66°02'30" 02' 01'30" 01' 00'30" 66°00'

18°16'00"

16'30"

17'

17'30"

18'

18'30"

19'

18°19'30"

0 2,500 5,000 FEET

0 1,000 2,000 METERS

15.0

14.0

13.0

6.0

9.0

4.0

4.0

7.0

7.0

5.0

8.0

12.0

11.0

12.0

10.0

11.0

6.0

8.0

8.0

1.0

3.0

2.0

6.0

6.0

4.0

4.0

4.0

6.0

6.0

7.0

1.0

2.0

3.0

1.0

1.0

1.0

10.0

11.0

8.0

5.0

4.0

6.0

6.0

4.0

1.0

6.0

2.0

8.0

9.0

12.0

5.0

7.0

5.0

5.0

6.0

7.0

7.0

7.0

8.0

10.0

11.0

Dam

15.0 RESERVOIR’S BOTTOM CONTOUR —Depth of the reservoir bottom

on July 2009, in meters below maximum pool elevation of

41.14 meters above mean sea level. Contour interval 1 meter. 

Shoreline shown as red line equal to zero depth or 

maximum pool elevation. 

EXPLANATION

Shoreline modified from the U.S. Geological Survey quadrangle

Aguas Buenas, Puerto Rico, topography from photogrametric methods

from aerial photographs taken in 1941. Field cheked in 1943.

Revised from aerial photographs taken in 1967. Field cheked in 1969.

Lambert conformal conic projection.

Puerto Rico state plane coordinate system (Puerto Rico datum, 1940 adjustment).

EXPLANATION

2004

2009

Depth, in meters bellow maximum pool 

Elevation of 41.14 meters above mean sea level

~ 1.5 m

~ 1.5 m ~ 2.0 m

~ 2.0 m

~ 2.0 m

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 9,000 10,000

Distance upstream from the dam, in meters

0

2

4

6

8

14

12

10

25.00 16

27.00

29.00

31.00

33.00

35.00

37.00

39.00

41.00

43.00

0.00 5.00 10.00 15.00 20.00

2004

2009

EXPLANATION

Maximum pool elevation = 41.14 m

Storage volume, in million cubic meters

Elevation, in meters above mean sea level

66°02'30" 01'30" 00'30" 66°00' 65°59'30"

18°20'30"

19'30"

18'30"

17'30"

16'30"

18°15'30"

0 2,500 5,000 FEET

0 1,000 2,000 METERS

DAM

Figure 2. Bathymetric map of Lago Loíza, Puerto Rico, for July 2009.

Figure 5. Aerial photograph of Lago Loíza showing its long geomorphology resembling a natural river channel.

Photograph from the U.S. Army Corps of Engineers, Puerto Rico, 2007.

Figure 4. Longitudinal profiles along the thalweg of Lago Loíza, for 2004 and 2009.

Introduction

 Lago Loíza is a reservoir formed at the confluence of Río Gurabo and Río Grande de Loíza in the municipality 

of Trujillo Alto in central Puerto Rico, about 10 kilometers (km) north of the town of Caguas, about 9 km northwest of 

Gurabo, and about 3 km south of Trujillo Alto (fig. 1). The Carraizo Dam is owned and operated by the Puerto Rico 

Aqueduct and Sewer Authority (PRASA), and was constructed in 1953 as a water-supply reservoir for the San Juan 

Metropolitan area. The dam is a concrete gravity structure that is located in a shallow valley and has a gently sloping left 

abutment and steep right abutment. Non-overflow sections flank the spillway section. Waterways include an intake 

structure for the pumping station and power plant, sluiceways, a trash sluice, and a spillway.

 The reservoir was built to provide a storage capacity of 26.8 million cubic meters (Mm3) of water at the 

maximum pool elevation of 41.14 meters (m) above mean sea level (msl) for the Sergio Cuevas Filtration Plant that 

serves the San Juan metropolitan area. The reservoir has a drainage area of 538 square kilometers (km2) and receives an 

annual mean rainfall that ranges from 1,600 to 5,000 millimeters per year (mm/yr). The principal streams that drain into 

Lago Loíza are the Río Grande de Loíza, Río Gurabo, and Río Cañas. Two other rivers, the Río Bairoa and Río Cagüitas, 

discharge into the Río Grande de Loíza just before it enters the reservoir. The combined mean annual runoff of the Río 

Grande de Loíza and the Río Gurabo for the 1960-2009 period of record is 323 Mm3. Flow from these streams 

constitutes about 89 percent of the total mean annual inflow of 364 Mm3 to the reservoir (U.S. Geological Survey, 2009). 

Detailed information about Lago Loíza reservoir structures, historical sediment accumulation, and a dredge conducted 

in 1999 are available in Soler-López and Gómez-Gómez (2005).

 During July 8–15, 2009, the U.S. Geological Survey (USGS) Caribbean Water Science Center (CWSC), in 

cooperation with PRASA, conducted a bathymetric survey of Lago Loíza to update the reservoir storage capacity and 

estimate the reservoir sedimentation rate by comparing the 2009 data with the previous 2004 bathymetric survey data. 

The purpose of this report is to document the methods used to update and present the results of the reservoir storage 

capacity, sedimentation rates, and areas of substantial sediment accumulation since 2004.

Method of Survey and Analysis

 The field techniques and bathymetric data reduction processes used for the 2009 survey were performed 

following procedures established by the CWSC, and described in the previous bathymetric survey report for Lago 

Loíza (Soler-López and Gómez-Gómez, 2005). The July 2009 bathymetric survey was conducted using a 

Bathymetric/Land Survey System, developed by Specialty Devices Inc., that consists of a global positioning system 

(GPS) coupled to a digital depth sounder similar to the setup used for the survey conducted in January 2004 

(Soler-López and Gómez-Gómez, 2005). A total of 221 survey navigation lines were established at a spacing of about 

50 m, beginning at the dam and continuing upstream along the different river branches of the reservoir. Geographic 

position (accurate to within 2 m) and water depths (accuracy of 2 centimeters +/-1 percent of the measured depth) 

were acquired simultaneously using the GPS interfaced to the depth sounder. The pool elevation of the reservoir was 

recorded continuously at USGS lake-level station 50059000, Lago Loíza at Damsite near Trujillo Alto. The soundings 

were subsequently adjusted to represent water depths below the maximum pool elevation. 

 The 2009 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 January 2004 

bathymetric map of Lago Loíza (Soler-López and Gómez-Gómez, 2005). Data from the July 2009 survey were then 

used to generate a bathymetric map representing the reservoir bottom (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 Loíza was generated from the bathymetric map, and the 

reservoir volume was calculated using the GIS. The TIN surface model was used with the 2004–2009 digital data to 

generate a stage-storage table and curves as well as longitudinal bottom profiles along the thalweg of Lago Loíza 

(table 2, figs. 3 and 4).

Storage Capacity, Sedimentation Rate, and Useful Life 

 Bathymetric data indicate that the storage capacity of Lago Loíza decreased slightly from 17.53 Mm3 in 

January 2004 (Soler-López and Gómez-Gómez, 2005) to 16.42 Mm3 in July 2009 (table 1). This reduction of 1.11 

Mm3 between 2004 and 2009 (6.3 percent) is equivalent to an annual storage-capacity loss rate of about 222,000 cubic 

meters per year (m3/yr) for the period. Although the intersurvey sedimentation rate decreased by about 39 percent, the 

long-term rate has not changed substantially (table 1). As noted in Soler-López and Gómez-Gómez (2005), the 

long-term sedimentation rate of Lago Loiza has varied little over its history, remaining near 310,000 m3/yr; this finding 

supports their conclusion that under normal rainfall conditions, the sedimentation rate of Lago Loíza has reached an 

equilibrium state in which the reservoir will continue to lose capacity at a rate of about 310,000 m3/yr. Intense rainfall 

events may exacerbate this process, however, as demonstrated in Soler-López (2001). 

 A comparison of longitudinal bottom profiles for 2004 and 2009 indicates that sediment accumulation in 

Lago Loíza for this period averaged about 1.5 m from 0 to 2,500 m upstream from the dam, and about 2.0 m from 

2,800 to 5,600 m upstream (fig. 4). Sediment deposition from 5,600 m farther upstream was minimal. 

 The long-term sediment yield for the Lago Loíza drainage area has been calculated historically by dividing 

the sediment accumulation in the reservoir by the sediment trapping efficiency, reservoir drainage area, and years since 

the sediment accumulation began (typically the final construction date). The reservoir sediment trapping efficiency 

was usually determined by using the empirical relation established by Brune (1953), which uses the ratio of the 

reservoir water-storage capacity to mean annual inflow volume (C/I). Heinemann (1981), however, associates a 

reservoir trapping efficiency to the following series of physical properties and operational procedures: (1) sediment 

particle-size distribution, (2) the time and rate of water inflow to the reservoir, (3) the reservoir size and shape, (4) the 

location of the outlet structure, and (5) the location and mechanism of discharge schedules. Of these, two are of 

primary importance in the sediment trapping efficiency of Lago Loíza: the reservoir size and shape, and the location 

and mechanism of water releases. Lago Loíza is a long and relatively shallow reservoir that resembles a natural river 

channel (fig. 5). This morphological characteristic imposes hydraulic conditions that increase water velocities during 

floods, preventing sediment particles from settling on the reservoir bottom. In addition, water discharges during these 

events are made through eight radial gates that open upwards. According to the 2009 bathymetry, the Lago Loíza 

bottom is at an elevation of about 26 m above msl. Based on the concrete spillway elevation of 

31.0 m above msl, the Lago Loíza reservoir bottom is 5 m below the structure where water overflows 

during floods. These morphological and structural conditions probably prevent fine and mid-size 

sediment particles from being deposited, keeping them in suspension and facilitating sediment 

pass-through over the spillway during floods. If these assumptions are correct, the Lago Loíza 

sediment trapping efficiency may have been overestimated previously consequently underestimating 

the drainage area sediment yield. 

 Quiñones (2009) estimated empirically high sediment loads that could have entered Lago 

Loíza during September 21–24, 2008, when a trough (a low-pressure atmospheric system) affected 

most of Puerto Rico. The estimate was based the previously published sediment trapping efficiency 

of 76 percent (Soler-López and Gómez-Gómez, 2005), historical USGS suspended sediment 

transport curves of the Lago Loíza tributaries, and peak discharge data compiled from USGS 

streamflow records. It was estimated that for the aforementioned event, a total of 1.22 metric tonnes 

of sediment (suspended and bed load) were deposited in the reservoir; this sediment load was 

equivalent to 1.39 Mm3 (Quiñones, 2009). In the paper (Quiñones, 2009), the USGS reported a 

long-term sedimentation rate of 0.31 million cubic meters per year (Mm3/yr) between 2004 and 2008 

was applied to estimate a Lago Loíza storage capacity before the September 21–24 trough, of 16.29 

Mm3 [17.53 – (0.31 × 4) = 16.29]. By subtracting 1.39 Mm3 from the prorated 2008 volume of 16.29 

Mm3, the Lago Loíza storage capacity was reduced to 14.90 Mm3 by the September 2008 trough 

according to Quiñones (2009). 

 The empirically estimated Lago Loíza storage capacity, however, is in disagreement with 

the 2009 bathymetric survey results by 1.52 Mm3. A possible explanation for the discrepancy 

between the estimated and the measured storage capacity of Lago Loíza is that the sediment trapping 

efficiency of the reservoir is lower than that estimated using the C/I ratio of Brune (1953), which 

does not take into account the findings of Heinemann (1981). For that reason, an adjusted sediment 

trapping efficiency of Lago Loíza was estimated for this study by dividing the 1.52 Mm3 difference 

by 16.42 Mm3 (1.52/16.42 = 0.09) and subtracting the result from the previously estimated trapping 

efficiency of 76 percent, yielding a new estimated sediment trapping efficiency of 67 percent

(76 – 9 = 67). The estimate by Quiñones (2009), therefore, was useful for determining a more 

realistic sediment trapping efficiency for Lago Loíza, which reflects the newly considered 

morphological characteristics and location and mechanism of discharge schedules.

 In addition to the corrections just described, the corrected sediment trapping efficiency of 

Lago Loíza also increases the reservoir-drainage-area sediment-yield by approximately 10 percent 

(table 1). The Lago Loíza sedimentation rate between 2004 and 2009 decreased from 364,000 m3/yr 

to 222,000 m3/yr (table 4), a 39 percent reduction. This trend in intersurvey drainage area sediment 

yield during the late 2000s has also been observed at nearby basins, such as the Lago Patillas and 

Lago de Cidra drainage areas where reductions ranged from 75 to 13 percent, respectively 

(Soler-López, 2010a, b). On a long-term basis the inter-survey drainage area sediment yield has 

remained nearly constant as has the reservoir sedimentation rate. The drainage area sediment yield 

was 882 cubic meters per square kilometer per year (m3/km2)/yr in 2004 and decreased slightly to 

854 (m3/km2)/yr in 2009, a difference of only 3 percent, which is not statistically significant. Based 

on a long-term storage-capacity loss of about 310,000 m3/yr, the projected useful life of Lago Loíza 

is about 53 years, ending in 2062 (table 1).

Summary and Conclusions

 During July 2009, the U.S. Geological Survey, in cooperation with the Puerto Rico 

Aqueduct and Sewer Authority, conducted a bathymetric survey of Lago Loíza to update the 

reservoir storage capacity and update the reservoir sedimentation rate by comparing 2009 

bathymetric survey data with 2004 data.

 The Lago Loíza storage capacity was 17.53 million cubic meters in January 2004, which 

decreased to 16.42 million cubic meters by July 2009. The inter-survey (2004-2009) storage 

capacity loss is about 6 percent, equivalent to a decrease of about 1.2 percent per year. This loss 

represents a reservoir sedimentation rate of about 222,000 cubic meters per year between 2004 and 

2009. On a long-term basis (1953–2009), however, the sedimentation rate has remained constant at 

about 310,000 cubic meters per year.

 Most of the storage capacity loss of Lago Loíza (averaging about 2 meters) occurred 

approximately 2,800 to 5,600 meters upstream from the dam. Sediment accumulation was observed 

to be 1.5 meters from approximately 0 to 2,500 meters upstream of the dam, however sediment 

deposition was practically unnoticeable from 5,600 meters to the lake headwaters. The long-term 

sediment yield of the Lago Loíza drainage area has remained nearly constant, totaling about 854 

cubic meters per square kilometer per year. Based on a long-term storage capacity loss of about 

310,000 m3/yr, the projected useful life of Lago Loíza is about 53 years, ending in 2062.

References Cited

Brune, G.M., 1953, Trap efficiency of reservoirs: Transactions of the American Geophysical Union, 

v. 34, p. 407–418.

Heinemann, H.G., 1981, New sediment trap efficiency curve for small reservoirs: Water Resources 

Bulletin, v. 7, p. 825–830.

Quiñones, Ferdinand, 2009, Análisis de la reducción en la capacidad del embalse Loíza debido a los 

sedimentos descargados durante la vaguada del 21 al 24 de septiembre de 2008, Dimensión, 

Colegio de Ingenieros y Agrimensores de Puerto Rico, División de Ingeniería y Agrimensura, 

año 23, vol. 2. 

Soler-López, Luis R., 2001, Sedimentation survey results of the principal water-supply reservoirs of 

Puerto Rico, in Sylva, W.F., ed., Proceedings of the Sixth Caribbean Islands Water Resources 

Congress, Mayagüez, Puerto Rico, February 2001, unpaginated cd.

Soler-López, Luis R., 2010b, Sedimentation survey of Lago de Cidra, Puerto Rico, August 2007: 

U.S. Geological Survey Scientific Investigations Map 2010–3118.

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

Geological Survey Scientific Investigations Map 2010–3128.

Soler-López, Luis R., and Gómez-Gómez, Fernando, 2005, Sedimentation survey of Lago Loíza, Puerto Rico, January 

2004: U.S. Geological Survey Scientific Investigations Report 2005–5239, 26 p.

U.S. Geological Survey, 2009, Caribbean Water Science Center Water Data Report— Water year 2009: Accessed 

October 2010 at http://wdr.water.usgs.gov/wy2009 .

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

Lago Loíza for 2004 and 2009.

Sedimentation Survey of Lago Loiza, Puerto Rico, July 2009

By Luis R. Soler-López and Nargies A. Licha-Soler

2012

Prepared in cooperation with the

PUERTO RICO AQUEDUCT AND SEWER AUTHORITY

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

SCIENTIFIC INVESTIGATIONS MAP 3219

 Sedimentation Survey of Lago Loíza, Puerto Rico, July 2009

By Soler-López, L.R., and Licha-Soler, N.A. 2012

0 1

1

2

2

3

3

4

0 4 5 KILOMETERS

5 MILES

Aguas Buenas

Caguas

Juncos

Sierra de Cayey

San Lorenzo

Las Piedras

Gurabo

Lago Loíza

Municipio

Drainage basin

boundary

EXPLANATION

Carraízo Dam

18°15'

18°07'30"

18°22'30"

66°07'30" 66°00' 65°52'30"

Location of the upper Río Grande

 de Loíza basin in Puerto Rico

San Juan

Mayagüez

Lago Loíza Basin

Ponce

50059000

Lake-elevation station 

and number

Río

Grande

de

Loíza

Luquillo

Sierra de

Trujillo Alto

50059000

Río

Río

Río

Río

Cañas

Bairoa

Caguitas

Gurabo

Base from U.S. Geological Survey digital data.

Figure 1. Location of Lago Loíza in the Río Grande de Loíza Basin, Puerto Rico.

Table 1. Comparison between the 2004 and the July 2009 sedimentation surveys of Lago Loíza, Puerto Rico. 

[ —, not applicable].

Descriptor Year of survey

2004 2009

Storage 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 cubic meters 

Annual loss of capacity, in percent

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

Storage capacity added by dredging, in million cubic meters 

Inter-survey drainage area sediment yield,

 in cubic meters per square kilometer per year

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

Dredged volume loss, in million cubic meters

Percent of dredge volume loss since 1999

Percent of annual dredge volume loss from 2004 to 2009

Year that the reservoir would fill with sediment

per year

17.53

17.28

 .25

51

16.12

1.82

60

316,078

1.4

364,000

5.78

1,015

882 

1.82

31

6.2

2060

16.42

16.34

 .08

56

17.23

1.11

64

307,679

 .8

222,000

—

616 

854 

2.93

51

3.8

2062

Table 2. 

July 2009.

Storage capacity for Lago Loíza, Puerto Rico, 

Pool elevation, in 

above mean sea 

meters 

level in 

Storage capacity,

million cubic meters

41.14 

40.14 

39.14

38.14

37.14

36.14

35.14

34.14

33.14

32.14

31.14

30.14

29.14

28.14

28.14

27.14

26.14

16.42 

14.05 

11.85

9.83

8.00

6.39

5.01

3.88

2.91

2.06

1.32

.74

.31

.08

.01

.01

.00