Piñones National Forest, which extends through Carolina, Loíza, and part of San Juan

Piñones National Forest, which extends through Carolina, Loíza, and part of San Juan

Surface waters are Puerto Rico’s most important hydrological resource. Surface water is found on the surface of the ground in bodies of water such as streams, rivers, gullies, reservoirs, lakes, canals, estuaries, wetlands, and springs. There are 134 primary hydrographic basins in Puerto Rico (Figure 1) [Figura1], basins being the area of land that contributes to the collection of rainwater and drains toward bodies of water. The amount of rain that falls on the basins of Puerto Rico, and the great number of rivers and streams that drain the basins, contribute to a significant volume of surface runoff during most of the year. Puerto Rico’s reservoirs – artificial lakes built in the 20th century – are among the bodies of surface water that store surface runoff. These are the main source of water for the water treatment plants of Puerto Rico and they provide water for the generation of electricity and agricultural irrigation.

Rivers and streams

In Puerto Rico there are 224 rivers and over 4,000 streams that drain the basins through which they flow. The rivers are dynamic systems that change constantly due to the high energy potential of the runoff in the mountain ranges where they originate. As they flow down from the mountain ranges, the rivers form rapids and pools separated by deposits of gravel and sand that move downriver with each rise in water level and with flooding. Along the stretches where there are rapids, the speed of the water increases, as does its power to erode and carry large volumes of sediment toward the pools.

The rivers of the northern and western regions flow constantly; this is in contrast to the rivers of the southern region, due to their having a larger catchment area and receiving a greater quantity of water in their basins. Most of the rivers and streams in the southern region flow intermittently. They maintain constant flow only during the rainy season or when there are abnormal weather events that produce heavy rains during the dry season.

Even though some rivers on the high slopes of the southern region flow most of the time, as the water reaches the alluvial valleys, seepage and the combined action of evaporation and transpiration consume most of the water, leaving the river beds dry. The construction of reservoirs and the extraction of water for domestic use and irrigation from the rivers of the southern region also contribute to diminished water flow, even during the rainy season.

Some of the principal rivers of the southern region that have reservoirs are the Río Grande of Patillas (Patillas Reservoir), Jacaguas River (the Toa Vaca and Guayabal Reservoirs), Bucaná River (Cerrillos Reservoir), Yauco River (Lucchetti Reservoir), and the Loco River (Loco Reservoir). In the northern region, there are major reservoirs on the Guajataca River (Guajataca Reservoir), Río Grande of Arecibo (Caonillas, Dos Bocas, and Garzas Reservoirs), Río Grande of Manatí (Guineo and Matrullas Reservoirs), La Plata River (Carite, Comerío I and II, and La Plata Reservoirs), Bayamón River (Cidra Reservoir), and Río Grande of Loíza (Carraízo Reservoir). In the western region, the Yahuecas and Guayo Reservoirs are in the basin of the Río Grande of Añasco, though the water is diverted to the basin of the Yauco River in the southern region. In the eastern region, the Fajardo Reservoir is being built on the Fajardo River, but outside of the river’s course.

The rivers are one of the most important natural factors in the evolution of the alluvial valleys along the coasts of Puerto Rico. The slope of the river beds in the coastal valleys is much less steep than on the slopes of the mountains. As the sediment is carried from the mountains to the coast and the sea, it precipitates out of the natural current as the current loses energy with the slope of the river course declines. In the coastal valleys, sediment accumulates in the riverbeds, reducing their depth and drainage capacity.

The rivers of Puerto Rico also have an important function in relation to the penetration of saline or brackish water into the aquifers. In the coastal valleys, mainly in the northern region, the scant elevation of the land over the sea level makes it possible, during high tides, for a wedge of sea water to run up into the mouths of the rivers. They can go upriver for several miles, as is the case of the Rio Grande of Arecibo in Arecibo, the Rio Grande of Manatí near Barceloneta, the La Plata River in Dorado, the Rio Grande of Loíza in Loíza, and the Espíritu Santo River in Río Grande, as well as others. This phenomenon contributes to the infiltration of saline waters into the alluvial aquifers and limestone of the northern region. These wedges of saltwater are displaced in turn when frequent rains produce significant flows of water, particularly in the northern region. Periodic formation of banks of sand at the mouth of the rivers affects this dynamic equilibrium between sea and river, sometimes preventing the wedges of saltwater from running inland, even during high tides. This phenomenon is not significant in the rivers of the southern, eastern or western coasts, because the slopes of the rivers in these regions are greater than in the north, which limits the effect that the tides have on the course of the rivers. The principal characteristics of the most important rivers in Puerto Rico in regard to water resources are summarized in Table 1 [Tabla1].

Lake Luchetti in Yauco, Puerto Rico

Lake Luchetti in Yauco, Puerto Rico

There are 36 main reservoirs in Puerto Rico that belong to the Commonwealth and its agencies, in addition to several smaller private ones. Of the 36 publicly owned reservoirs, the Department of Natural and Environmental Resources considers 21 to be of the greatest significance based on their capacity and importance, as can be seen in Figure 2[Figura2]. The publicly owned reservoirs were built during the 20th century for a variety of uses, including agricultural irrigation, hydroelectrical power generation, drinking water, flood control, and recreation. although in Puerto Rico these are generally called “lakes,” there are no natural lakes on the island, all the “lakes” being artificial reservoirs formed by dams to store part of the surface runoff carried by the rivers.

The reservoirs are the principal source of raw water that is used by the Autoridad de Acueductos y Alcantarillados (AAA, Aqueducts and Sewers Authority) to produce drinking water as well as to supply irrigation water, mainly in the coastal valleys of the southern region. The reservoirs supply approximately 373 mgd (million gallons per day) of raw water to filtration plants operated by the AAA, which is 62 percent of the drinking water produced on the island. At the same time, the reservoirs provide approximately 37 mgd of water for agricultural irrigation in the coastal valleys of the northern and southern regions. In addition, they provide water to generate approximately 1.7 percent of the electrical energy produced by the Autoridad de Energía Eléctrica (AEE, Electrical Energy Authority) in Puerto Rico. They also serve as recreational facilities for thousands of residents who fish and sail in the mountainous areas. And finally, they serve as refuges for birds and aquatic wildlife, including fish, shrimp, and turtles.

Except for the new Fajardo Reservoir, all the main reservoirs in Puerto Rico have been built in the mountainous areas of the island. This location is preferable because it is possible to store relatively large quantities of surface runoff produced by the abundant rain in those areas.

Accumulation of sediment in the reservoirs is considered one of the most important problems in the management of Puerto Rico’s water resources. Sedimentation has reduced the storage capacity of all the reservoirs.

It is important to point out that over 95 percent of the sediment deposited takes place during climatic events such as hurricanes: Hortensia in 1996 and Georges in 1998. This natural process occurs mainly in the river basins in the northern and eastern regions of Puerto Rico, where rain is most abundant, the slopes of the basins have the greatest incline, and development and deforestation are most intense. Even so, though the rate of sedimentation of several of the reservoirs is high, in general the useful life of the existing reservoirs is appreciable. Loss in their storage capacity has led to changes in the design of the new reservoirs in order to avoid excessive accumulation of sediment. It will be necessary to implement an aggressive program of protection of the basins to minimize erosionand the movement of sediment.

  • The Caonillas and Dos Bocas system of reservoirs, in Utuado, supply up to 100 mgd of raw water to the north coast Super Aqueduct, which provides drinking water to municipalities from Arecibo to the San Juan metropolitan area, including Caguas. In addition, at Dos Bocas, the AEE generates 5,000 kilowatts of electricity per hour and adequate water levels are maintained for navigation and recreation. Dos Bocas has lost 56percent of its original storage capacity, which threatens the capacity of the Super Aqueduct in the short term to supply the amount of water committed in the northern region. The Caonillas Reservoir has lost 26 percent of its original capacity, and urban development of its basin threatens to increase its rate of sedimentation.
  • Carraízo Reservoir (Loíza), in Trujillo Alto, supplies up to 100 mgd of water to the Sergio Cuevas filtration plant in Trujillo Alto, which serves the San Juan metropolitan area, including sectors of Trujillo Alto and Carolina. Although the Carraízo Reservoir was dredged in 1997-98 at a cost of approximately $60 million, preliminary data from the U.S. Geological Survey (USGS) indicate that the sediment carried down during the flooding caused by Hurricane Georges in 1998 reduced the capacity restored by dredging.
  • The La Plata Reservoir, in Toa Alta, supplies up to 67 mgd of water to the San Juan metropolitan area, principally to sectors of Toa Alta, Toa Baja and Bayamón. That reservoir has lost 15 percent of its original capacity.
  • The Guajataca Reservoir, between Camuy and San Sebastián, is part of the northwestern irrigation District operated by the AEE and is the principal source of drinking water for approximately 150,000 residents of the municipalities of Isabela, Aguadilla, Rincón, Aguada and sectors of Moca. In the zone from Isabela to Aguadilla, there are no alternative sources of water, which is why the reservoir is critical to supplying the residential and commercial demand for water. The reservoir supplies up to 20 mgd to the aqueducts of Isabela, Quebradillas, Ramey and Aguadilla, as well as providing for agricultural uses. The current capacity of the reservoir is 86percent of its original capacity.
  • The Toa Vaca and Guayabal Reservoirs, in the south-central region, supply up to 20 mgd to Ponce, Juana Díaz and Santa Isabel. Guayabal has lost 51 percent of its original capacity due to sedimentation. Even though Toa Vaca, the reservoir with the largest capacity, has lost only 7 percent of its original capacity, its sedimentation rate exceeds that of all the other reservoirs on the island.
  • The Lucchetti and Loco Reservoirs, which are part of the Lajas Valley irrigation district in the southwestern region, receive water from the Yahuecas and Guayo Reservoirs, located on the northern slopes of the island. This system is the most important source of water for the southwestern region and supplies up to 8 mgd of water to the AAA filtration plants that serve 160,000 residents in the municipalities of Lajas, San Germán, Sabana Grande, Guánica and sectors of Hormigueros. The district also supplies up to 12 mgd for agricultural irrigation in the Lajas Valley. In addition, the AEE generates electricity at two hydroelectric plants in the system, producing an average of 38,000 kilowatts annually. The Loco Reservoir has lost 59 percent of its original capacity, but due to its smaller size, it is not a significant water storage resource. The Lucchetti Reservoir has lost 40.6percent of its original capacity.

The most important characteristics of the 21 largest reservoirs in Puerto Rico, including that in Fajardo, are summed up in Table 2 [Tabla2].

Lagoons Puerto Rico’s lagoons are not a source of water supply given their salinity, level of pollution, or ecological value. The principal lagoons include those of the San Juan bay estuary system (Condado, San José, Torrecillas and Piñones); Tortuguero in Vega Baja and Manatí; Joyuda in Mayagüez; Grande and Aguas Prietas in Fajardo; Santa Teresa I and II and Mandry in Humacao; Salinas (La Matilde) in Ponce; Las Salinas and Guanaquilla in Cabo Rojo; San Jacinto in Guánica; Carrizales in Hatillo; and Cartagena in Lajas. Caño Tiburones, between Manatí and Barceloneta, forms a lagoon (La Tembladera) between the canals that drain it.

The lagoons of Puerto Rico are in relatively flat lands and serve as the habitat for wildlife species (flora and fauna), many classified as in danger of extinction. The lagoons also provide great scenic value and are ideal areas for quiet moments and rest for the spirit. All of this gives them a special value, which is the reason that they are protected by the Commonwealth and the DRNA through special designation.


In Puerto Rico, there are both fresh water and saline springs, primarily in the karst region in the northern region and in the mountainous area of the interior region. Though most of the fresh water springs discharge only limited amounts of underground water at the surface, there are springs that keep up an appreciable constant flow. The USGS evaluated Puerto Rico’s principal springs in 1982-84 (Figure 3) [Figura3] and concluded that the average flow was approximately 23.5 mgd. Some of these are the main source of the public water supply for rural communities that operate their own drinking water systems (known as community or “Non-PRASA” systems).

Springs are underground waters rising to the surface. Rain water that filters down into the soil continues to move through the subsoil until it reaches the water level of an aquifer’s saturation zone. In the mountainous areas and on the slopes of the central mountain range, the water generally accumulates in fractures or cracks, and in porous materials that are volcanic in origin. Where the freatic layer reaches the surface, water pours out in the form of springs. In the karst region, there are many conduits – between mogotes (haystack-shaped limestone hills), from banks and in riverbeds – through which water comes to the surface. In the coastal valleys, where the freatic level reaches the surface, springs pour into coastal lagoons and wetlands. Tortuguero Lagoon (Vega Alta) and Caño Tiburones (Manatí to Barceloneta) receive significant volumes of water from springs through alluvial deposits and outcroppings of limestone formations in the Karst Region. The main fresh water springs in Puerto Rico and their estimated volumes are summed up in Table 3 [Tabla3].

In Puerto Rico, there are also saline springs, primarily in the coastal valleys of the northern region. These appear in areas where the elevation of the land is below the average level of the sea, and the geology of the materials allows sea water to flow inland. The area of Caño Tiburones receives significant discharges from several saline springs that are fed by conduits in the limestone. These include La Cambija and Zanja Fría in the Caño Tiburones. Tortuguero Lagoon also receives discharge from smaller freshwater and saline springs.

In the Southern Region, there are several thermal springs, including those in Coamo (Baños de Coamo, the Coamo Baths) and Ponce (Quintana). These springs are fed by waters issuing from cracks that extend deep into the earth’s crust, where the temperature of the rock is high. This causes the water that filters down from the surface to evaporate. The vapor rises to the surface through the cracks and condenses again before pouring out in the form of springs. Quintana Springs has practically disappeared, possibly due to blockage of the cracks that feed them.

Though the potential of springs as a source of water is limited, these systems are a valuable resource that needs to be preserved and utilized to supplement alternative sources of water to meet public needs, and studies of these systems need to be updated and expanded.


Author: Sigfredo Torres González
Published: August 27, 2014.

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