15.0

14.0

7.0

14.0

12.0

9.0

5.0

6.0

5.0

7.0

3.0

5.0

1.0

8.0

3.0

4.0

6.0

6.0

2.0

7.0 1.0 4.0

14.0

14.0

13.0

3.0

2.0

5.0

4.0

13.0

3.0

14.0

13.0

12.0

11.0

1.0

11.0

11.0

Sedimentation Survey of Lago Loíza, 

Puerto Rico, January 2004 

U.S. Geological Survey 

Scientific Investigations Report 2005-5239

Prepared in cooperation with the 

Puerto Rico Aqueduct and Sewer Authority 

U.S. Department of the Interior 

U.S. Geological Survey 
Sedimentation Survey of Lago Loíza, 

Puerto Rico, January 2004

By Luis R. Soler-López and Fernando Gómez-Gómez

U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the 

PUERTO RICO AQUEDUCT AND SEWER AUTHORITY

Scientific Investigations Report 2005-5239
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

P. Patrick Leahy, Acting Director

U.S. Geological Survey, Reston, Virginia: 2005

For sale by U.S. Geological Survey, Information Services

Box 25286, Denver Federal Center

Denver, CO 80225

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

World Wide Web: http://www.usgs.gov/

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Soler-López, L.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., 2 pls.
 iii

Contents

Abstract. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Dam, Reservoir, and Basin Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Method of Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Field Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Data Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Historical Sediment Accumulation and Effect on Reservoir Storage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Sediment Trapping Efficiency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Sediment Yield and Reservoir Life Expectancy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Cited References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Plates

[Plates are in pocket]

1. Lago Loíza, Puerto Rico, Bathymetry, January 2004, Northern Section

2. Lago Loíza, Puerto Rico, Bathymetry, January 2004, Southern Section
iv 

Figures

1–6. Maps showing:

1. Location of Lago Loíza in the Río Grande de Loíza Basin, Puerto Rico. . . . . . . . . . . . . . . . . . . . . . . . . .3

2. Mean annual rainfall distribution in Puerto Rico. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

3. Planned cross-section locations for the January 2004 bathymetric survey of 

Lago Loíza, Puerto Rico. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

4. Actual track lines of the January 2004 bathymetric survey of Lago Loíza, 

Puerto Rico. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

5. Triangulated irregular network surface model of Lago Loíza, Puerto Rico, 

for January 2004. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

6. Reference longitudinal distance along the thalweg of Lago Loíza, Puerto Rico. . . . . . . . . . . . . . . 10

7–11. Graphs showing:

7. Relation between water-storage capacity and pool elevation of Lago Loíza, 

Puerto Rico, for 1994, 1999, and 2004 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

8. Selected cross sections generated from the TIN surface models of Lago Loíza, 

Puerto Rico, for 1994, 1999, and 2004 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

9. Longitudinal profiles along the thalweg of Lago Loíza, Puerto Rico, generated 

from the TIN surface models for 1994, 1999, and 2004. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

10. Lago Loíza storage capacities and sediment accumulation rates from 1953 

to 2004. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

11. Reservoir trapping efficiency as a function of the ratio between water-storage 

capacity and annual water inflow volume. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Tables

1. Principal characteristics of Lago Loíza and structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4

2. Comparison between the 1994, 1999, and 2004 sedimentation surveys of Lago Loíza, 

Puerto Rico . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3. Storage capacity for Lago Loíza, Puerto Rico, January 2004. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

4. Suspended-sediment loads at tributary streams to Lago Loíza for water years 

1996 to 2002 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
 v

Conversion Factors and Datum, Acronyms, and Translations

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F = (1.8 x °C) + 32

Datum:

Horizontal Datum - Puerto Rico Datum, 1940 Adjustment

Sea level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929) - 

a geodetic datum derived from a general adjustment of the first-order level nets of the United States and Canada, 

formerly called "Sea Level Datum of 1929". 

Acronyms used in this report:

BLASS Bathymetric/Land Survey System

DGPS Differential Global Positioning System

GIS Geographic Information System

GPS Global Positioning System

PRASA Puerto Rico Aqueduct and Sewer Authority

PRIFA Puerto Rico Infrastructure Financing Authority

TIN Triangulated Irregular Network

USGS U.S. Geological Survey

Translations:

Spanish English

Lago Lake (in Puerto Rico, also reservoir)

Río River

municipio municipality

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft)

kilometer (km) 0.6214 mile (mi) 

Area

square meter (m2) 0.0002471 acre

square kilometer (km2) 247.1 acre

square meter (m2) 10.76 square foot (ft2)

Volume

cubic meter (m3) 35.31 cubic foot (ft3) 

cubic meter (m3) 0.0008107 acre-foot (acre-ft) 

Flow rate

cubic meter per year (m3/yr) 0.000811 acre-foot per year (acre-ft/yr) 

cubic meter per day (m3/d) 35.31 cubic foot per day (ft3/d)

millimeter per year (mm/yr) 0.03937 inch per year (in/yr)

Mass

gram (g) 0.03527 ounce, avoirdupois (oz)

megagram (Mg) 1.102 ton, short (2,000 lb)

megagram (Mg) 0.9842 ton, long (2,240 lb)

metric ton per day 1.102 ton per day (ton/d)
vi 
Sedimentation Survey of Lago Loíza, Puerto Rico, 

January 2004

By Luis R. Soler-López and Fernando Gómez-Gómez

Abstract

A sedimentation survey of Lago Loíza reservoir was 

conducted during January 2004 to determine the reservoir 

water-storage capacity. After a bathymetric survey performed 

on November 1994, the drainage basin was impacted by 

Hurricanes Hortense on September 10, 1996, and Georges on 

September 21, 1998. In addition, dredging was conducted 

between 1997 and 1999 to increase the storage capacity by 

about 5.8 million cubic meters to increase the public-supply 

water safe yield at the Sergio Cuevas Filtration Plant. 

The results of the 2004 bathymetric survey indicate that 

the storage capacity of Lago Loíza reservoir was 17.53 million 

cubic meters or 3.34 million cubic meters greater than the 

November 1994 capacity. The long-term reservoir 

sedimentation rate is about 310,000 cubic meters per year; 

however, higher annual rates have occurred since the reservoir 

construction, but these rates are considered short-term, low 

occurrence, and principally associated with land-use 

disturbances and extreme runoff events.

The estimated sediment trapping efficiency of Lago Loíza 

was as follows: 72 percent in 1994, 78 percent in 1999, and 76 

percent in 2004. The Lago Loíza drainage area sediment yield 

for the period 1994 to 2004 was 859 cubic meters per square 

kilometer per year, about one-and-a-half times higher than the 

previously (1990-1994) estimated value of 600 cubic meters per 

square kilometer per year.

Based on the 2004 bathymetric survey, the continued 

utility of Lago Loíza as a water-supply reservoir is about 56 

more years. Based on the average long-term reservoir 

sedimentation rate of 310,000 cubic meters per year, however, 

the storage added by the dredging, which was conducted from 

1997 to 1999, will be lost by 2018.

Introduction

The Lago Loíza reservoir has a drainage area of 538 square 

kilometers (km2) and constitutes the principal source of public 

water supply to the eastern part of the San Juan metropolitan 

area. During 2003, the reservoir supplied an average of 394,000 

cubic meters per day (m3

/d) to the Puerto Rico Aqueduct and 

Sewer Authority (PRASA) Sergio Cuevas Filtration Plant 

(W. Molina, U.S. Geological Survey, written commun., 2004). 

The ability to maintain this draft rate, however, is determined 

primarily by the available storage, which by 1994 on a longterm basis (1953-1994) was reduced, on average, by about 

337,000 cubic meters per year (m3

/yr) by sedimentation, based 

on several sedimentation surveys conducted in the past (Webb 

and Soler-López, 1997). The effect of sedimentation became 

evident during the drought of 1994, when rainfall in 

northwestern Puerto Rico was about 30 percent below normal 

between August 1993 and September 1994. At the time of the 

drought, the available storage capacity in Lago Loíza was about 

14.2 million m3

, which was about 7 million m3 below the 

required storage capacity to have sustained a draft rate of about 

360,000 m3

/d (Rodríguez-Martínez and others, 2002). In order 

to alleviate a future water supply short-fall, the Puerto Rico 

Infrastructure Financing Authority (PRIFA) authorized the 

dredging of Lago Loíza between 1997 and 1999, to increase the 

reservoir storage capacity by about 5.8 million m3

.

Between the drought of 1994 and the completion of the 

dredging in 1999, two hurricanes affected the island (Hurricane 

Hortense on September 10, 1996, and Hurricane Georges on 

September 21, 1998). Because of the high sediment loads 

associated with hurricanes (Gellis, 1993), Hurricanes Hortense 

and Georges negatively impacted the reservoir by 

sedimentation. Sedimentation surveys conducted at other 

reservoirs in Puerto Rico indicated that these hurricanes 

reduced the water-storage capacities of the reservoirs in excess 

of 10 percent of their original volumes (Soler-López, 2001a, 

2001b). In order to determine the available storage in Lago 

Loíza, the PRASA requested the USGS to conduct a 

bathymetric survey. The purpose of this report is to document 

the results of the bathymetric survey and to provide the PRASA 

officials with the necessary information to effectively manage 

the reservoir.

The bathymetric survey was conducted using a global 

positioning system (GPS) interfaced to a depth sounder. The 

field data were then transferred into a geographic information 

system (GIS), which was used to determine the existing storage 

capacity, the sedimentation rates, sediment distribution, and to 

anticipate the useful life of the reservoir as a major public 

water-supply source. Data from the 2004 bathymetric survey 

were compared with the 1999 post-dredge storage capacity of 

Lago Loíza derived from information provided by the PRIFA, 

and with the previous USGS 1994 bathymetric survey of Lago 

Loíza.
2 Sedimentation Survey of Lago Loíza, Puerto Rico, January 2004

From the 1994, 1999, and 2004 data sets, longitudinal 

bottom profiles along the central portion of Lago Loíza, as well 

as transverse cross sections representing the reservoir’s bottom 

from shore to shore, were generated and overlaid to locate areas 

of sediment accumulation from 1999 to 2004. In addition, the 

reservoir pool elevation and storage capacity relation were 

generated at 1-meter (m) elevation intervals for each year to 

generate a graph representing the reservoir volume at different 

elevations and to show the volume variations during these 

surveys.

Dam, Reservoir, and Basin Characteristics

The Río Grande de Loíza was impounded in March 1953 

with the completion of the Carraízo Dam, 21.7 kilometers (km) 

upstream from the river’s outlet to the Atlantic Ocean. The 

reservoir is located between the municipios of Caguas , Gurabo, 

and Trujillo Alto in northeastern Puerto Rico, and the dam 

structure is about 10 km north of Caguas, about 9 km northwest 

of Gurabo, and about 3 km south of Trujillo Alto (fig. 1).

The dam is a concrete gravity structure located in a shallow 

valley with a gently sloping left abutment and a 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 dam was modified in 1977 with eight radial gates 

installed above the spillway, each 11.88 m wide by 9.14 m high. 

The gates had a 10.67-m radius skinplate of riveted fabrication 

with upper and lower arms at each side supported by pin 

bearings on the downstream portion of the piers. In 1977, flash 

boards were added to raise the maximum pool elevation from 

40.14 to 41.14 m above mean sea level. The additional storage 

capacity provided by the flash boards was in the range of 

about 2.7 million m3 (Webb and Soler-López, 1997). Recently 

(2001-2002), as part of a major maintenance and safety 

overhaul, all radial gates were replaced with new structures 

having the same dimensions. The principal dam and reservoir 

characteristics are given on table 1.

The reservoir was built to provide a storage capacity of 

26.80 million m3 of water at an elevation of 41.14 m above 

mean sea level for the Sergio Cuevas Filtration Plant, serving 

the San Juan metropolitan area. Although penstocks and 

turbines for power generation were originally included to 

provide an independent source of power for dam operations, 

hydroelectric power generation was later abandoned due to the 

infrequency of reservoir water releases (Ivari, 1981).

 The reservoir has a drainage area of 538 km2 making it the 

largest drainage basin of any reservoir in Puerto Rico. Mean 

annual rainfall in the Lago Loíza Basin ranges from 1,600 

millimeters (mm) in Juncos and Caguas (which lie in the rain 

shadow of the mountain range) to as much as 5,000 millimeters 

per year (mm/yr) on the slopes of the Sierra de Luquillo 

mountains (Calvesbert, 1970) (fig. 2). The principal streams 

draining 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 period of 

record from 1960 to 2002 is 305 million m3

. Flow from these 

streams constitute about 84 percent of the total mean annual 

inflow of 363 million m3

 to the reservoir (Díaz and others, 

2002).

The Lago Loíza basin is underlain primarily by 

granodiorite of the San Lorenzo Batholith, volcanic rocks, and 

alluvium (Briggs and Akers 1965; Cox and Briggs, 1973; 

Bawiec, 2001; Gellis and others, 1999). The basin is composed 

mainly of the Mucara-Caguabo soils series, followed by the 

Mabi-Río Arriba soil series, and of the Pandura-Lirios soil 

series. These soils have slopes that range from 0 to 60 percent 

with the flat lands containing the Mabi-Río Arriba series, which 

is the portion of the Río Gurabo valley (Boccheciamp, 1978).

The 1987 land use within the basin was classified as 

pasture (50 percent), forested areas (20 percent), rural areas (11 

percent), croplands (10 percent), and urban areas (6 percent). 

The rest of the areas (constituting about 3 percent) were 

classified as disturbed ground and other uses (Birdsey and 

Weaver, 1987). These land-use classifications and percentages 

could have changed as a result of new urban developments that 

have taken place during the past 17 to 18 years.

Method of Survey

The 2004 bathymetric survey of Lago Loíza involved 

planning, data collection, data processing, and analysis. An 

Arc/Info GIS was used to establish the survey lines and to 

analyze the collected data. The survey lines were planned at a 

spacing of 50 m, starting at the dam and continuing upstream 

along the two branches of Lago Loíza (fig. 3). The bathymetric 

data were collected during January 26 to 28, 2004, using a depth 

sounder coupled to a GPS that recorded the horizontal position 

of the survey boat. A geo-referenced digital map of the reservoir 

shoreline and planned survey lines were loaded into a portable 

computer and served as a guide for the bathymetric data 

collection.

The reservoir pool elevation was monitored at the 

continuous recording USGS lake-elevation station Lago Loíza 

at damsite near Trujillo Alto, Puerto Rico (station number 

50059000, fig. 1). The pool elevation of Lago Loíza was not at 

the crest top of the gates, therefore, the depth sounding data 

were adjusted using a time-elevation correction factor to 

represent depths at maximum pool elevation of 41.14 m above 

mean sea level.
Method of Survey 3

 Figure 1. Location of Lago Loíza in the Río Grande de Loíza Basin, Puerto Rico.
4 Sedimentation Survey of Lago Loíza, Puerto Rico, January 2004

Table 1. Principal characteristics of Lago Loíza and structures (modified from Sheda and Legas, 1968; Puerto Rico Electric Power 

Authority, 1980).

[amsl, above mean sea level; TIN, Triangulated Irregular Network; GIS, Geographic Information System]

Total length of dam at top (spillway and non-overflow sections) 210 meters

Length of spillway section 95.1 meters

Elevations (above mean sea level):

Top of dam (maximum flooded elevation) 44.0 meters

Top of gates (maximum pool elevation) 41.14 meters

Spillway crest 31.0 meters

Crown of water intake 30.0 meters

Invert of water intake 28.0 meters

Maximum width at base 30.0 meters

Diameter of penstocks 1.67 meters

Installed power-generating capacity 3,000 kilowatts

Discharges with gates completely open

Maximum pool elevation (41.14 meters amsl) 6,590 cubic meters per second

Maximum design elevation (43.0 meters amsl) 8,835 cubic meters per second

Top of dam (44.0 meters amsl) 9,970 cubic meters per second

Maximum discharge recorded at dam site (September 10, 1996) 6,314 cubic meters per second

Original storage at pool elevation of 41.14 meters amsl from the 1947 TIN 26.80 million cubic meters

Surcharge storage for flood control (from 41.14 to 43.0 meters amsl) 8.0 million cubic meters

Drainage area at dam site 538 square kilometers

Flooded area at elevation of 41.14 meters (derived from GIS) 2.67 square kilometers

Maximum original depth at pool elevation of 41.14 meters amsl 25 meters

Maximum depth during 1999 post-dredge survey 18 meters

Maximum depth during 2004 survey 15 meters

Maximum length of normal pool (to the confluence of the Río Grande de Loíza

 and the Río Gurabo)

10 kilometers
Method of Survey 5 Figure 2. Mean annual rainfall distribution in Puerto Rico (modified from Calvesbert, 1970).
6 Sedimentation Survey of Lago Loíza, Puerto Rico, January 2004

 Figure 3. Planned cross-section locations for the January 2004 bathymetric survey of Lago Loíza, Puerto Rico.
Method of Survey 7

A total of 19,442 data points (depth soundings) were 

collected over the entire reservoir, while navigating along the 

planned survey lines (fig. 4, track lines). The depths along the 

cross sections were plotted and 1-m-interval contour lines of 

equal depth were drawn from the shoreline to the deepest parts 

of the reservoir (plates 1 and 2). The procedure used to contour 

the reservoir’s bottom is explained in the data processing 

section of this report. These contour lines were then converted 

into a triangulated irregular network (TIN) describing the 

reservoir’s bottom (fig. 5). The TIN represents the reservoir 

bottom surface model as thousands of adjoining triangles with 

x, y, and z coordinates assigned to all vertices (Environmental 

Systems Research Institute, Inc., 1992). The longitudinal 

distance of the reservoir along the thalweg is shown on figure 6. 

The GIS uses the TIN to calculate the storage capacity and 

thickness of sediment accumulation. 

To obtain an estimate of the sediment accumulation rate 

between 1999 and 2004, and to estimate the useful life of the 

reservoir based on current data, the 1999 post-dredge storage 

capacity of Lago Loíza provided by the PRIFA was compared 

with the 2004 calculated storage capacity. 

Selected cross sections depicting the reservoir’s bottom 

from shore to shore, as well as longitudinal profiles of the 

reservoir’s bottom along the thalweg of Lago Loíza were 

generated for 1994, 1999, and 2004 from their TIN surface 

models. The relation between pool elevation and water-storage 

capacity for 1994, 1999, and 2004 was generated by calculating 

the reservoir volume at 1-m elevation intervals and is shown in 

graphical form on figure 7. 

Field Techniques

The bathymetric survey of Lago Loíza was conducted 

during January 26 to 28, 2004. Data were collected using the 

bathymetric/land survey system (BLASS) developed by 

Specialty Devices, Inc. Previous bathymetric surveys 

conducted by the USGS used a differential global positioning 

system (DGPS), which consisted of two GPS receivers that 

operated simultaneously in order to maintain the positional data 

accuracy within 2 m. For the bathymetric survey of Lago Loíza, 

however, the BLASS system was upgraded from a 12-channel 

satellite receiver to a 24-channel satellite receiver, which 

enabled for one stand-alone GPS unit to maintain the 2-m 

positional data accuracy. The system consists of a Novatel 

global GPS receiver coupled to a Depth Sounder model 

SDI-IDS Intelligent. The depth sounder vertical accuracy is 

2 centimeters (cm) more or less 1 percent of the measured 

depth. The GPS receiver monitors the horizontal position of the 

survey boat, whereas the depth sounder measures water depths. 

To verify the accuracy of the system upgrade, the stand-alone 

GPS unit was placed at the known benchmark, USGS roof 

(latitude 18°25’N, longitude 66°06’W), which was calculated 

using the post-processing software, CENTIPOINT, giving a 

maximum error of 10 cm. The positional data were collected 

with the upgraded system for about 5 minutes and a total of 

1,049 positional data points were analyzed and the standard 

deviation was determined. The resulting data were compared 

with the post-processed position coordinates and the outcome 

indicated that the minimum latitude difference was 1.65 m and 

the maximum difference was 2.05 m, for an average of 1.85 m. 

The minimum longitude difference was 2.11 m and the 

maximum difference was 2.48 m, averaging 2.29 m. The overall 

positional difference between the post-processed coordinates 

and the stand-alone coordinates average was 2.07 m, which is in 

very close agreement with the positional accuracy obtained 

using the DGPS.

The system upgrade resulted in a more effective data 

collection process, because it eliminated the need to have a base 

or reference GPS station deployed at several sites overlooking 

the reservoir. The reservoir water level varied less because of 

the reduced data collection time, therefore, the time spent 

collecting data was less and the time spent post-processing data 

was reduced.

The bathymetric survey software HYPACK was used to 

navigate and to collect data. The software integrates the position 

and depth data, storing the x, y (geographic locations), and z 

(depths) coordinates in a portable computer.

A total of 221 survey sounding lines were originally 

planned using the GIS (fig. 3); however, vegetation growth and 

water hyacinths (Eichornia crassipes) in some areas limited the 

data collection to only 118 cross sections (fig. 4). Nonetheless, 

the vast majority of the unsurveyed lines were located in areas 

where runoff is negligible, thus sediment influx is also minimal. 

It is reasonable to conclude that the bottom topography in these 

areas has changed little, if any, over the years. The bottom 

features at these areas were estimated by using previous surveys 

and the original Lago Loíza pre-impounded topography.

Data Processing

The initial editing of the 2004 data was performed using the 

HYPACK software. The positions were corrected to eliminate 

anomalies that occur when the satellite signal is lost, because of 

local topographic features or electromagnetic interference. The 

position errors were corrected by interpolating back to the middle 

point between the correct preceding and posterior position. The 

depth data were also corrected to eliminate incorrect depth 

readings that can result from insufficient signal gain or when 

floating debris interferes with the transducer face. Incorrect depth 

readings were interpolated between the correct preceding and 

posterior depth readings. The edited data were then transferred 

into the Arc/Info GIS database for further processing. The Arc/Info 

software was customized to produce color-coded depth data, 

according to different depth values. The data points of the same 

color were connected by adding a line between them, and a 

contour map of the reservoir’s bottom depth was generated (plates 

1 and 2).
8 Sedimentation Survey of Lago Loíza, Puerto Rico, January 2004

 Figure 4. Actual track lines of the January 2004 bathymetric survey of Lago Loíza, Puerto Rico. 
Method of Survey 9

 Figure 5. Triangulated irregular network surface model of Lago Loíza, Puerto Rico, for January 2004.
10 Sedimentation Survey of Lago Loíza, Puerto Rico, January 2004

 Figure 6. Reference longitudinal distance along the thalweg of Lago Loíza, Puerto Rico.
Method of Survey 11

The bathymetric contour map was used to create the TIN surface 

model of the reservoir bottom for 2004 (fig. 5). In addition, the 1994, 

1999, and 2004 TIN surface models of Lago Loíza were used to 

generate transverse cross sections and longitudinal profiles of the 

reservoir’s bottom. Sampling the TIN every 5 m along selected 

cross sections generated transverse cross sections representing 

the reservoir’s bottom from shore to shore for 1994, 1999, and 2004 

(fig. 8). The same procedure used in generating the selected cross 

sections was employed to generate the longitudinal profile along 

the thalweg of Lago Loíza for the same years (fig. 9). The selected 

cross sections were located to represent flooded areas of the 

reservoir, whereas the longitudinal profiles were located at the 

deepest part of the reservoir’s bottom from the dam upstream to 

the confluence of the reservoir tributaries (Río Grande de Loíza 

and Río Gurabo).

 Figure 7. Relation between water-storage capacity and pool elevation of Lago Loíza, Puerto Rico, for 1994, 1999, and 2004.
12 Sedimentation Survey of Lago Loíza, Puerto Rico, January 2004

Figure 8. Selected cross sections generated from the TIN surface models of Lago Loíza, Puerto Rico, for 1994, 1999, and 2004. Refer to 

figure 3 for cross-section locations.
Method of Survey 13

Figure 8. Selected cross sections generated from the TIN surface models of Lago Loíza, Puerto Rico, for 1994, 1999, and 2004. Refer to 

figure 3 for cross-section locations.—Continued
14 Sedimentation Survey of Lago Loíza, Puerto Rico, January 2004

Figure 8. Selected cross sections generated from the TIN surface models of Lago Loíza, Puerto Rico, for 1994, 1999, and 2004. Refer to 

figure 3 for cross-section locations.—Continued
Method of Survey 15

Figure 8. Selected cross sections generated from the TIN surface models of Lago Loíza, Puerto Rico, for 1994, 1999, and 2004. Refer to 

figure 3 for cross-section locations.—Continued
16 Sedimentation Survey of Lago Loíza, Puerto Rico, January 2004

Figure 8. Selected cross sections generated from the TIN surface models of Lago Loíza, Puerto Rico, for 1994, 1999, and 2004. Refer to 

figure 3 for cross-section locations.—Continued
Method of Survey 17

Figure 8. Selected cross sections generated from the TIN surface models of Lago Loíza, Puerto Rico, for 1994, 1999, and 2004. Refer to 

figure 3 for cross-section locations.—Continued
18 Sedimentation Survey of Lago Loíza, Puerto Rico, January 2004

Figure 9. Longitudinal profiles along the thalweg of Lago Loíza, Puerto Rico, generated from the TIN surface models for 1994, 1999, and 2004.
Historical Sediment Accumulation and Effect on Reservoir Storage 19

Historical Sediment Accumulation and 

Effect on Reservoir Storage

The Lago Loíza reservoir has been affected by a relatively 

high sedimentation rate (compared to regional rates, SolerLópez, 2001b), and the resulting storage-capacity loss since its 

construction in 1953. The original installed storage capacity 

was 26.80 million m3

 at 41.14 m above mean sea level in 1953, 

decreasing to 23.41 million m3 by 1963 and to 20.00 million m3

by 1971. The storage capacity continued decreasing to 16.38 

million cubic meters in 1979, 15.20 million cubic meters in 

1990, and to 14.19 million cubic meters in 1994 (Webb and 

Soler-López, 1997). The reservoir was dredged between 1997 

and 1999 to augment the water-storage capacity, as a result of 

the reduced storage capacity of the reservoir, increase in draft 

rate to supply the Sergio Cuevas Filtration Plant and the drought 

of 1994, which led to water rationing during a 136-day period. 

The storage capacity of Lago Loíza before the dredging 

started in 1997, was estimated by applying the 1953-1997 

sedimentation rate of about 310,000 m3

 for the period of 

1994 to 1997, resulting in an estimated volume of about 

13.26 million m3.

The 1999 PRIFA reservoir storage capacity after dredging 

was reported as 19.35 million m3 (Puerto Rico Infrastructure 

Financing Authority, written commun., 2004) or an increase of 

36 percent compared with the 1994 storage capacity. Detailed 

information on the historical sedimentation trends of Lago 

Loíza from 1953 to 1994 was published in Webb and SolerLópez (1997). Table 2 summarizes the 1994, 1999, and 2004 

results of the respective sedimentation surveys of Lago Loíza. 

The Lago Loíza storage capacity was increased by 

dredging from 14.19 million m3 in 1994 to 19.35 million m3

 in 

1999, for an increase of 36 percent compared with the 1994 

volume. This 1999 storage capacity of Lago Loíza is about 

72 percent of the original 1953 volume of 26.80 million m3. 

Based on these values, the actual dredged-volume loss after the 

hurricanes is about 31 percent (1.82 out of 5.78 million m3

). 

Table 2. Comparison between the 1994, 1999, and 2004 sedimentation surveys of Lago Loíza, Puerto Rico. All the 1994 data 

are derived from Webb and Soler-López (1997). 

[---, undetermined or unavailable]

Year 1994 1999 2004

Reservoir surface area, in square kilometers 2.67 2.67 2.67

Storage capacity, in million cubic meters 14.19 19.351

1 Reported in storage-capacity table provided by the Puerto Rico Infrastructure Financing Authority.

17.53

Live storage, in million cubic meters 14.15 17.80 17.28

Dead storage, in million cubic meters 0.04 1.55 0.25

Years since construction 41 46 51

Sediment accumulation since construction, in million cubic meters 12.61 14.302

2

 Sediment accumulation by 1994 (12.61 million cubic meters) plus the average annual sedimentation rate of about 337,000 cubic meters

(as reported in Webb and Soler-López, 1997) applied from 1994 to 1999 (1.69 million cubic meters).

16.123

3

 Sediment accumulation by 1999 (14.30 million cubic meters) plus the loss between 1999 and 2004 (1.82 million cubic meters).

Inter-survey sediment accumulation, in million cubic meters 1.01 1.69 1.82

Long-term storage capacity loss, in percent 47 53 60

Long-term annual storage capacity loss, in cubic meters 303,000 310,870 316,078

Inter-survey storage capacity loss, in percent 3.8 6 7

Inter-survey annual storage capacity loss, in percent 1.0 1.2 1.4

Inter-survey annual storage capacity loss, in cubic meters 242,000 338,000 364,000

Storage capacity added by dredging, in million cubic meters --- 5.784

4

 Reported by the Puerto Rico Infrastructure Financing Authority.

---

Dredged volume loss, in million cubic meters --- --- 1.82

Percent of dredged volume loss --- --- 31

Percent of annual dredge volume loss from 1999 to 2004 --- 0 6.2
20 Sedimentation Survey of Lago Loíza, Puerto Rico, January 2004

This represents a dredged-volume loss of about 364,000 

m3/yr (6.2 percent per year) between 1999 and 2004. Figure 10 

summarizes the Lago Loíza storage capacity variations and 

sedimentation rates from 1953 to 2004. As indicated by figure 

10, high sedimentation rates occurred for the period of 1971 to 

1979. This particularly high sedimentation rate of about 

452,000 m3

/yr occurred during the construction of Highway 

PR-52 and the subsequent passage of Hurricane David and 

Tropical Storm Frederic in 1979. These high sedimentation 

rates, however, are considered to be short-term, low-occurrence 

events associated with land-use disturbances and extreme 

runoff events.

The sedimentation rate between 1999 and 2004 of 364,000 

m3

/yr is 17 percent higher than the average long-term rate of 

about 310,000 m3

/yr, indicating that dredging activities alone 

may not be sufficient to increase substantially the life 

expectancy of Lago Loíza. Gellis (1991) documented the 

importance of land use in delivering sediment to Lago Loíza 

and, therefore, another alternative to reducing the sediment 

transport to Lago Loíza is upland erosion-control strategies. 

Based on the data presented, it is uncertain that the existing 

sedimentation rate of Lago Loíza can be reduced substantially 

in the foreseeable future. The increased sedimentation rate after 

1999 (0.36 million m3/yr) probably reflects the sediment load 

transported into Lago Loíza by the flooding caused by 

Hurricane Georges on September 21, 1998. It is important to 

understand that the net effect of major floods may not be 

completely observed at the reservoirs until several years after 

the event occurred, as soil eroded by these extreme events tends 

to remain stored in the river channels upstream from the 

reservoir with the potential of rapid mobilization even with 

runoff events of minor intensity (Soler-López, 2001a, 2001b). 

The greater mass of sediment mobilization and deposition 

occurs primarily during rainfall-runoff events of substantial 

magnitude capable of mobilizing sand and coarse-grained 

streambed sediments, which make up the bed material in all 

inflowing tributaries. In the Lago Loíza basin, the source of 

sand and coarse material is the extensive granodiorite intrusive 

rock outcrop, which covers about 50 percent of the basin 

(Gellis, 1991).

It is reasonable to conclude that sediment accumulation in 

Lago Loíza tends towards an equilibrium state in which the 

reservoir loses storage capacity to a rate close to the historical 

long-term average sedimentation rate of about 310,000 m3/yr. 

Higher sedimentation rates, however, can be induced by 

extreme rainfall-runoff events and land-use disturbances as 

shown on figure 10.

The Lago Loíza reservoir water-intake structure used for 

public-supply water withdrawals is located at an invert 

elevation of 28.0 m above mean sea level. The volume of water 

contained above the elevation of the intake structure is referred 

to as the live (useful) storage, and the volume below it is 

referred to as the dead storage (the dead storage is designed to 

accommodate sediment without disabling reservoir structures). 

The 2004 bathymetric data indicate that the reservoir’s bottom 

in the vicinity of the water intake tower was at an approximate 

elevation of 26.0 m above mean sea level. This indicates that the 

reservoir’s bottom is about 2 m below the invert (lower) 

elevation of the structure. Using the intake structure invert 

elevation of 28.0 m above mean sea level, the 2004 live storage 

of Lago Loíza is about 17.28 million m3

 and the dead storage is 

about 0.25 million m3

. The 2004 storage capacity of Lago Loíza 

at 1-m elevation intervals is listed on table 3.

The sediment deposition and distribution pattern in Lago 

Loíza, although appreciable in selected cross sections (fig. 8), is 

more distinguishable in the longitudinal bottom profiles of the 

reservoir (fig. 9). Sediment accumulation along the thalweg 

from the dam to the Río Grande de Loíza and Río Gurabo 

confluence since 1999 can be summarized as follows: a 

1-m-thick layer of sediment has accumulated in the immediate 

vicinity of the dam; a 2-m-thick layer of sediment has 

accumulated slightly upstream from the dam to about 1,250 m 

upstream; a more uniform (overall elevation) sediment layer 

ranging from 3.5 to 4.0 m thick extends from about 1,750 to 

about 5,000 m upstream from the dam; and, a sediment layer of 

2.5 m thick extends from about 6,500 to 7,000 m upstream from 

the dam. The sediment deposition rates in these specific areas 

from 1999 to 2004 are equivalent to 20, 40, 70, 80, and 50 

centimeters per year (cm/yr), respectively. Sediment deposition 

rates increase with distance from the dam because coarse 

sediment settles in the bottom faster than finer sediment as 

sediment-laden water loses velocity. The relatively lower 

deposition rate of 50 cm/yr for the portion, about 7,000 m 

upstream from the dam, could be the effect of the narrow, 

relatively shallow reservoir morphology, which tends to 

maintain water velocities similar to the tributary rivers during 

high runoff events. It is precisely at about 5,000 to 6,000 m 

upstream from the dam that deposition rates are higher, where 

the reservoir morphology transforms from a narrow, relatively 

shallow channel to a wider, deeper channel.

Based on the sediment deposition rate of 20 cm/yr near the 

dam, the reservoir’s bottom of Lago Loíza could reach the 

public-supply water intake structure in about 10 years, thus 

affecting the normal water withdrawals. The inflow towards the 

intake structure of about 4.38 cubic meters per second (m3/s), 

however, makes the sediment deposition and settling 

improbable.

Based on the mean annual inflow to Lago Loíza of 

363 million cubic meters and the 2004 water-storage capacity 

of 17.53 million m3, the Lago Loíza drainage area provides 

enough water to completely fill the reservoir about 21 times per 

year. Therefore, Lago Loíza can store only about 5 percent of 

the mean annual inflow, and the percentage of basin runoff 

storage will continue to decrease as the water-storage capacity 

decreases, owing to sedimentation.
Historical Sediment Accumulation and Effect on Reservoir Storage 21 Figure 10. Lago Loíza storage capacities and sediment accumulation rates from 1953 to 2004.
22 Sedimentation Survey of Lago Loíza, Puerto Rico, January 2004

Table 3. Storage capacity for Lago Loíza, Puerto Rico, 

January 2004.

Sediment Trapping Efficiency

Heinemann (1981) considered trapping efficiency to be the 

most informative descriptor of a reservoir, because it basically 

provides information on the useful life of a reservoir based on 

the mean annual sediment influx and the amount that is actually 

deposited in the reservoir. This value is the proportion of the 

incoming sediment that is deposited or trapped in a pond, 

reservoir, or lake. The trapping efficiency is dependent on 

several properties, including sediment particle-size distribution, 

the time and rate of water inflow to the reservoir, the reservoir 

size and shape, the location of the outlet structure, and the 

location and discharge schedules (Verstraeten and Poesen, 

2000).

Many empirical studies showing the relation between 

reservoir storage capacity, water inflow, and trapping efficiency 

have been conducted in the past, of which Brune’s (1953) is the 

most widely used and accepted. Brune developed a curve 

(fig. 11) that estimates the trapping efficiency of a reservoir 

based on the ratio of storage capacity to annual water inflow 

volume. The trapping efficiency of Lago Loíza was estimated 

using the relation established by Brune (1953). In addition, a 

suspended-sediment budget for Lago Loíza was estimated for 

the period of 1996 to 2002, using the USGS suspendedsediment stations network and compared with the estimate 

derived from Brune’s curve. 

Based on historical data, the estimated long-term mean 

annual runoff for the entire Lago Loíza drainage area is 

363 million m3, and the 2004 reservoir storage capacity is 

17.53 million m3. Therefore, the ratio of capacity to inflow 

(C/I) of Lago Loíza is 0.048 (17.53/363). Using the median 

curve of Brune’s relation, the Lago Loíza sediment trapping 

efficiency was estimated to be 72 percent for 1994, 78 percent 

for 1999, and 76 percent for 2004. The increase in sediment 

trapping efficiency recorded from 1994 and 1999 is the result of 

the increased reservoir storage resulting from reservoir 

dredging operations between 1997 and 1999. Conversely, the 

sediment trapping efficiency decreased from 1999 to 2004, as a 

result of the reservoir storage-capacity decrease.

Streamflow and suspended sediment have been monitored 

at each of the five tributaries draining into the reservoir. Table 

4 summarizes the total suspended-sediment load for each water 

year from 1996 to 2002.

The data presented on table 4 indicate that two major 

suspended-sediment loads entered the reservoir, when 

Hurricanes Hortense (1996) and Georges (1998) made landfall 

in Puerto Rico. For water year 1996, about 78 percent 

(753,508 metric tons) of the total suspended-sediment load of 

all tributaries were generated during the passage of Hurricane 

Hortense on September 10, 1996 (Díaz and others, 1996-2002). 

For water year 1998, about 61 percent (512,523 metric tons) of 

the total suspended-sediment load was generated during the 

passage of Hurricane Georges on September 20-21, 1998 (Díaz 

and others, 1997-2004). These suspended sediment loads 

represent 144 and 98 percent, respectively, of the average 

suspended-sediment load of 524,585 metric tons for a 5-year 

period (1996-2000).

Sediment Yield and Reservoir Life 

Expectancy

Sediment yield has been defined by the American Society 

of Civil Engineers as the total sediment outflow measurable at 

a point of reference, for a specified period of time, per unit of 

surface area contributing to the point of reference (McManus 

and Duck, 1993). For the period of 1994 to 1999, the total 

estimated volume of sediment contributed to Lago Loíza from 

a drainage area of 538 km2

 is estimated at 2.17 million cubic 

meters. This volume is estimated by dividing 1.69 million cubic 

meters of sediment accumulation from 1994 to 1999 (table 2) by 

the estimated sediment trapping efficiency of 78 percent for 

1999. This estimated rate of sediment influx (2.17 million cubic 

meters) divided by the number of years between 1994 and 1999 

(5 years), results in an average of about 434,000 cubic meters 

per year. This sediment volume eroded from the Lago Loíza 

drainage area divided by the net sediment contributing area 

of 535 km2, (the total drainage area of 538 km2 minus the 2.67 

km2 surface area of the reservoir) results in an average basin 

sediment yield, or area-normalized reservoir storage loss of 811 

cubic meters per square kilometer per year (m3

/km2/yr). The 

sediment yield from 1999 to 2004 is about 907 m3

/km2/yr. The 

average drainage area sediment yield for the period 1994-2004 

is about 859 m3

/km2/yr.

Pool elevation, in meters 

above mean sea level

Storage capacity, 

in million cubic meters 

 41.14 17.53

 40.14 15.07

 39.14 12.84

 38.14 10.80

 37.14 9.95

 36.14 7.31

 35.14 5.90

 34.14 4.76

 33.14 3.78

 32.14 2.91

 31.14 2.12

 30.14 1.41

 29.14 0.76

 28.14 0.25

 27.14 0.03

 26.14 0.00
Sediment Yield and Reservoir Life Expectancy 23

Figure 11. Reservoir trapping efficiency as a function of the ratio between water-storage capacity and annual water inflow volume 

(Brune, 1953).

Table 4. Suspended-sediment loads at tributary streams to Lago Loíza for water years 1996 to 2002, in metric tons per year (Díaz and 

others, 1997-2004).

[---, no data available; N/A, not applicable]

Tributary 1996 1997 1998 1999 2000 2001 2002

Total for 

the period 

1996-2000

Río Grande de Loíza 494,992 68,745 548,820 159,086 131,140 87,124 71,567 1,402,783

Río Gurabo 276,172 23,215 182,983 100,431 64,236 37,274 7,201 647,037

Río Cagüitas 38,801 3,625 68,262 49,537 114,326 --- --- 274,551

Río Bairoa 36,720 358 5,475 19,459 13,807 1,887 --- 75,819

Río Cañas 116,414 9,193 33,731 31,496 31,902 6,676 --- 222,736

Total for the year 963,099 105,136 839,271 360,009 355,411 N/A N/A Average 524,585
24 Sedimentation Survey of Lago Loíza, Puerto Rico, January 2004

These sediment yields are about one-and-a-half times 

higher than the value of 600 m3

/km2/yr reported for the period 

from 1990 to 1994, possibly reflecting the impact of Hurricanes 

Hortense in 1996 and Georges in 1998 as evidenced by the 

suspended-sediment loads calculated for these years.

Sediment cores of about 2 m in depth were collected from 

four locations in Lago Loíza in December 1990, as part of a 

study conducted by the USGS on the effects of land use in the 

Lago Loíza basin on upland erosion, sediment transport, and 

reservoir sedimentation (Gellis and others, 1999). These cores 

were taken for the determination of many properties and 

characteristics, but for the published report, the sediment drybulk density data were not published. The average dry-bulk 

density of the cores was roughly 1 gram per cubic centimeter. 

Therefore, an estimate of the sediment yield from the drainage 

area of Lago Loíza on a mass basis was estimated by using the 

sediment dry-bulk density of 1 gram per cubic centimeter 

determined for that study. Thus, the Lago Loíza sediment yield 

on a mass basis is equivalent to 859 and 907 megagrams per 

square kilometer per year for 1994-2004, and 1999-2004, 

respectively. These drainage area-normalized storagecapacity loses of Lago Loíza are about half the average of 

1,578 m3/km2/yr for14 reservoirs studied in Puerto Rico 

(Soler-López, 2001b).

The life expectancy of Lago Loíza or any other reservoir 

can be estimated by dividing the remaining storage capacity by 

the annual storage-capacity loss. Using the average long-term 

(1953-1997) reservoir storage loss rate of 310,000 m3

/yr as 

previously indicated, the reservoir would completely fill in 

about 56 years or by the year 2060. More important, however, 

is the fact that the gain in storage remaining from the dredging 

operations between 1997 and 1999 could be lost to 

sedimentation in about 13 years (3.96 million m3

 [amount of 

remaining water storage] divided by 0.31 million m3 [the longterm sedimentation rate]). This dredging operation was 

conducted to augment the “safe yield” of Lago Loíza as the 

public-water supply source of the Sergio Cuevas Filtration 

Plant serving the eastern part of the San Juan metropolitan area. 

At the time of the drought in 1994, the filtration plant was 

unable to maintain a production rate of 340,000 m3/d (90 

million gallons per day) leading to water rationing during a 

136-day period (Rodríguez-Martínez and others, 2002).

Summary and Conclusions

The Carraízo Dam was constructed in 1953 and constitutes 

a major public-supply water source for the San Juan 

metropolitan area. In 1977, as a result of loss to sediment 

accumulation, flash boards were installed to increase the 

storage capacity to about 2.7 million cubic meters and increase 

the “safe yield” of the reservoir. The reservoir storage capacity, 

however, continued to decrease as documented by several 

subsequent sedimentation surveys. The consequences of lost 

storage became evident during 1994, when a major drought 

resulting from a rainfall deficit of about 30 percent between 

August 1993 and September 1994 led to a 136-day water 

rationing schedule affecting the eastern part of the San Juan 

metropolitan area. 

Prompted by this water-supply shortfall, the reservoir was 

dredged between 1997 and 1999, to increase the storage 

capacity by about 5.8 million cubic meters. Prior to initiating 

the dredging project, however, Hurricane Hortense impacted 

the Lago Loíza watershed in 1996 and Hurricane Georges in 

1998. Both of these hurricanes caused major flooding in the 

basin and undoubtedly contributed to reservoir storage 

depletion through sedimentation.

During January 2004, the U.S. Geological Survey in 

cooperation with the Puerto Rico Aqueduct and Sewer 

Authority conducted a bathymetric survey of Lago Loíza 

reservoir to assess the impact of these two storms on the waterstorage capacity of the reservoir and to investigate the possible 

impact on dredging performed from 1997 to 1999.

The bathymetric survey results indicate that (1) the storage 

capacity of Lago Loíza in 2004 was 17.53 or 3.34 million cubic 

meters greater than that calculated in 1994 (14.19 million cubic 

meters); (2) the sedimentation rate of the reservoir on a longterm basis (1953-1997) is about 310,000 cubic meters per year, 

(3) the sedimentation rate between 1999 and 2004 was 

estimated at about 364,000 cubic meters per year, and (4) based 

on this sedimentation rate, the reservoir storage could reach the 

volume of 1994 by about the year 2015 and to pre-dredge 

conditions (1997) by the year 2018.

The long-term sedimentation rate of Lago Loíza of about 

310,000 cubic meters per year may be increased by major landuse changes and rainfall-runoff events of the magnitudes 

generated by hurricanes. Between 1994 and 1999, the 

sedimentation rate was estimated at 338,000 cubic meters per 

year and it is likely that this sediment loading rate increased as 

a result of the impact of Hurricane Hortense in 1996 and 

Georges in 1998. Also the high rate of 364,000 cubic meters per 

year from 1999 to 2004 could be the repercussion of these 

storms. Considering that about 50 percent of the drainage basin 

lies within a granodiorite intrusive rock outcrop, the sand 

portion of the sediment eroded from the stream banks or land 

surface by major storm events can remain stored longer than 

finer sediment in the river channels upstream from the reservoir 

with the potential of being mobilized by subsequent high runoff 

events. Therefore, the full impact of a major storm on the 

reservoir may not be completely observed until a few years 

after. As an example, the 1990 to 1994 drainage area sediment 

yield was in the range of 600 cubic meters per square kilometer 

per year. The yield increased, however, to about 811 cubic 

meters per square kilometer per year during the period from 

1994 to 1999, and to 907 cubic meters per square kilometer per 

year for the period from 1999 to 2004, possibly reflecting the 

impact of Hurricanes Hortense and Georges. Other possible 

causes that could contribute to the increase in annual sediment 

yields could be land-use disturbances.
Cited References 25

According to the 2004 bathymetric survey, Lago Loíza 

would be completely silted by the year 2060. The life 

expectancy of Lago Loíza was about 50 years, based on the 

1994 sedimentation survey, and continues to be close to that 

(56 years) according to recent data. Dredging had the net effect 

of providing additional water for periods of low runoff and 

increased the reservoir life expectancy by about 18 years; 

however, the fact remains that the reservoir storage will likely 

reach the 1994 volume (when water rationing plans were 

implemented) by about 2015, cancelling out the safe-yield 

increase resulting from the dredging operations between 1997 

and 1999.

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Research Note SO-331, New Orleans, Southern Forest 

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