Prepared in cooperation with the

PUERTO RICO AQUEDUCT AND SEWER AUTHORITY

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

SCIENTIFIC INVESTIGATIONS MAP 3218

 Storage Capacity and Sedimentation Trends

of Lago Garzas, Puerto Rico, 1996-2007

By Luis R. Soler-López, 2012

Storage Capacity and Sedimentation Trends of Lago Garzas, Puerto Rico, 1996-2007

By Soler-López, L.R. 

2012

Figure 3. Bathymetric map of Lago Garzas, Puerto Rico, July 2007.

66°45'00" 66°44'30"

18°08'00"

18°08'30"

1.0

1.0

4.0

8.0

21.0

14.0

12.0

18.0

30.0

17.0

9.0

5.0

29.0

26.0

22.0

13.0

4.0

3.0

1.0

9.0

Shoreline modified from the U. S. Geological Survey quadrangle Adjuntas, Puerto Rico, 1960, photo revised 1977. 

Lambert Conic Conformal Projection. Puerto Rico State Plane coordinate system (Puerto Rico Datum, 1940 adjustment)

RESERVOIR BOTTOM CONTOUR- Depth of the reservoir bottom 

in 2007, in meters below spillway elevation of 736.12 meters above 

mean sea level. Contour interval 1-meter. 

Shoreline shown as red line equal to zero depth or

spillway elevation of 736.12 meters above mean sea level.

Spillway structure

0 1,000 2,000 FEET

0 250 500 METERS

Río Vacas branch

Southeastern branch

Northwestern branch

Northern branch

EXPLANATION

Basin area

Study

area

PUERTO RICO

EXPLANATION

Lago Garzas basin area

Lago Garzas 

near Adjuntas

Station 50020100

Drainage basin boundary

Caño Tiburones flow direction

Tunnel flow direction 

Town or city

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"

Figure 1. Location of Lago Garzas in the upper Río Grande de Arecibo basin, Puerto Rico.

Base from U.S. Geological Survey digital data.

HATILLO

BARCELONETA

JAYUYA

ARECIBO

ADJUNTAS

UTUADO

FLORIDA

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

Lago

Dos Bocas

Lago

Pellejas

Lago

Adjuntas

Lago Viví

Lago

Garzas

Lago

Garzas

Lago

Jordan

Caño Tiburones

Tanamá

Limón

Limón

La

Venta

Yunes

Viví

Tunnel

Tunnel

Tunnel

Tunnel

Tunnel

Viví

Vacas

Pellejas

Introduction

 Lago Garzas is located in west-central Puerto Rico, about 3.5

kilometers southwest of the town of Adjuntas, in the confluence of the Río 

Vacas and three other unnamed tributaries (fig. 1). The dam is owned and 

operated by the Puerto Rico Electric Power Authority (PREPA), and was 

constructed in 1943 for hydroelectric power generation and municipal

water use along the southern coast. 

 The dam is a semi-hydraulic earthfill embankment lined with

boulders, and has a height of 61.57 meters, a top width of 9.14 meters, a 

base width of 365.76 meters, and a crest length of 227.37 meters; State 

Road PR-518 crosses the top of the dam. A morning-glory-type spillway is 

located near the west abutment of the dam at an elevation of 736.12 meters 

above mean sea level (Puerto Rico Water Resources Authority, 1969).

Figure 2 shows an aerial photograph of the Lago Garzas earthfill dam and 

the morning-glory spillway section. Additional information and

operational procedures are provided in Soler-López and others (1999).

 During July 17–18, 2007, the U.S. Geological Survey (USGS)

Caribbean Water Science Center, in cooperation with the Puerto Rico

Aqueduct and Sewer Authority, conducted a bathymetric survey of Lago 

Garzas to update the reservoir storage capacity and update the reservoir 

sedimentation rate by comparing the 2007 data with the previous 1996 

bathymetric survey results. The purpose of this report is to describe and 

document the USGS sedimentation survey conducted at Lago Garzas 

during July 2007, including the methods used to update the reservoir 

storage capacity, sedimentation rates, and areas of substantial sediment 

accumulation since 1996.

Method of Survey and Analysis

 The field techniques and bathymetric data reduction processes 

used for the 2007 survey were performed following procedures

established by the USGS Caribbean Water Science Center and described 

in the previous bathymetric survey report of Lago Garzas (Soler-López 

and others, 1999). The July 2007 bathymetric survey was conducted using 

a differential global positioning system (DGPS) coupled to a digital depth 

sounder similar to the setup used for the survey conducted in September 

1996 (Soler-López and others, 1999). Survey navigation lines were 

66°45'00" 44'50" 44'40" 44'30" 66°44'20"

18°08'50"

08'40"

08'30"

08'20"

08'10"

08'00"

07'50"

18°07'40" 0 0.125 0.25 0.5 KILOMETERS

0 0.125 0.25 0.5 MILES

Figure 4. Aerial photograph of the Lago Garzas and its immediate surroundings. 

Photograph from the U.S. Army Corps of Engineers, Puerto Rico.

~6.0 m

~5.0 m

0

5

10

15

20

25

30

35

40

0

5

10

15

20

25

30

35

40

0

5

10

15

20

25

30

35

40

0

5

10

15

20

25

30

35

40

Vertical scale greatly exaggerated

0 250 500 750 1,000 1,250 1,500 1,750 2,000 2,250 2,500

2007

EXPLANATION

1996

Figure 6. Longitudinal bottom profiles along the thalweg of Lago Garzas, within the northern, northwestern, Río Vacas, and 

southeastern branches of the reservoir, for 1996 and 2007.

Northern branch

Northwestern branch

Río Vacas branch

Southeastern branch

Depth, in meters below spillway elevation of 736.12 meters above mean sea level

Distance from the dam, in meters

network (TIN) surface model of Lago Garzas was then created from the 

bathymetric map, and the reservoir volume was calculated using GIS. The 

TIN surface model was used with the digital data for 1996 and 2007 to 

generate a stage-storage curve and table, as well as longitudinal profiles 

along the different tributary branches of Lago Garzas.

Storage Capacity, Sedimentation Rates, and Useful Life

 The storage capacity of Lago Garzas decreased from 5.11 million 

cubic meters in 1996 (Soler-López, 1999), to 4.99 million cubic meters by 

July 2007 (table 1). This decrease represents a reduction from 1996 to 

2007 of 120,000 cubic meters (or 2.3 percent over 11 years), for an annual 

storage-capacity loss rate of about 10,909 cubic meters. The long-term 

sedimentation rate of Lago Garzas has decreased slightly from 13,019 

cubic meters per year in 1996 to 12,656 cubic meters per year in 2007 

(table 1). The nearly unchanged sedimentation rate of Lago Garzas from 

1996 to 2007 (table 1) may be due to the largely undisturbed Lago Garzas 

drainage area (fig. 4). An actualized storage capacity table and curve were 

generated by calculating the TIN volume at 1-meter elevation intervals 

(table 2, fig. 5). 

 Sediment accumulation within the majority of Lago Garzas 

between 1996 and 2007 has been minimal, with the exception of one 

deposition area along the northwestern branch of the reservoir (fig. 6). The 

profiles shown in figure 6 indicate that a patch of sediment about 5 meters 

thick has accumulated along the northwestern branch of the reservoir 

between about 550 to 650 meters upstream of the dam. The Lago Garzas 

long-term drainage area sediment yield rate has remained relatively 

unchanged, decreasing slightly from 565 to 550 cubic meters per square 

kilometer per year from 1996 to 2007 (table 1; Soler-López and others, 

1999).

 As of 2007, Lago Garzas has lost 14 percent of its original storage 

capacity of 5.80 million cubic meters (Soler-López and others, 1999). 

Based on the current long-term storage-capacity loss of 12,656 cubic 

meters per year estimated for 2007, the Lago Garzas has an estimated 

useful life of about 394 years, which may end by the year 2401.

Summary and Conclusions

 During July 2007, the U.S. Geological Survey, in cooperation 

with the Puerto Rico Aqueduct and Sewer Authority, conducted a 

bathymetric survey of Lago Garzas to update the reservoir storage 

capacity and estimate the reservoir sedimentation rate by comparing the 

2007 data with the previous 1996 bathymetric survey results.

 The Lago Garzas storage capacity in September 1996 was 5.11 

million cubic meters, and decreased to 4.99 in million cubic meters by 

July 2007. This difference represents a reservoir storage capacity loss 

of 120,000 cubic meters between 1996 and 2007. The long-term 

(1943-2007) sedimentation rate of the reservoir is 12,656 cubic meters 

per year. As of 2007, the reservoir has experienced a total storage loss 

of 14 percent compared to its original storage capacity of 5.80 million 

cubic meters. 

 Most of the storage capacity loss of Lago Garzas is distributed 

uniformly across the reservoir bottom; however, one area of sediment 

deposition is evident along the northwestern tributary branch, where an 

average of about 5 meters of sediment has accumulated between 1996 

and 2007. The long-term Lago Garzas drainage area sediment yield has 

remained relatively unchanged from 1996 to 2007, decreasing slightly 

from 565 to 550 cubic meters per square kilometer per year.

 The 2007 life expectancy of about 394 years for Lago Garzas is 

not a pressing concern, assuming a long-term storage-loss rate of about 

12,656 cubic meters per year. Sediment accumulation along the 

northwestern and southeastern branches of the reservoir, however, will 

continue forward towards the deepest parts of Lago Garzas, which will 

eventually start filling the reservoir near the dam. If the long-term 

sedimentation rate of 12,656 cubic meters estimated for 2007 

continues, the useful life of Lago Garzas may end by the year 2401.

References Cited

Puerto Rico Water Resources Authority, 1969, Engineering and 

Construction Division: Inspection and Maintenance Reports of the 

Garzas Dam and Related Facilities, 8 p., 4 pls.

Soler-López, L.R., Webb, R.M.T., and Pérez-Blair, Francisco, 1999, 

Sedimentation survey of Lago Garzas, Puerto Rico, September 1996: 

U.S. Geological Survey Water-Resources Investigations Report 

99-4143, 20 p., 2 pls.

established at 50-meter intervals and

oriented parallel to the dam face of the 

reservoir, continuing upstream to the

reservoir tail along the different river 

branches. Geographic positions and water 

depths were acquired simultaneously

using a DGPS interfaced to a depth 

sounder. The pool elevation of the

reservoir was measured at the USGS 

lake-level station 50020100, Lago Garzas 

near Adjuntas, Puerto Rico. The soundings 

were subsequently adjusted to represent 

water depths below the spillway elevation. 

 The 2007 data were stored 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 1996

bathymetric map of Lago Garzas reservoir 

(Soler-López and others, 1999). The field 

data were used to generate a bathymetric 

map representing the reservoir bottom in 

July 2007 (fig. 3). A triangulated irregular 

Table 1. Lago Garzas data comparison for 1996, and July 2007. 

Data descriptor Year of survey

1996 2007

Years since construction1

Years since previous survey

Total storage capacity, 

in million cubic meters

Sediment accumulated since construction, 

in million cubic meters

Sediment accumulation since previous survey,

in million cubic meters

Storage loss since construction, in percent

Annual loss of capacity since construction, 

in cubic meters

Annual loss of capacity since construction, 

in percent

Estimated sediment trapping efficiency,

in percent

Sediment yield, in cubic meters 

per square kilometer per year

Estimated year the reservoir would fill

with sediments

53

18

5.11

.69

.47

12

13,019

.23

95

565

2388

64

11

4.99

.81

.12

14

12,656

.22

95

550

2401

1 Using the construction date of 1943.

Table 2. 

July 2007, 

Storage capacity for Lago Garzas, Puerto Rico, 

Pool elevation, in 

above mean sea 

meters 

level

Storage capacity, in 

million cubic meters

736.12 

735.12 

734.12 

733.12 

732.12 

731.12 

730.12 

729.12 

728.12 

727.12 

726.12 

725.12 

724.12 

723.12 

722.12 

721.12 

720.12 

719.12 

718.12 

717.12 

716.12 

715.12 

714.12 

713.12 

712.12 

711.12 

710.12 

709.12 

708.12 

707.12 

706.12 

 4.99

 4.56

 4.18

 3.82

 3.49

 3.20

 2.92

 2.66

 2.41

 2.19

 1.98

 1.79

 1.61

 1.44

 1.28

 1.13

 1.00

 .87

 .76

 .65

 .55

 .46

 .38

 .31

 .24

 .19

 .13

 .09

 .05

 .02

 .00

Normal pool elevation = 736.12 meters above mean sea level

740.00

700.00

705.00

710.00

715.00

720.00

725.00

730.00

735.00

0.0 1.00 2.00 3.00 4.00

Storage volume, in million cubic meters

5.00 6.00

Elevation, in meters above mean sea level

Figure 5. Relation between pool elevation and storage capacity of 

Lago Garzas for 1996 and 2007.

2007

EXPLANATION

1996

Figure 2. Aerial photograph of the Lago Garzas earthfill dam and spillway section, taken during a dam safety inspectio

in May 2006. Photograph courtesy of the Puerto Rico Electric Power Authority. 

Spillway

Earthfill dam

Spillway

Earthfill dam

n