Dam

RESERVOIR BOTTOM CONTOUR -- Depth of the reservoir bottom 

on January 2010, in meters above mean sea level. 

Contour interval 1 meter. Shoreline shown as red line 

equal to zero depth or spillway elevation.

21.0

Río Limón branch

Río Caonillas branch

Río Grande de Arecibo branch

0 1,250 2,500 3,750 5,000 FEET

0 500 1,000 1,500 2,000 METERS

66°40' 39'30" 66°39'

18°20'

19'30"

19'

18°18'30"

21.5

21.0

20.0

19.0

18.0

17.0

16.0

15.0

14.0

13.0

12.0

11.0

10.0

9.0

8.0

7.0

6.0

6.0

5.0

4.0

3.0

2.0

1.0

12.0

11.0

10.0

9.0

8.0

7.0

6.0

5.0 4.0

4.0

3.0

2.0

1.0

21.0

20.0

19.0

18.0

17.0

16.0

15.0

14.0

13.0

12.0

11.0

10.0

9.0

8.0

7.0

6.0

4.0

3.0

2.0

1.0

5.0

EXPLANATION

2010

2005

EXPLANATION

Storage volume, in million cubic meters

Elevation, in meters above mean sea level

90

85

80

75

70

65

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

0

Río Grande de Arecibo branch

10

Depth, in meters below spillway elevation of 89.92 meters above mean sea level

20

30

0

Río Caonillas branch

10

20

30

0

Río Limón branch

10

EXPLANATION 20

2010

2005

30

0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000 4,500 5,000 5,500 6,000

Distance upstream from the dam, in meters

Figure 1. Location of Lago Dos Bocas within the Río Grande de Arecibo basin. 

Figure 2. Bathymetric map of Lago Dos Bocas, Utuado, Puerto Rico for January 2010. 

Figure 4. Longitudinal bottom profiles along the thalweg of the Río Grande de Arecibo, Río Caonillas, and Río Limón branches of Lago Dos Bocas, 

Puerto Rico, for 2005 and 2010.

Figure 3. Relation between water-storage capacity and pool elevation

of Lado Dos Bocas, for 2005 and 2010.

Introduction

 Lago Dos Bocas reservoir was completed in 1942 to provide water for hydroelectric power 

generation along the northern coast of Puerto Rico. The reservoir had an original storage capacity of 37.50 

million cubic meters (Mm3). The dam is located about 9 kilometers (km) northeast of the town of Utuado, 

immediately downstream of the original confluence of the Río Grande de Arecibo and the Río Caonillas 

(fig. 1).

 The Puerto Rico Electric Power Authority (PREPA) owns and operates the Lago Dos Bocas 

reservoir, and since 1996, the reservoir has become an essential part of the Puerto Rico Aqueduct and 

Sewer Authority (PRASA) North Coast Superaqueduct Project. The Superaqueduct is supplied by 

controlled releases for hydroelectric power generation that replenish the public-supply raw-water intake 

pool located about 10 km downstream from the Lago Dos Bocas Dam (fig. 1). As of 2005, the 

Superaqueduct supplies about 4.03 cubic meters per second (m3/s) (348,192 cubic meters per day [m3/d]) of 

potable water to communities along the northern coast, from Arecibo to the San Juan metropolitan area.

 Because of the importance of the reservoir to the North Coast Superaqueduct, the U.S. Geological 

Survey (USGS), in cooperation with PRASA, conducted a sedimentation survey of Lago Dos Bocas in 

January 2009. The results of this survey were used to estimate the useful life and the firm yield of the 

reservoir, and evaluate the need to dredge the reservoir.

Method of Survey and Analysis

 The field techniques and bathymetric data reduction processes used for the 2010 survey were 

performed following procedures established by the USGS Caribbean Water Science Center (CWSC) and 

described in a previous survey report of Lago Dos Bocas (Soler-López, 2007). The January 2010 

bathymetric survey was conducted using a Bathymetric/Land Survey System developed by Specialty 

Devices Inc., consisting of a global positioning system (GPS) coupled to a digital depth sounder similar to 

the setup used for the survey conducted in August 2005 (Soler-López, 2007). A total of 178 survey 

navigation lines were established at a spacing of about 50 meters (m), beginning at the dam and continuing 

upstream along the different river branches of the reservoir. Geographic position (accuracy of less than 2 m) 

and water depths (accuracy of 2 centimeters [cm] ± 1 percent of the measured depth) were acquired 

simultaneously using the GPS interfaced to the depth sounder. The reservoir pool elevation was monitored 

at the continuous recording USGS lake-level monitoring station Lago Dos Bocas at dam site near Utuado, 

Puerto Rico, station number 50027100 (fig. 1). The fathometer was calibrated prior to each day of survey 

by using a bar check.

 The pool elevation of Lago Dos Bocas was below the crest of the spillway structure; therefore, the 

sounding data were adjusted using a time-elevation correction factor to represent depths relative to the crest 

of spillway elevation of 89.92 m above mean sea level. A total of 17,039 data points were collected over the 

entire reservoir. The 2010 data were stored in a portable computer and transferred into the USGS 

geographic information system (GIS) where final analysis and volume calculations were made following 

procedures similar to those used to develop the August 2005 bathymetric map of Lago Dos Bocas 

(Soler-López, 2007). Field data from the January 2010 survey were used to generate a bathymetric map 

representing the reservoir bottom in January 2010 (fig. 2). Data points were color-coded according to their 

specific depths, and same-color points were connected with a line to draw depth contours. A triangulated 

irregular network (TIN) surface model of Lago Dos Bocas was then generated from the bathymetric map, 

and the reservoir volume was calculated using the GIS. The TIN surface model was used with the digital 

data for 2005 and 2010 to generate a stage-storage curve and longitudinal profiles along the different river 

branches of Lago Dos Bocas. A storage capacity table and curve were generated by calculating the TIN 

volume at 1.00-m elevation intervals and are shown on table 2 and figure 3, respectively.

Storage Capacity, Sedimentation Rate, and Useful Life 

 The storage capacity of Lago Dos Bocas decreased slightly from 17.26 Mm3 in August 2005 

(Soler-López, 2007) to 16.74 Mm3 in January 2010 (table 1). The inter-survey (2005–2010) storage 

capacity loss was about 3 percent, equaling a decrease of about 0.6 percent per year. This decrease 

represents a reduction of 520,000 cubic meters (m3) between 2005 and 2010, for an annual storage capacity 

loss rate of about 104,000 m3. 

 The Lago Dos Bocas long-term capacity loss rate due to sedimentation has not changed 

substantially, decreasing from 321,000 cubic meters per year (m3/yr) in 2005 to 305,000 m3/yr in 2010, for 

a reduction of only 5 percent (table 1). Although the long-term sedimentation rate has remained nearly 

unchanged (321,000 compared to 305,000 m3/yr), the annual loss rate of Lago Dos Bocas for the last 5 

years has shown a gradual decrease (Soler-López, 2007), from 130,000 m3/yr in 2005 to 104,000 m3/yr in 

2010. This trend may be due to sediment depletion in the Lago Dos Bocas Basin after Hurricane Georges 

(1998) in which the sedimentation rate increased threefold (1,090,000 m3/yr). Hurricane Georges flushed 

material stored upstream of the Lago Dos Bocas Basin and not only deposited it within the reservoir, but 

also reduced readily available sediments for transport. Similar trends were observed after Hurricane 

Georges at three other reservoirs in Puerto Rico, Lago Patillas, Lago de Cidra, and Lago Loíza 

(Soler-López, 2010a, b; 2012). 

 The comparison of longitudinal bottom profiles for 2005 and 2010 indicates that sediment 

accumulation in Lago Dos Bocas for this period was insignificant (less than 1 m at limited locations, fig. 4). 

The greatest buildup occurred at the Río Grande de Arecibo delta where sediment accumulation has 

reduced the flooded area of Lago Dos Bocas by about 800 m in length and about 100,000 square meters 

(m2) in area since 2005 (figs. 4 and 5). In the Río Limón delta, sediment accumulation reduced the flooded 

area by about 150 m in length and about 1,700 m2 in area (fig. 4). Sediment accumulation was minimal in 

the Río Caonillas branch of Lago Dos Bocas; previous survey data indicate that the Lago Caonillas Dam 

acts as a sediment trap upstream of the branch (Soler-López, 2007). 

 The long-term drainage area sediment yield rate of Lago Dos Bocas has remained nearly constant. 

Based on a long-term sediment trapping efficiency of 79.4 percent (Soler-López, 2007), the sediment yield 

of the reservoir basin has decreased from 1,312 to 1,247 cubic meters per square kilometer, per year 

[(m3/km2472)/yr] from 2005 (Soler-López, 2007) to 2010 (table 1). The inter-survey drainage area sediment 

yield, however, decreased from 567 to 466 (m3/km2)/yr, a decline of about 18 percent. 

 Based on the current long-term storage-capacity loss of about 305,000 m3 per year estimated for the 

2005–2010 period, and assuming a constant sedimentation rate, the projected useful life of Lago Dos Bocas 

is about 55 years, ending in 2065. This predicted useful life of Lago Dos Bocas is nearly equal to the 

previously estimated useful life of 54 years (Soler-López, 2007).

Summary and Conclusions

 During January 2010, the U.S. Geological Survey, in cooperation with the Puerto Rico Aqueduct 

and Sewer Authority, conducted a bathymetric survey of Lago Dos Bocas to update the reservoir storage 

capacity and actualize the reservoir sedimentation rate by comparing the 2010 bathymetric survey data with 

2005 data. 

 The Lago Dos Bocas storage capacity was 17.26 million cubic meters in August 2005, which 

decreased to 16.74 million cubic meters by January 2010. The inter-survey (2005–2010) storage capacity 

loss was 1 percent, equaling a decrease of about 0.2 percent per year; this loss represents a reservoir 

sedimentation rate of about 104,000 cubic meters per year between 2005 and 2010. On a long-term basis 

(1942–2010), however, the sedimentation rate has remained nearly constant, decreasing from about 321,000 

to 305,000 cubic meters per year from 2005 to 2010. Most of the storage capacity loss of Lago Dos Bocas 

has occurred along its Río Grande de Arecibo and Río Limón branches in the form of flooded-area 

reduction. Sediment accumulation has been minimal along the Río Caonillas branch. The Lago Dos Bocas 

drainage area sediment yield has decreased by about 18 percent, from 567 cubic meters per square 

kilometer per year in 2005 to 466 cubic meters per square kilometer per year in 2010. On a long-term basis, 

however, the sediment yield has remained relatively unchanged, decreasing from 1,312 to about 1,247 

cubic meters per square kilometer, per year from 2005 to 2010. If the long-term sedimentation rate of 

305,000 cubic meters per year remains constant, the useful life of Lago Dos Bocas may end in 2065.

References Cited

Soler-López, L.R., 2007, Sedimentation history of Lago Dos Bocas, Puerto Rico, 1942-2005: U.S. 

Geological Survey Scientific Investigations Report 2007-5053, 36 p., 1 pl. 

http://pubs.usgs.gov/sir/2007/5053/.

Soler-López, 2010a, Sedimentation Survey of Lago Patillas, Puerto Rico, March 2007: U.S. 

Geological Survey Scientific Investigations Map 2010–3128, 1 plate., available at 

http://pubs.usgs.gov/sim/3128/.

Soler-López., 2010b, Sedimentation Survey of Lago de Cidra, Puerto Rico, August 2007: U.S. 

Geological Survey Scientific Investigations Map 3118, 1 plate., available at 

http://pubs.usgs.gov/sim/3118/.

North Coast

Superaqueduct Project 

Intake Pool

Basin area

Study

area

PUERTO RICO

EXPLANATION

Lago Dos Bocas

Station 50027100

Drainage basin boundary

Caño Tiburones flow direction

Tunnel flow direction 

Town or city

Lake-level monitoring station 

and identifier

ATLANTIC OCEAN

0

0 1

1

2

2

3

3

4

4 5 MILES

5 KILOMETERS

66°47'30" 66°32'30"

18°27'30"

18°15'00"

Base from U.S. Geological Survey digital data.

Río

Río

Río

Río

Río

Río

Caonillas

Caonillas

Río Río

Jauca

Río

Río

Río

Cidra

Río

Río

Grande

Río

Grande

Río

Río

Caricaboa

Saliente

Grande

de

de

de

Jayuya

Arecibo

Arecibo

Río

Río

Lago

Caonillas

Dam

Dam

Lago

Pellejas

Lago

Adjuntas

Lago Viví

Lago

Garzas

Lago

Dos Bocas

Lago

Jordan

Caño Tiburones

Tanamá

Limón

Limón

La

Venta

Yunes

Viví

Tunnel

Tunnel

Tunnel

Tunnel

Tunnel

Viví

Vacas

Pellejas

San Juan

HATILLO

BARCELONETA

JAYUYA

ARECIBO

ADJUNTAS

UTUADO

FLORIDA

Sedimentation Survey of Lago Dos Bocas, Utuado, Puerto Rico, January 2010

By Luis R. Soler-López

2012

Prepared in cooperation with the

PUERTO RICO ELECTRIC POWER AUTHORITY

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

SCIENTIFIC INVESTIGATIONS MAP 3217

 Sedimentation Survey of Lago Dos Bocas, Utuado, Puerto Rico, January 2010

By Luis R. Soler-López, 2012

66°40'30" 66°40' 39'30" 66°39'

18°20'

19'30"

18°20'30"

18°19'

18°18'30"

0 0.5

0 0.5 1 KILOMETER

1 MILE

Río Limón branch

Río Caonillas branch

Río Grande de Arecibo branch

Figure 5. Aerial photograph of Lago Dos Bocas and surroundings areas showing development.

Photograph from the Centro de Recaudaciónes de Ingresos Municipales (CRIM) 2007 Puerto Rico.

Table 1. Comparison between the August 2005 and the January 2010 sedimentation surveys of Lago 

Dos Bocas, Puerto Rico.

Data descriptor Year of survey

2005 2010

Capacity, in million cubic meters

Live storage, in million cubic meters

Dead storage, in million cubic meters

Years since construction 

Sediment accumulated, in million cubic meters

Inter-survey sediment accumulation, in million cubic meters

Long-term storage loss, in percent

Long-term annual loss of capacity, in million cubic meters

Annual loss of capacity, in percent

Inter-survey loss of capacity, in million cubic meters per year

Sediment trapping efficiency, in percent

Long-term sediment yield, in cubic meters per square kilometer 

Inter-survey sediment yield, in million cubic meters per year 

Year reservoir would fill with sediments 

per year

17.26

16.50

.76

63

20.24

.78

54

.321

.86

.13

72

1,312

567

2059

16.74

16.30

.44

68

20.76

.52

55

.305

.81

.104

72

1,247

466

2065

Table 2. Storage capacity for Lago Dos Bocas, Puerto Rico, 

January 2010.

Pool elevation, in meters Storage capacity,

above mean sea level in million cubic meters

89.92 

88.92 

87.92

86.92

85.92

84.92

83.92

82.92

81.92

80.92

79.92

78.92

77.92

76.92

75.92

74.92

73.92

72.92

71.92

70.92

69.92

68.92

67.92

16.74 

15.21 

13.79

12.46

11.20

10.01

8.89

7.85

6.89

5.98

5.14

4.36

3.64

3.05

2.49

1.99

1.52

1.10

.74

.43

.19

.05

.00