 34. Traffic flow in Puerto Rico (Picó 1969). The width of the 

lines is proportional to the number of vehicles using the particular 

route. The San Juan Metropolitan Area is connected to the rest of 

the island by three main roads, one each from the west, east, and 

south.

Figure 33. Population changes in the urban core and municipality 

of Río Piedras between 1774 and 2000 (from Sepúlveda Rivera 

2004a).
36 The Río Piedras Watershed and Its Surrounding Environment

Table 21. Land cover-class area (hectare) for each municipality of the San Juan Metropolitan Area. 

Land cover Bayamón Cataño Guaynabo San Juan Trujillo Alto Carolina Total

Urban, high density 3,911 656 1,771 7,298 857 3,281 17,775

Urban, low density 863 10 508 588 698 634 3,300

Barren, artificial 202 11 163 225 53 286 940

Barren, natural 26 8 16 21 12 36 118

Nonforest vegetation 2,902 307 1,673 1,961 1,850 4,209 12,902

Forested vegetation 3,567 92 2,554 2,147 1,934 2,882 13,176

Wetlands 2 136 18 95 0 462 713

Water 71 89 29 339 141 567 1,237

Total area 11,545 1,308 6,731 12,674 5,544 12,357 50,160

Total urban 4,774 666 2,279 7,885 1,555 3,915 21,075

Total vegetation 6,472 535 4,245 4,203 3,784 7,553 26,791

Notes: The unaccounted area corresponds to cloud cover. Data are rounded to the nearest hectare.

Figure 35. Historic changes in land cover for the San Juan 

Metropolitan Area. Covers between 1900 and 1970 are from Webb 

and Gómez Gómez (1998). Post-1970 covers are from Universidad 

Metropolitana and Estudios Técnicos, Inc. (2001). 

Figure 37. Percent of land cover for the municipalities of the San 

Juan Metropolitan Area in November 1999 (Ramos González et al. 

2005).

Figure 36. Land cover for the San Juan Metropolitan Area in 

November 1999 (Ramos González et al. 2005).

Urban or built-up, 

high-density

Urban or built-up, 

low-density

Non-forest vegetation

Forested

Wetland

26 5 34 23 4

15 13 33 35 0

57 5 15 17 1

25 7 24 36 0

49 1 23 7 10

34 7 25 31 0

Carolina

Trujillo Alto

San Juan

Guaynabo

Cataño

Bayamón

0% 25% 50% 75% 100%
The Río Piedras Watershed and Its Surrounding Environment 37

Box 3. Methods for Land Cover Analysis of the San Juan Metropolitan Area

A Landsat ETM+ scene dated November 27, 1999, with 

less than 10 percent cloud coverage was acquired, terraincorrected, and georeferenced through the U.S. Geological 

Survey EROS Data Center in Sioux Falls, SD (fig. B3-1). We 

used six of the 30 m resolution spectral bands (bands 1 to 

5 and 7) and the 15 m panchromatic band (band 8) from this 

satellite sensor. The scene was subset to the study region, 

preprocessed for clouds and cloud-shadows, and pansharpened with the panchromatic band. A 6-band composite 

image resulted, incorporating the 15 m spatial resolution 

and the spectral information (visible to near infrared spectra) 

from the original scene.

The image was further segmented using a first-cut, 

unsupervised ISODATA clustering procedure into vegetation, 

nonvegetation, and water. This split reduced confusion 

among spectral classes and provided the benefit of running 

further clustering routines individually by general land 

cover element in separate images. Each of these images 

was iteratively processed using both unsupervised and 

supervised clustering procedures until separation of clear 

land cover was achieved, most specifically for the vegetation 

segment. Ancillary data such as aerial photographs 

(1:48,000 NOAA, 1999 and 

1:20,000 DTOP, 1997–

2000) and other available 

Geographic Information 

System (GIS) data sets (U.S. 

Fish and Wildlife Service 

National Wetland Inventory 

and San Juan Bay Estuary) 

were used as reference 

information to manually 

edit and refine the resulting 

classified image.

The following land cover 

classification scheme was 

used for the assessment: (1) 

high-density urban or builtup (more than 80-percent 

built-up land), (2) low-density 

urban or built-up, (3) natural 

barren (sand, gravel, bare 

rock, or sediment), (4) 

artificial barren (artificially 

exposed soil), (5) nonforest 

vegetation (pasture, grassy 

areas, pasture with shrub), 

(6) forested (closed forest 

canopy), (7) wetland (forested 

and herbaceous), (8) water 

(inland water bodies or ocean), (9) clouds, and (10) no data 

(outside satellite coverage).

The minimum mapping unit chosen for the analysis was 0.5 

ha. Total land area classified in the San Juan Metropolitan Area 

(SJMA) was 50,588 ha. When estimating the percentage of each 

land cover class, however, we did not include cloud cover or 

areas with no satellite coverage. This exclusion reduced the total 

area assessed to 50,160 ha. All images were processed using 

ERDAS Imagine 8.4, whereas map output was generated using 

ESRI Arc View 3.2a GIS software or ArcGIS 9.3. We conducted 

an initial accuracy assessment by comparing results with aerial 

photography. The labeling is correct to within 80 to 90 percent 

accuracy. The map date corresponds to November 27, 1999.

The following analysis perspectives were used to uncover 

the spatial distribution patterns of forest fragments in the 

SJMA (fig. B3-1): Total land cover area in SJMA and percent 

land cover within each municipality, land cover within 50 and 

100 m elevation ranges, land cover within high-density vs. lowdensity urban areas, and number and characterizations by size 

and location of forest fragment classes. In addition, area and 

percent land cover were calculated by major watersheds above 

and below 100 m elevation bands for the entire study area.

Fig. B3-1. Original Landsat ETM+ satellite image and stages of interpretation of the image were used 

to study land cover for the San Juan Metropolitan Area.
38 The Río Piedras Watershed and Its Surrounding Environment

Figure 39. Land cover for the Municipality of San Juan in November 1999 (Ramos González et al. 2005). Cover area and its percentage are 

reported separately for below and above 100 m elevation.

cover. Cataño had the largest percentage of wetlands and the 

lowest of forest cover than any municipality. Cataño was the 

second most urbanized municipality in the metropolitan area. 

The percentage of barren land was similar in all municipalities.

The 1999 land cover results for the municipality of San Juan 

(fig. 37) match the 17-percent forest cover figure reported for 

1995 by J. Molineli of the University of Puerto Rico. Ramos 

González et al. (2005) found that regionalizing the results 

improved the description of the spatial distribution of the forest fragments in the region. Forest fragments tend to be sparse 

and isolated in the lowlands, whereas above 100 m in elevation, forest fragments are larger and more connected (fig. 38). 

The southern areas in San Juan contain about one-half of the 

vegetation cover for the whole municipality (fig. 39). The difference in spatial distribution of these fragments is striking, 

although similar in number of hectares as shown in the table 

within fig. 39. Most of the municipalities, in general, fall short 

of healthier tree cover goals such as those proposed by American Forests; that is, 40 percent for general tree cover and 25 

percent for urban residential areas. (http://www.americanforests.org/resources/urbanforests/treedeficit.php).

The metropolitan area still contains significant areas of vegetation other than forests. Ramos González et al. (2005) recognized 

that some of this vegetation (that is, managed grass and pasture) 

is indicative of urban development. This class includes vegetation 

in cemeteries, golf courses, grass along roads, and in parks and 

other recreation areas. Nevertheless, this cover class and other 

Figure 38. Forest cover in the San Juan Metropolitan Area in 

November 1999 (Ramos González et al. 2005). Dark green 

represents the largest forest fragments, which are concentrated 

and contiguous only above 100 m in elevation. Below 100 m, forest 

patches tend to be more isolated and much smaller in extension. All 

are less than 200 ha in area and are displayed in red and orange. 

Some of these patches are currently in protected status. Bosque 

San Patricio (approx. 25 ha) and the University of Puerto Rico 

Botanical Gardens within the newly designated San Juan Ecological 

Corridor (approx. 200 ha) are examples. 

Below 100 m in elevation

Class name Percent Area (ha)

Urban or built-up, high density 67.9% 7149.3

Urban or built-up, low density 3.0% 311.2

Artificial barren 1.7% 176.9

Natural barren 0.2% 19.5

Non-forest vegetation 12.7% 1342.5

Forested 10.4% 1095.4

Wetland 0.9% 94.8

Water 3.2% 342.0

No data 1 (outside sat cov) 0.0% 0.0

No data 2 (cloud and unk) 0.0% 0.0

Total Area 10531.7

Above 100 m in elevation

Class name Percent Area (ha)

Urban or built-up, high density 7.8% 170.7

Urban or built-up, low density 12.8% 281.4

Artificial barren 2.2% 49.5

Natural barren 0.0% 1.0

Non-forest vegetation 28.7% 631.1

Forested 48.4% 1065.2

Wetland 0.0% 0.0

Water 0.2% 3.6

No data 1 (outside sat cov) 0.0% 0.0

No data 2 (cloud and unk) 0.0% 0.0

Total Area 2202.5

Total San Juan 

area

12734.2
The Río Piedras Watershed and Its Surrounding Environment 39

Figure 40. Distribution of land cover classes in (a) the lowlands, 

below 100 m in elevation, and (b) the uplands, above 100 m in 

elevation, by major watershed in the San Juan Metropolitan Area 

(Ramos González et al. 2005).

Table 22. Land cover (percent) at three spatial scales above a 

Río Piedras River station located at the intersection of highway 

PR-176 and the river channel.

Parameter Río Piedras

Mean of 

46 rivers

30-m 

riparian 

buffer

Agriculture 30.1 37.4

Forest 31.6 43.9

Shrub and woodland 11.1 14.8

Urban 24.3 3.5

100-m 

riparian 

buffer

Agriculture 27.3 25.3

Forest 28.8 56.9

Shrub and woodland 9.3 13.6

Urban 33.0 4.0

Watershed

Agriculture 24.8 40.1

Forest 26.3 42.1

Freshwater 0.7 0.1

Shrub and woodland 8.8 13.4

Urban 39.4 4.2

Source: Kwak et al. (2007).

pasture and shrub classes contribute to the green infrastructure 

of the city and represent opportunities for restoring forests where 

appropriate. Inhabitants in the city derive benefits from all the 

green areas and the built-up land. Only San Juan and Cataño 

have more land without vegetation than with vegetation, and the 

metropolitan area as a whole is at 44 percent of the area without 

vegetation. The distribution of vegetated areas provides options 

for city planners to maintain a mix of vegetation and nonvegetation zones to optimize the benefits of the green infrastructure.

Land cover data by watershed (fig. 40) show how prevalent 

forested vegetation is on the upper portions of all watersheds 

(compare fig. 40a for lowland portions of watersheds with 

fig. 40b for the corresponding uplands). The Río Piedras 

Watershed is part of the San Juan Bay Estuary Watershed in 

these figures. Table 22 contains information on land cover for 

the Río Piedras Watershed only. It shows that in comparison 

with other watersheds in the island, the Río Piedras Watershed 

is heavily urban with equal proportions of nonforest and forest 

cover. Even near the Río Piedras River channel, the urban cover 

is significant.

Overview of the Effects 

of Human Activity in the 

San Juan Bay Area

Seguinot Barbosa (1983) conducted a comprehensive analysis of 

coastal modifications and land transformation in the San Juan 

Bay Area, which we now summarize for its value as an overview 

of the effects of human activity in the metropolitan area. The 

early presence of humans in the region was before 350 AD, and 

the climax of indigenous activity by the Taíno was in 1200 AD. 

This activity was followed by 400 years of land transformation by the Spanish colonial period and more than 100 years of 

accelerated change under the influence of the United States of 

America.

The port of San Juan became the most significant development nucleus of Puerto Rico that attracted rural populations 

and the formation of extensive slums throughout the first half 

of the 20th century. As an industrial model of development 

progressively substituted the agricultural model, urban sprawl 

began to take hold, particularly after the 1960s. Deterioration 

of the urban core accelerated the population shift towards 

the periphery of the city and adjacent municipalities. Such 

40a.

40b.

Urban or built-up, high-density

Urban or built-up, low-density

Artificial barren

Non-forest vegetation

Forested

Wetland

Río La Plata

Río Bayamón

San Juan Bay Estuary

Río Grande de Loiza

Río La Plata

Río Bayamón

San Juan Bay Estuary

Río Grande de Loiza

0 5000 10000 15000 20000

Area (ha)

Urban or built-up, high-density

Urban or built-up, low-density

Artificial barren

Non-forest vegetation

Forested

0 1000 2000 3000

Area (ha)

4000 5000 6000
40 The Río Piedras Watershed and Its Surrounding Environment

dramatic shifts in human activity and population distribution 

had considerable effects on the physical environment.

The San Juan Bay experienced few changes between 1518 

and 1790. Land reclamation processes of coastal areas were the 

greatest effects documented up to the turn of the 20th century. 

Between 1912 and 1980, however, the changes were dramatic. 

The coastal configuration of the Bay changed with significant 

effects on ocean currents and wave refraction, reflection, erosion, deposition, and circulation patterns. About 19,158 m2

 of 

the Bay were filled or dredged. Coastal erosion was evident at 

rates of 0.6 m/y and 2.9 m/yr in different locations. These erosion rates were associated with hurricanes, other natural events, 

and synergy with dredging, filling, and channelization of 

streams and rivers. Humans have constructed artificial beaches; 

contributed to the formation of the Palo Seco spit; and changed 

deposition patterns, allowing mangroves to develop where sediments accumulate. High-wave energy areas have been converted to low-energy coastal areas.

Large areas of mangroves on the eastern shore of the Bay 

were filled and converted to urban built-up land such as the 

Puerto Nuevo area, which now floods with high frequency. 

The filled area in the San Juan Bay Watershed was equivalent 

to 12 percent of the watershed (table 2). Vegetation cover was 

reduced by 80 percent between 1508 and the 1980s. Ciénaga Las 

Cucharillas lost 50 percent of its 1508 area and 90 percent of 

its mangroves. In 1888, the average depth of San Juan Bay was 

about 10 ft (3.1 m); it increased to 12 ft (3.7 m) by 1912, and was 

25 ft (7.6 m) in the 1980s. Shoals and benthic communities disappeared. The extension of the flood plains of the Río Bayamón 

and Río Piedras Rivers was reduced by 95 percent.

Discussion

Our analysis has uncovered a number of issues that deserve additional attention. The Río Piedras Watershed has been heavily 

modified by human activity, and it is very difficult to establish 

its hydrologic boundaries (box 2). We found different delineation by different government agencies, some in clear inconsistency with the hydrologic definition of what a watershed is. The 

connection of the original channel of the Río Piedras River with 

the Caño Martín Peña and water from the Bayamón River make 

the mouth of the river a very complex hydrological system that 

requires further attention.

The proposal of the U.S. Army Corps of Engineers to channelize the Río Piedras and Río Puerto Nuevo Rivers (fig. 17) 

will further complicate the hydrologic behavior of the Río 

Piedras River by introducing hypercritical flows to the watershed. These measures were necessary due to the lack of effective 

implementation of land use planning in the metropolitan area. 

We found that in the 1940s to the 1960s (Picó 1950, 1969), 

the Planning Board had made land development plans for the 

region (fig. 41) by regulating the conversion of rural lands to 

urban uses. These plans were apparently abandoned after the 

Figure 41. Land use plan for the 1970s in the San Juan 

Metropolitan Area (Picó 1969). This plan regulated the development 

and conversion of rural lands to urban uses.

1970s as the Planning Board abandoned its efforts to plan the 

development of rural lands in the island. Instead, the Planning 

Board simply permitted urban developments and zoned urban 

areas but did not develop a land use plan for rural lands (see 

Padín et al. N.d.).

The rapid expansion of urban uses in the metropolitan area 

negated many of the assumptions that led to the planning of 

the channelization of the Río Piedras. For example, constructions of large buildings near vital river intersections with main 

roads limited the width of the proposed canal, thus leading 

to the need to incorporate hypercritical fluxes into the design 

of the channelization. This design, in turn, not only made the 

project more expensive but also demanded more land for water 

storage in the basin during periods of high rainfall. It also made 

the project more dangerous, and required the isolation of the 

channel from human use of the river to avoid fatalities during 

the periods of critical discharges. When the channelization was 

conceived, however, the expectations for urban expansion and 

population density in the metropolitan area were overestimated, 

as no one could predict in the 1960s and 1970s the movement 

of population away from urban cores and into the rural areas of 

Puerto Rico away from the metropolitan area. Thus, the upper 

portions of the Río Piedras River watershed were not developed 

as expected, which allows for more flexibility for the design 

of the proposed concrete channels. Moreover, today the value 

of the river for uses other than concrete channels is a public 

concern as alternative technologies exist to reduce storm runoff. 

There appears to be sufficient cause to revisit the management 

alternatives for the floodwaters in the Río Piedras Watershed.

As an example of the increased recognition of the value 

of rivers and riparian zones in Puerto Rico, we quote here a 

section of the Law of Flood Prevention and Conservation of 

Streams (Ley 49, 2003, Art 2):

Se dispone que en cualquier obra de urbanización o cualquier lotificación colindante con un río, quebrada, laguna o 

cualquier cuerpo de agua se dedicará a uso público, en interés 

general de la conservación del cuerpo de agua, mediante 

inscripción en el Registro de Propiedad, una faja de terreno 
The Río Piedras Watershed and Its Surrounding Environment 41

con un ancho mínimo de cinco metros lineales a ambos lados 

del cauce del río, arroyo o quebrada o del lecho de la laguna o 

lago. Esta faja se mantendrá expedita y no podrá ser utilizada para usos distintos al propósito de conservación. Sólo se 

permitirán usos recreativos pasivos que no conlleven obstrucción, no conflijan con funciones de conservación y limpieza y 

estén relacionados con el disfrute del cuerpo de agua. Cuando 

se trate una quebrada o arroyo, la faja deberá será cedida al 

Municipio con jurisdicción.

San Juan Bay is the economic hub of Puerto Rico, as most 

of the shipping and tourism vessels require use of the Bay. 

The large sediment and nutrient loads that are transported 

by the Río Piedras River to San Juan Bay (table 7) deteriorate 

water quality and sediment the Bay, however, which requires 

more frequent dredging and maintenance at a high cost to the 

Federal Government. These costs could be reduced with better 

regulation of land movement in the Río Piedras Watershed, 

but such regulations do not exist or are not enforced. Allowing developers to sediment the river is a subsidy from the 

Federal Government to the construction industry that saves 

money for developers who do not have to be careful with land 

movement activities.

We were surprised by the high species richness in the Río 

Piedras River itself and its surrounding environment (tables 

8–10) and the health and species richness (tables 11–19, photos 

4, 5, and 7) of urban forests and vegetation throughout the city. 

This high level of biotic activity reflects a functioning river system and watershed despite the highly polluted conditions of its 

waters, which are classified as unacceptable to humans in terms 

of fecal bacterial counts (table 6, fig. 21). This situation appears paradoxical, although the Río Piedras River might be an 

example of the resilience of riverine systems in urban environments, or the fact that what is poor water quality for humans 

might be acceptable water quality to wild organisms, or perhaps 

both. On the other hand, terrestrial vegetation and wetlands 

in the metropolitan area must be playing important ecological 

roles that sustain economic activity and life quality, although 

these ecosystem services are not generally recognized.

We also found a reduction in monitoring activities by U.S. 

Government agencies in the metropolitan area (see appendix). 

Many of the studies conducted in the coastal lagoons, aquifers, 

rivers, and streams of San Juan could not be conducted today 

because a network of stations gaging these natural systems no 

longer exists. A system of rainfall, stream gaging, water quality, 

tidal surges, biological diversity, and aquifer behavior is critically needed for this overcrowded part of Puerto Rico.

Finally, our review only briefly addressed the issue of the 

large use of energy in the metropolitan area, which leads to the 

development of a heat island effect, but which also powers the 

economy. A clear need exists to gain more understanding on 

the metabolism of the SES of San Juan, with particular focus 

on the material and energy balances that support such high 

levels of human activity in such a small area.
42 The Río Piedras Watershed and Its Surrounding Environment

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Webb, R.M.T.; Gómez Gómez, F. 1998. Synoptic survey of 

water quality and bottom sediments, San Juan Bay estuary system, Puerto Rico, December 1994–July 1995. Water 

Resources Investigations Report 97-4144. San Juan, PR: U.S. 

Department of the Interior, U.S. Geological Survey.

Yépez, A. 2008. Cartoon of the shrimp Palaemon pandaliformis. 

Editorial page, El Vocero, November 26: 58 

Zapp, A.D.; Berquist, H.R.; Thomas, C.R. 1948. Tertiary geology of the coastal plains of Puerto Rico. U.S. Geological 

Survey Oil and Gas Investigations Preliminary Map 85, San 

Juan, PR. San Juan, PR: U.S. Department of the Interior, 

U.S. Geological Survey.
The Río Piedras Watershed and Its Surrounding Environment 45

1. U.S. Geological Survey gaging stations (http://waterdata.usgs.gov/nwis/).

Station number Station name

Latitude

(NAD 27)

Longitude

(NAD 27)

Drainage area 

(km2)

Datum of gage

 (m asl NGVD 29*)

50048680 Lago Las Curias at Damsite near 

Río Piedras River

18o

20'40" – 66o03’03" 2.8 99.97

50048690 Quebrada Las Curias BLW

Las Curias outflow

18o

20'42" – 66o03'08" 2.85 80.00

50048770 Río Piedras River at El Señorial 18o

21'51" – 66o03'56" 19.40 60.02

50048800 Río Piedras near Río Piedras River 18o

22'15" – 66o03'40" 21.2 29.99

50049000 Río Piedras at Río Piedras 18o

23'48" – 66o03'24" 32.4 50.02

50049100 Río Piedras River at Hato Rey 18o

24'34" – 88o04'10" 39.37 0.98

50999971 Lago Las Curias Raingage Near Cupey 18°20'05" – 66°02'23" n/a 47.24

* m asl NGVD 29 = meters above sea level National Geodetic Vertical Datum of 1929

Appendix. Environmental Monitoring Stations Operated 

by the Government in the San Juan Metropolitan Area. 

Note: Empty cells mean that data are not available. For more information on the stations, see the indicated Web pages.

2. National Oceanographic and Atmospheric Administration climate stations (http://www.ncdc.noaa.gov/oa/climate/stationlocator.html).

Station number Station name Latitude Longitude Elevation (m) Period of record*

668814** San Juan WFO (Weather Forecast Office) 18.43 – 66.00 2.59 2006 to 2009

20023983 Río Piedras 18.40000 – 66.06667 29.87 1/1/55 to 1/31/62

20023985 Río Piedras, Experiment Station 18.39056 – 66.05417 28.04 1/1/59 to 2009

20023988 San Juan International Airport 18.43250 – 66.01083 2.74 1/1/55 to 2009

20023989 San Juan Naval Station 18.43333 – 66.08333 13.11 1/1/39 to 12/31/45

20023994 San Juan Isla Grande 18.45000 – 66.10000 18.90 12/1/47 to 5/31/55

20023999 San Juan City 18.46667 – 66.10000 6.10 1/11/1888 to 

5/31/77

20024000 San Juan AFF 

(Autoridad de Fuentes Fluviales)

18.46667 – 66.11667 24.99 6/1/41 to 6/30/43

30001966 San Juan FAA 

(Federal Aviation Administration)

18.11750 – 66.07861 851.61 7/7/97 to 2009

*As of 2009.

** COOP (cooperative agreement) number.

Note: Station numbers correspond to those of the National Climate Data Center.
46 The Río Piedras Watershed and Its Surrounding Environment

3. Environmental Protection Agency Air Quality System (http://www.epa.gov/ttn/airs/airsaqs/detaildata/downloadaqsdata.htm).

Site ID

(72-127-) Address

Latitude

(WGS84)

Longitude

(WGS84) Period of record

0001 Fire Department, Muñoz Rivera and Manuel 

Domenech

18.416761 – 66.055011 1/1/62 to 12/31/64

0002 Stop 22, Ponce de León, Santurce 18.447222 – 66.067500 1/1/81 to 2009

0003 Baldorioty de Castro Ave. 18.449814 – 66.052510 10/26/82 to 2009

0004 Gandara Ave., University of Puerto Rico 18.403889 – 66.052778 3/18/81 to 8/30/02

0005 65th Infantry and Verona 18.396667 – 66.026111 1/1/82 to 2009

0006 High School Trina Padilla, Hato Rey 18.408056 – 66.073056 12/8/84 to 5/14/85

0007 Post Office, Juan B. Huyke St. 18.423333 – 66.059722 4/2/86 to 12/19/98

0008 William Jones St., Río Piedras 18.401944 – 66.046389 2/1/95 to 2009

0009 St. 16, NE-5, Puerto Nuevo 18.418889 – 66.087500 2/20/97 to 2009

0010 Covadonga Terminal, Fernández Juncos and Comer 18.466760 – 66.112793 11/29/96 to 2009

0011 Fernández Juncos Ave. 18.449859 – 66.073426 5/14/04 to 2009

1001 Edificio Margarida, 562 Muñoz Rivera Ave. 18.406944 – 66.054167 1/1/74 to 12/31/81

1002 Shopping Center FD Roosevelt and Muñoz Rivera 18.422500 – 66.056667 1/1/76 to 12/31/81

1003 FD Roosevelt Ave., Hato Rey 18.422778 – 66.069722 1/1/78 to 12/31/81

International Institute of Tropical Forestry

USDA Forest Service

1201 Ceiba Street

Jardín Botánico Sur

Río Piedras, PR 00926–1115