Scientific Investigations

Report 2004-5035
Sedimentation Survey of Toa Vaca, 

Puerto Rico, June-July 2002

By Luis R. Soler-López

U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the

PUERTO RICO INFRASTRUCTURE FINANCING AUTHORITY

Scientific Investigations Report 2004-5035
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2004

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/

Suggested citation:

Soler-López, L.R., Sedimentation survey of Lago Toa Vaca, Puerto Rico, June-July 2002, U.S. Geological Survey Scientific 

Investigations Report 2004-5035, 32 p.

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.
 iii

Contents

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

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

Dam, Reservoir, and Basin Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Method of Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Field Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Data Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Storage Capacity and Sediment Accumulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Sediment Trapping Efficiency and Water Renewal Rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Sediment Yield and Reservoir Life Expectancy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

References Cited. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Plates

[In pocket]

1. Lago Toa Vaca, Puerto Rico, Bathymetry, 2002

2. Lago Toa Vaca, Puerto Rico, Topography, 1972

Figures

1-3. Map showing

1. Location of Lago Toa Vaca in the upper Río Jacaguas and Río Toa Vaca 

Basins, and areas served by the Juana Díaz Irrigation District in 

southern Puerto Rico. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2

2. Mean-annual rainfall distribution in Puerto Rico . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4

3. Lago Toa Vaca drainage area showing the reservoir shoreline and tributaries 

to Lago Toa Vaca . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

4. Aerial photograph of part of Lago Toa Vaca drainage area showing the location 

of the quarry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

5-8. Map showing

5. Planned location of sounding lines for the 2002 bathymetric survey of Lago 

Toa Vaca, Puerto Rico . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

6. Actual survey sounding lines of the 2002 bathymetric survey of Lago Toa 

Vaca, Puerto Rico. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

7. Triangulated irregular network (TIN) surface model of Lago Toa Vaca, 

Puerto Rico, for 2002 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

8. Reference longitudinal distance along the thalweg of Lago Toa Vaca, 

Puerto Rico . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
iv 

9-11. Graph showing

9. Selected cross sections generated from the TIN surface models of Lago 

Toa Vaca, Puerto Rico, for 1972 and 2002 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

10. Longitudinal profiles generated from the TIN surface models along the 

thalweg of Lago Toa Vaca, Puerto Rico, for 1972 and 2002 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

11. Lago Toa Vaca capacity curves for 1972 and 2002. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

12. Aerial photograph of the Lago Toa Vaca dam area showing the approximate extent 

of the borrow area used in construction of the earthfill dam as shown in the 

Unites States Bureau of Reclamation plans. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

13. Aerial photograph of the Lago Toa Vaca dam area showing the rock-source area and 

the resulting new shoreline. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

14. Front-view perspective of the rock-source area and drill marks visible during the 2002 

bathymetric survey of Lago Toa Vaca, Puerto Rico. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

15. Map showing planned location of sounding lines for the 1985 bathymetric survey of 

Lago Toa Vaca, Puerto Rico . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

16. Reservoir trapping efficiency as a function of the ratio between storage capacity and 

annual water inflow volume . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Tables

1. Principal characteristics of Lago Toa Vaca and Toa Vaca dam, Puerto Rico . . . . . . . . . . . . . . . . . . . . . . . .3

2. Comparison of the 1972 topographic survey with the 2002 bathymetric survey of 

Lago Toa Vaca, Puerto Rico . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

3. Storage capacity of Lago Toa Vaca, Puerto Rico, June-July 2002 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
 v

Conversion Factors, Datums, Acronyms, and 

Translations

Datums

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 “Seal Level Datum of 1929”.

Acronyms used in this report

BLASS Bathymetric/Land Survey System

DGPS Differential Global Positioning System

DOQ Digital Orthophoto Quadrangle

GIS Geographic Information System

GPS Global Positioning System

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

Multiply By To obtain

Length

millimeter 0.03937 inch

centimeter 0.03281 foot

meter 3.281 foot

kilometer 0.6214 mile

 Area

square meter 10.76 square foot

square kilometer 0.3861 square mile

square kilometer 247.1 acre

Volume

cubic meter 35.31 cubic foot

cubic meter 0.0008107 acre-foot

million cubic meters 810.7 acre-foot

Volume per unit time (includes flow)

cubic meter per second 35.31 cubic feet per second

cubic meter per second 15,850 gallon per minute

cubic meter per second 22.83 million gallons per day

Mass per area (includes sediment yield)

megagram per square kilometer 2.855 ton per square mile 
Sedimentation Survey of Lago Toa Vaca, Puerto Rico, 

June-July 2002

By Luis R. Soler-López

Abstract

The Lago Toa Vaca dam is located in the municipality of 

Villalba in southern Puerto Rico, and is owned and operated by 

the Puerto Rico Aqueduct and Sewer Authority. Construction 

was completed in 1972 as the first phase of a multi-purpose 

project that contemplated four possible diversions from other 

basins to mitigate the rapid storage capacity loss of Lago 

Guayabal, located immediately downstream of the Toa Vaca 

dam. The latter phases of the intra-basin diversions were 

cancelled, and currently, the reservoir receives runoff from only 

56.8 square kilometers of its drainage area.

Lago Toa Vaca reservoir when constructed was to be used 

for irrigation of croplands in the southern coastal plain. The 

reservoir had an original storage capacity of 68.94 million cubic 

meters. Sedimentation has reduced the storage capacity by only 

7 percent between 1972 and 2002 to 64.08 million cubic meters. 

This represents a long-term sedimentation rate of about 162,000 

cubic meters per year.

Based on the 2002 sedimentation survey, Lago Toa Vaca 

has a sediment trapping efficiency of about 98 percent and a 

drainage area-normalized sedimentation rate of about 3,086 

cubic meters per square kilometer per year between 1972 and 

2002. At the current long-term sedimentation rate the reservoir 

would lose its storage capacity by the year 2400.

Introduction

The southern coastal plain of Puerto Rico has traditionally 

been used for agricultural purposes (fig. 1). Because rainfall 

along the southern coastal plain averages 900 millimeters per 

year (Calvesbert, 1970), a major irrigation infrastructure project 

to convey water for the cultivation of crops along this coastal 

plain was completed in 1914. The Lago Guayabal reservoir 

constituted the principal source of surface-water irrigation 

within the western half of the southern coastal plain. This 

reservoir was losing storage capacity rapidly, and after many 

efforts to restore storage capacity by dredging and installation 

of flashboards, managing authorities decided to construct the 

Toa Vaca dam on the Río Toa Vaca, the northeastern tributary 

of Lago Guayabal. The new dam was built to impound water 

from the Río Toa Vaca, Río Coamo, and Río Jacaguas located 

in the south-central part of the island, and to receive water 

transferred from the headwaters of the Río Grande de Arecibo, 

Río Grande de Manatí, and Río Saliente, located north of the 

insular hydrologic divide (Puerto Rico Water Resources 

Authority, 1967). However, after the completion of the Toa 

Vaca dam in 1972, the project of the intra-basin diversions was 

cancelled and currently the reservoir receives only the runoff 

from its 56.8 square-kilometer drainage basin. The construction 

of Lago Toa Vaca reservoir, nevertheless, has benefitted the 

immediate areas by: (1) reducing the rapid storage capacity loss 

of Lago Guayabal; (2) providing additional surface-water 

storage that otherwise would have been impossible to contain in 

Lago Guayabal; and (3) providing flood control protection to 

the population downstream of Lago Guayabal.

The Lago Toa Vaca was to be used primarily as a water 

source for irrigation in southern Puerto Rico. However, 

immediately after construction in 1972, its use was designated 

primarily for public-water supply purposes and its management 

was transferred from the Puerto Rico Electric Power Authority 

(PREPA) to the Puerto Rico Aqueduct and Sewer Authority 

(PRASA).

The U.S. Geological Survey (USGS), in cooperation with 

the Puerto Rico Infrastructure Financing Authority (PRIFA) 

conducted a bathymetric survey of Lago Toa Vaca during JuneJuly 2002. The bathymetric survey was conducted using a 

differential global positioning system (DGPS) 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 predict the useful life of the 

reservoir. 

This report provides PRIFA and PRASA officials with the 

necessary information to more effectively manage the water 

resources of Lago Toa Vaca. Data from the 2002 bathymetric 

survey were compared with the 1972 pre-impoundment storage 

capacity of Lago Toa Vaca as defined from the 1:20,000 USGS 

Río Descalabrado topographic quadrangle. A high-resolution 

bathymetric survey was completed to define the historical longterm sedimentation rate, the storage capacity loss, and to 

provide an accurate bathymetric map for future reference.
50111210 50111210

CARIBBEAN SEA

SANTA SALINASISABEL

JUANADÍAZ

VILLALBA

COAMO

AIBONITO

BARRANQUITAS

CAYEY

PUERTO RICO

Part of the soin the Juana 

Upper Río JRío Toa Vac

Lake elevatioand number

Town or city

Drainage baboundary

EXPLANATION

Figure 1. Location of Lago Toa Vaca in the upper Río Jacaguas and Río Toa Vaca Basins, and areas served by the Juana Díaz Irrigation D
Dam, Reservoir, and Basin Characteristics 3

Dam, Reservoir, and Basin Characteristics

The Lago Toa Vaca dam structure was completed in 1972. 

It is located on the Río Toa Vaca in the municipality of Villalba 

in southern Puerto Rico, about 5 kilometers north of Juana Díaz 

and about 4 kilometers south of Villalba (fig. 1). The reservoir 

has a drainage area of 56.8 square kilometers. Although the dam 

was originally designed to provide about 68.94 million cubic 

meters of water storage for irrigation of sugarcane crops in 

southern Puerto Rico, its principal use was changed in 1972 as 

an infrastructure improvement for public-water supply 

purposes (U.S. Department of Agriculture, 1987).

Lago Toa Vaca was to be a major part of the largest 

irrigation-water supply infrastructure project in southern Puerto 

Rico. Source waters were to include the area draining to Lago 

Toa Vaca and headwaters of the Río Coamo, Río Jacaguas, Río 

Grande de Arecibo, Río Grande de Manatí, and Río Saliente, 

and so was provided with the largest water storage capacity in 

Puerto Rico at the time of construction. However, the plans for 

the mega project were later abandoned.

The dam is an earthfill structure with a height of 65.53 

meters above the original Toa Vaca river bed, a length of 530.35 

meters, and has a spillway elevation of 154.83 meters above 

mean sea level. Flashboards were installed on top of three radial 

gates (with dimensions of 9.14 meters wide by 9.26 meters 

high) to raise the normal pool elevation of the reservoir to 

164.90 meters above mean sea level. The spillway was designed 

to discharge 2,180 cubic meters per second at the maximum 

pool elevation of 166.48 meters above mean sea level. The 

outlet works of the dam are located at the right abutment and 

consist of a diversion inlet, an intake tower, an intake shaft, a 

circular tunnel, a gate chamber, an access house, and a flow 

meter.

Water for public use is withdrawn from the intake tower 

located 130 meters upstream from the dam, near the right bank. 

The tower has six intakes at different elevations. The lowest 

intake has an invert elevation of 121.00 meters above mean sea 

level, the next intake is at an invert elevation of 124.05 meters 

above mean sea level, and the remaining four intakes are spaced 

9.14 meters apart. Part of this information and other 

characteristics of the Lago Toa Vaca dam are summarized in 

table 1.

Mean-annual rainfall in the Lago Toa Vaca Basin is about 

1,800 millimeters (Calvesbert, 1970) (fig. 2). The Caguabo and 

Humatas soils, with the moderate erosion hazard are the 

predominant soils in the basin. Slopes vary from 20 to 60 

percent. The solum of the Caguabo series is 36 to 61 centimeters 

thick, and the Humatas series solum is 76 to 132 centimeters 

thick. Surface runoff in these soils is rapid, thus the moderate 

susceptibility to erosion (Gierbolini, 1979). The steep slopes, 

thickness of the solum and the climatic conditions all contribute 

to the high susceptibility of the basin to erosion and the 

consequent storage capacity loss of Lago Toa Vaca.

Table 1. Principal characteristics of Lago Toa Vaca and Toa Vaca dam, Puerto Rico (modified from the Puerto 

Rico Water Resources Authority, 1975).

Total length of dam, in meters 530.35

Maximum height of dam, in meters 65.53

Invert elevation of lower intake structure, in meters above mean sea level 121.00

Crest elevation of spillway structure, in meters above mean sea level 154.83

Normal pool elevation after the installation of flashboards, in meters above mean sea level 164.90

Reservoir length at normal pool elevation, in kilometers 6.75

Maximum discharge capacity, in cubic meters per second1

1

 Puerto Rico Water Resources Authority, 1975.

 2,180.0

Drainage area at dam site, in square kilometers1 56.8

Reservoir surface area at elevation of 164.90 meters above mean sea level, in square kilometers 3.22

Maximum depth after reservoir impoundment, in meters 54.90

Maximum depth during the June-July 2002 bathymetric survey, in meters 54.00
Laguna Tortuguero S 

ie 

rra de 

Río Culebrinas Río GuajatacaRío Camuy Río Grande de Arecibo Río Grande de Manatí Río Cibuco Río de la Plata Río Hondo Río de Bayamón Bahía de San Juan Río Piedras Laguna San José Laguna La Torrecilla Quebrada Blasina Río Grande de Lo

Río Herre

Río 

Río Guayabo Río Grande Río Grande de Añasco Río Yagüez Río Guanajibo

Río Loco Río YaucoRío Guayanilla Río TallaboaRío Matilde Río Portugués Río Bucaná Río Inabón Río Jacaguas Río Cañas Río Descalabrado Río Coamo Río Jueyes Río Salinas Río Seco Río GuamaníRío Nigua Río Grande de Patillas Río Chico Río Jacaboa Río Maunabo

R

Río Gu

ATLANTIC OCEAN

CARIBBEAN SEA

67 ̊15'

18 ̊30'

15'

18 ̊00'17 ̊45'

67 ̊00' 45' 30' 15' 66 ̊00' 

00 10

10 20 KILOMETERS

20 MILES

EXPLANATION

1,524

1,778

2,032

2,540 2,286

2,540

3,810

889

1,016

1,270

1,524

1,778

2,286

2,540 2,286

2,286

2,540 5,080

2,032

1,778

2,540

LINE OF EQUARAINFALL--VaInterval is varia

Lago Toa Vaca

Figure 2. Mean-annual rainfall distribution in Puerto Rico (modified from Calvesbert, 1970).
Method of Survey 5

The Caguabo soils, for the most part, have remained in 

pasture, brush, with some shallow-rooted crops that include 

pigeon peas. The Humatas soils are typically planted with a 

wide variety of crops, including coffee, yams, plantains, and 

tanniers. Some areas are covered with native pasture 

(Gierbolini, 1979).

Human development in the basin as of 1995 has been 

minimal (fig. 3). However, some climatic and anthropogenic 

conditions within and around the basin, could be affecting the 

sediment yields from the Toa Vaca drainage area. There are 

intensively cultivated private croplands in the immediate 

periphery of the reservoir; wildfires are common, especially 

during the relatively dry months, which extend from February 

to July, and there is a quarry within the headwaters of the Río 

Toa Vaca drainage basin. This quarry, although originally 

outside of the Río Toa Vaca drainage basin, has progressively 

mined resources northward across the hydrologic divide and 

could be contributing to additional erosion within the basin 

(fig. 4).

Method of Survey 

The 2002 bathymetric survey of Lago Toa Vaca involved 

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

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

the collected data. Survey lines were planned at a spacing of 50 

meters, beginning at the dam and continuing upstream along the 

two branches of Lago Toa Vaca (fig. 5). Bathymetric data were 

collected during June-July 2002 using a depth sounder coupled 

to a DGPS 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 personal 

(laptop) computer; these data served as the guide for 

bathymetric data collection. 

The reservoir pool elevation was monitored at the 

continuous recording USGS lake-level station Lago Toa Vaca 

at damsite near Villalba, Puerto Rico, station number 50111210 

(fig. 1). The pool elevation of Lago Toa Vaca was not at the 

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

adjusted using a time-elevation correction factor to represent 

depths at normal pool elevation of 164.90 meters above mean 

sea level.

A total of 17,345 data points (depth soundings) were 

collected over the entire reservoir while navigating along the 

planned survey lines (fig. 6). The depths along the cross 

sections were plotted, and 1-meter interval contour lines of 

equal depth were drawn from the shoreline to the deepest parts 

of the reservoir (plate 1). The procedure used to contour the 

reservoir bottom is explained further on in the Data Processing 

section. These contour lines were then converted into a 

triangulated irregular network (TIN) describing the reservoir 

bottom (fig. 7). 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 central portion is shown on figure 8. The 

original 1972 pre-impounded topography of the Lago Toa Vaca 

reservoir is shown on plate 2. The GIS utilized the TIN to 

calculate the storage capacity and thickness of sediment 

accumulation. The 1972 reservoir storage was compared with 

the 2002 calculated storage capacity to obtain an estimate of the 

historical sediment accumulation rate and to estimate the useful 

life of the reservoir based on historical and current data. 

Selected cross sections depicting the reservoir bottom 

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

reservoir bottom along the thalweg of Lago Toa Vaca were 

generated for 1972 and 2002 from their TIN surface models 

(figs. 9 and 10). The relation between pool elevation and 

reservoir storage capacity for 1972 and for 2002 was generated 

by calculating the reservoir volume at 1-meter elevation 

intervals and is shown in graphical form on figure 11.

Field Techniques

The bathymetric survey of Lago Toa Vaca was conducted 

during June-July, 2002. Data were collected using the 

bathymetric/land survey system (BLASS) developed by 

Specialty Devices, Inc. The system consists of two Novatel 

global positioning system (GPS) receivers coupled to a Depth 

Sounder model SDI-IDS Intelligent. The GPS receivers 

monitor the horizontal position of the survey boat while the 

depth sounder measures water depths. The GPS units were first 

used in static mode to establish a benchmark overlooking the 

reservoir. Satellite data were recorded simultaneously at the 

USGS District office benchmark (referred to as “USGS roof”) 

(latitude 18°25’N., longitude 66°06’W.) and at a site 

overlooking the reservoir. The benchmark coordinates for “Toa 

Vaca 1” (latitude 18°06’N., longitude 66°27’W.), were then 

calculated using the post-processing software CENTIPOINT. 

Although the new benchmark indicated a horizontal error of 

less than 10 centimeters, the coordinates listed previously are 

reported to the nearest minute. Once established, the “Toa Vaca 

1” benchmark was established as the reference station. One 

GPS unit was installed at the reference station; the other GPS 

unit was installed in the survey boat to be used as the mobile 

station. The GPS on board the survey boat independently 

calculated a position every second while receiving a set of 

correction signals from the reference station, converting the 

system into a DGPS. This combination maintained the data 

position accuracy within two meters. The bathymetric survey 

software HYPACK was used to navigate and to collect data. 

The software integrates the depth and position data, storing the 

x, y, (geographic locations) and z (depths) coordinates in a 

portable personal computer. 

A total of 159 survey sounding lines were planned using 

the GIS (fig. 5); however, sediment accumulation, low pool 

elevation, and vegetation growth in riverine areas limited the 

data collection to only 114 cross sections (fig. 6).
Shoreline Drainage area boundary Tributaries 18° 08 ́ 18° 09 ́ 18° 10 ́ 18° 11 ́

18° 07 ́

18° 06 ́

66° 30 ́ 66° 29 ́ 66° 28 ́ 66° 27 ́ 66° 26 ́ 66° 25 ́ 66° 24 ́ 

Approximate area ofquarry. Refer to figure 4 for details

Source: USGS Digital Orthophoto QuadrangleRío Descalabrado, November 13, 1995

EXPLANATION

00

1.0 2.0 KILOM

0.5 1.0 MILE

Figure 3. Lago Toa Vaca drainage area showing the reservoir shoreline and tributaries to Lago Toa Vaca.
Lago Toa VaBasin boundPortion of ar

0

0

100

250

200 METERS 200 METERS

500 FEET 500 FEET

EXPLANATION

Figure 4. Aerial photograph of part of Lago Toa Vaca drainage area showing the location of the quarry. Note that the resource exploitation has alt

boundary marked by a red line. (Photo taken on November 13, 1995.)
8 Sedimentation Survey of Lago Toa Vaca, Puerto Rico, June-July 2002

1 2

3

3

4

5 6

7 8

9

10

11

12

13 14 15 16 17 18 19

20

22

24

25

27 28

29 30 31 32

34 35 36 37

39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58

59 60 61 62 63

133

134

135

136

137

138 141

142

140 143

144

145

146

147

148 149

151

152153

154

157

158

159

155

156

139

64

65

66

67 6869

70

71

72

73

74

75

76

77 78

79

80

81 82

83 84

85

86

87

88

89

9091 92

93 94

96 95

97

98

99

100 101

103

113 102

115

116

117

118

119120

121

123 124

126

127

128

122

125

129

130

131

132

114

104

105

106

107

108

109

110

111 112

38

33

26

23

21

0

0 500 1,000 FEET

200 400 METERS

66°27 00 ′ ′′

Planned survey lines

and identification number

Río Toa Vaca branch

Quebrada Grande branch

Dam

150

EXPLANATION

66°29 30 ′ ′′ 66°29 00 ′ ′′ 66°28 30 ′ ′′ 66°28 00 ′ ′′ 66°27 30 ′ ′′

18°08 00 ′ ′′

18°07 30 ′ ′′

18°07 00 ′ ′′

18°06 30 ′ ′′

18°06 00 ′ ′′

18°05 30 ′ ′′

Figure 5. Planned location of sounding lines for the 2002 bathymetric survey of Lago Toa Vaca, Puerto Rico.
Method of Survey 9

Track lines where bathymetric

data were collected

EXPLANATION

0

0 500 1,000 FEET

200 400 METERS

Dam

Río Toa Vaca branch

Quebrada Grande branch

66°29 30 ′ ′′ 66°29 00 ′ ′′ 66°28 30 ′ ′′ 66°28 00 ′ ′′ 66°27 30 ′ ′′ 66°27 00 ′ ′′

18°08 00 ′ ′′

18°07 30 ′ ′′

18°07 00 ′ ′′

18°06 30 ′ ′′

18°06 00 ′ ′′

18°05 30 ′ ′′

Figure 6. Actual survey sounding lines of the 2002 bathymetric survey of Lago Toa Vaca, Puerto Rico.
10 Sedimentation Survey of Lago Toa Vaca, Puerto Rico, June-July 2002

0

0 500 1,000 FEET

200 400 METERS

Depth values, in meters

Triangulated irregular network 

of Lago Toa Vaca, Puerto Rico,

EXPLANATION

20.01-25.00

25.01-30.00

30.01-35.00

35.01-40.00

40.01-45.00

45.01-50.00

50.01-54.00

15.01-20-00

10.01-15.00

5.01-10.00

0.01-5.00

0

Dam

Río Toa Vaca branch

Quebrada Grande branch

66°29 30 ′ ′′ 66°29 00 ′ ′′ 66°28 30 ′ ′′ 66°28 00 ′ ′′ 66°27 30 ′ ′′ 66°27 00 ′ ′′

18°08 00 ′ ′′

18°07 30 ′ ′′

18°07 00 ′ ′′

18°06 30 ′ ′′

18°06 00 ′ ′′

18°05 30 ′ ′′

Figure 7. Triangulated irregular network (TIN) surface model of Lago Toa Vaca, Puerto Rico, for 2002.
Method of Survey 11

0

0 500 1,000 FEET

200 400 METERS

250

250

500

750

1,000

1,250

1,500

1,750

2,000

2,250

2,500

2,750

3,000

3,250

3,500 3,750

,0 4 00

4,250

4 5, 00

4,650

3,500

3,750

4,000

4,250

4,500

4,750

5,000

5,250

5,500 5,750

6,000

6,250

6,500

6,750

0

Dam

Distance from the dam,

in meters

Río Toa Vaca branch

Quebrada Grande branch

EXPLANATION

66°29 30 ′ ′′ 66°29 00 ′ ′′ 66°28 30 ′ ′′ 66°28 00 ′ ′′ 66°27 30 ′ ′′ 66°27 00 ′ ′′

18°08 00 ′ ′′

18°07 30 ′ ′′

18°07 00 ′ ′′

18°06 30 ′ ′′

18°06 00 ′ ′′

18°05 30 ′ ′′

Figure 8. Reference longitudinal distance along the thalweg of Lago Toa Vaca, Puerto Rico.
12 Sedimentation Survey of Lago Toa Vaca, Puerto Rico, June-July 2002

2002

SECTION 3

SECTION 10

SECTION 16

1972

0 164.90

164.90

164.90

10 154.90

154.90

154.90

20 144.90

144.90

144.90

30 134.90

134.90

134.90

40 124.90

124.90

124.90

50 114.90

114.90

114.90

60 104.90

104.90

104.90

0

10

20

30

40

50

60

0

10

20

30

40

50

60

0 100 200 300 400 500 600 700 800 900 1,000 1,100 1,200

DISTANCE FROM THE LEFT BANK, IN METERS 

DEPTH, IN METERS BELOW NORMAL POOL ELEVATION OF 164.90 METERS 

ABOVE MEAN SEA LEVEL

ELEVATION, IN METERS ABOVE MEAN SEA LEVEL

Vertical exaggeration x 6

Rockfill material for

dam construction

Figure 9. Selected cross sections generated from the TIN surface models of Lago Toa Vaca, Puerto Rico, for 1972 and 

2002. Refer to figure 5 for cross-section locations.
Method of Survey 13

0

10

20

30

40

50

60

0 100 200 300 400 500 600 700 800 900 1,000 1,100 1,200

0

10

20

30

40

50

60

0

10

20

30

40

50

60

2002

1972

DISTANCE FROM THE LEFT BANK, IN METERS 

Vertical exaggeration x 6

DEPTH, IN METERS BELOW NORMAL POOL ELEVATION OF 164.90 METERS 

ABOVE MEAN SEA LEVEL

164.90

164.90

164.90

154.90

154.90

154.90

144.90

144.90

144.90

134.90

134.90

134.90

124.90

124.90

124.90

114.90

114.90

114.90

104.90

104.90

104.90

ELEVATION, IN METERS ABOVE MEAN SEA LEVEL

SECTION 37

SECTION 31

SECTION 23

Figure 9. Selected cross sections generated from the TIN surface models of Lago Toa Vaca, Puerto Rico, for 1972 

and 2002. Refer to figure 5 for cross-section locations.—Continued
14 Sedimentation Survey of Lago Toa Vaca, Puerto Rico, June-July 2002

2002

1972

0

10

20

30

40

50

60

0 100 200 300 400 500 600 700 800 900 1,000 1,100 1,200

0

10

20

30

40

50

60

0

10

20

30

40

50

60

DISTANCE FROM THE LEFT BANK, IN METERS 

Vertical exaggeration x 6

DEPTH, IN METERS BELOW NORMAL POOL ELEVATION OF 164.90 METERS 

ABOVE MEAN SEA LEVEL

164.90

164.90

154.90

154.90

164.90

154.90

144.90

144.90

144.90

134.90

134.90

134.90

124.90

124.90

124.90

114.90

114.90

104.90

114.90

104.90

104.90

ELEVATION, IN METERS ABOVE MEAN SEA LEVEL

SECTION 44

SECTION 49

SECTION 56

Figure 9. Selected cross sections generated from the TIN surface models of Lago Toa Vaca, Puerto Rico, for 1972 and 

2002. Refer to figure 5 for cross-section locations.—Continued
Method of Survey 15

2002

1972

0

10

20

30

40

50

60

0

10

20

30

40

50

60

0 100 200 300 400 500 600 700 800 900 1,000 1,100 1,200

0

10

20

30

40

50

60

DISTANCE FROM THE LEFT BANK, IN METERS 

Vertical exaggeration x 6

DEPTH, IN METERS BELOW NORMAL POOL ELEVATION OF 164.90 METERS 

ABOVE MEAN SEA LEVEL

ELEVATION, IN METERS ABOVE MEAN SEA LEVEL

164.90

154.90

154.90

154.90

144.90

144.90

144.90

134.90

134.90

134.90

124.90

124.90

124.90

114.90

114.90

114.90

104.90

104.90

164.90

164.90

104.90

SECTION 74

SECTION 67

SECTION 62

Figure 9. Selected cross sections generated from the TIN surface models of Lago Toa Vaca, Puerto Rico, for 1972 and 

2002. Refer to figure 5 for cross-section locations.—Continued
16 Sedimentation Survey of Lago Toa Vaca, Puerto Rico, June-July 2002

2002

1972

0

10

20

30

40

50

60

0

10

20

30

40

50

60

0 100 200 300 400 500 600 700 800 900 1,000 1,100 1,200

0

10

20

30

40

50

60

DISTANCE FROM THE LEFT BANK, IN METERS 

Vertical exaggeration x 6

DEPTH, IN METERS BELOW NORMAL POOL ELEVATION OF 164.90 METERS 

ABOVE MEAN SEA LEVEL

164.90

164.90

164.90

154.90

154.90

154.90

144.90

144.90

144.90

134.90

134.90

134.90

124.90

124.90

124.90

114.90

114.90

114.90

104.90

104.90

104.90

ELEVATION, IN METERS ABOVE MEAN SEA LEVEL

SECTION 80

SECTION 86

SECTION 92

Figure 9. Selected cross sections generated from the TIN surface models of Lago Toa Vaca, Puerto Rico, for 1972 and 

2002. Refer to figure 5 for cross-section locations.—Continued
Method of Survey 17

2002

1972

0

10

20

30

40

50

60

0

10

20

30

40

50

60

0 100 200 300 400 500 600 700 800 900 1,000 1,100 1,200

0

10

20

30

40

50

60

DISTANCE FROM THE LEFT BANK, IN METERS 

Vertical exaggeration x 6

DEPTH, IN METERS BELOW NORMAL POOL ELEVATION OF 164.90 METERS 

ABOVE MEAN SEA LEVEL

164.90

164.90

164.90

154.90

154.90

154.90

144.90

144.90

144.90

134.90

134.90

134.90

124.90

124.90

124.90

114.90

114.90

114.90

104.90

104.90

104.90

ELEVATION, IN METERS ABOVE MEAN SEA LEVEL

SECTION 114

SECTION 107

SECTION 98

Figure 9. Selected cross sections generated from the TIN surface models of Lago Toa Vaca, Puerto Rico, for 1972 and 

2002. Refer to figure 5 for cross-section locations.—Continued
18 Sedimentation Survey of Lago Toa Vaca, Puerto Rico, June-July 2002

2002

1972

0

10

20

30

40

50

60

0

10

20

30

40

50

60

0 100 200 300 400 500 600 700 800 900 1,000 1,100 1,200

0

10

20

30

40

50

60

DISTANCE FROM THE LEFT BANK, IN METERS 

Vertical exaggeration x 6

DEPTH, IN METERS BELOW NORMAL POOL ELEVATION OF 164.90 METERS 

ABOVE MEAN SEA LEVEL

164.90

164.90

164.90

154.90

154.90

154.90

144.90

144.90

144.90

134.90

134.90

134.90

124.90

124.90

124.90

114.90

114.90

114.90

104.90

104.90

104.90

ELEVATION, IN METERS ABOVE MEAN SEA LEVEL

SECTION 135

SECTION 140

SECTION 144

Figure 9. Selected cross sections generated from the TIN surface models of Lago Toa Vaca, Puerto Rico, for 1972 

and 2002. Refer to figure 5 for cross-section locations.—Continued
Method of Survey 19

2002

1972

0

10

20

30

40

50

60

0

10

20

30

40

50

60

0 100 200 300 400 500 600 700 800 900 1,000 1,100 1,200

DISTANCE FROM THE LEFT BANK, IN METERS 

Vertical exaggeration x 6

DEPTH, IN METERS BELOW NORMAL POOL ELEVATION OF 164.90 METERS 

ABOVE MEAN SEA LEVEL

164.90

164.90

154.90

154.90

144.90

144.90

134.90

134.90

124.90

124.90

114.90

114.90

104.90

104.90

ELEVATION, IN METERS ABOVE MEAN SEA LEVEL

SECTION 155

SECTION 149

Figure 9. Selected cross sections generated from the TIN surface models of Lago Toa Vaca, Puerto Rico, for 1972 

and 2002. Refer to figure 5 for cross-section locations.—Continued
50

0

10

20

30

40

60

0

10

20

30

40

60

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 

50

2002

Río Toa Vaca branch

Quebrada Grande branch

1972

1972 

DISTANCE UPSTREAM FROM THE DAM, IN METERS 

DEPTH, IN METERS BELOW NORMAL POOL ELEVATION OF 164.90 METERS ABOVE MEAN SEA LEVEL

Vertical scale grea

~6 meters

~6 meters

~12 meters

~12 meters

~14 meters

~15 meters

~8 meters

~12 meters

Figure 10. Longitudinal profiles generated from the TIN surface models along the thalweg of Lago Toa Vaca, Puerto Rico, for 1972 and 2002
Method of Survey 21

100

110

120

130

140

150

160

170

VOLUME, IN MILLION CUBIC METERS

ELEVATION, IN METERS ABOVE MEAN SEA LEVEL

1972

2002

0 20 30 40 50 60 70 80 10

Normal pool elevation = 164.90 meters above mean sea level

Figure 11. Lago Toa Vaca capacity curves for 1972 and 2002.
22 Sedimentation Survey of Lago Toa Vaca, Puerto Rico, June-July 2002

Data Processing

Initial editing for the 2002 data was performed using the 

HYPACK software. Positions were corrected to eliminate 

anomalies that occur when the correction signal from the 

reference station is lost because of local topographic features or 

electromagnetic interference. Position errors were corrected by 

interpolating back to the middle point between the correct 

anteceding and preceding position. The depth data also were 

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 anteceding or preceding depth 

readings. Edited data were then transferred into the Arc/Info 

GIS database for further processing. The Arc/Info software was 

customized to color-code the depth data according to different 

depth intervals. Data points of the same color code were 

connected by adding a line between them, and a contour map of 

the reservoir bottom depth was generated (plate 1). The 

bathymetric contour map (plate 1) was used to create the TIN 

surface model of the reservoir bottom for 2002 (fig. 7). In 

addition, the 1972 pre-impoundment topography of Lago Toa 

Vaca, extracted from the 1:20,000 scale USGS Río 

Descalabrado topographic quadrangle, was converted into a 

TIN to re-calculate the original reservoir volume and to 

generate cross sections and longitudinal profiles.

Sampling the TIN every five meters along selected cross 

sections generated profiles representing the reservoir bottom 

from shore to shore for 1972 and 2002 (fig. 9). The same 

procedure used in generating the selected cross-section profiles 

was employed to generate the longitudinal profile along the 

thalweg of Lago Toa Vaca for the same years (fig. 10). 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 bottom from the dam upgradient to the river deltas of the tributaries.

The 1972 pre-impoundment topography of Lago Toa Vaca 

was scanned, geo-referenced (assign real-world coordinates and 

map projection) and converted into a TIN using the GIS. From 

this TIN, the 1972 original volume was calculated and yielded 

a volume of 64.16 million cubic meters or about 0.08 million 

cubic meters more than the 2002 calculated volume of 64.08 

million cubic meters. This minimal volume difference suggests 

that the reservoir essentially had not sedimented in 30 years, 

which is unrealistic. The 1972 original pre-impoundment 

topography of Lago Toa Vaca, obtained from the 1:20,000 

USGS Río Descalabrado topographic quadrangle was 

examined and no apparent error or inconsistency was found. To 

clarify any discrepancies, the original 1968 pre-impoundment 

engineering plans and topographic map used for the 

construction project was requested from the United States 

Bureau of Reclamation (USBR) to recalculate the original 

volume using the official project map. This map was essentially 

the same as the USGS topographic map with the same elevation 

intervals, with the exception that the USBR map had two 

distinct areas identified on the map: a rock quarry and a borrow 

area. These areas were mined to construct the earthfill dam. The 

storage capacity of the reservoir reported in the USBR map was 

68.94 million cubic meters. Therefore, the difference between 

the 1972 GIS-calculated and the 1968 USBR-reported volume 

(4.78 million cubic meters) could be the amount of rock and soil 

material removed from within the impoundment to construct 

the earthfill dam. The 1975 Puerto Rico Water Resources 

Authority (PRWRA), Phase I Inspection Report of the National 

Dam Safety Program informed that the Toa Vaca dam 

contained 2.17 million cubic meters of material. Therefore, the 

dam material volumes difference between the GIS calculated 

and USBR reported probably reflects the compaction of 

bedrock and soil material used to build the earthfill dam. 

Evidence of material removal scars was found when the 1968 

USBR reservoir shoreline was compared to the 1995 aerial 

photograph of the near-dam area, and while conducting the 

2002 bathymetric survey of Lago Toa Vaca (figs. 12, 13, and 

14).

Storage Capacity and Sediment 

Accumulation

The Lago Toa Vaca reservoir has been affected by a 

moderate sediment load and consequent storage loss since its 

construction in 1972. The storage capacity has decreased from 

68.94 million cubic meters in 1972 to 64.08 million cubic 

meters in 2002 or a total storage loss of 7 percent. The amount 

of sediment accumulated indicates an annual sedimentation or 

storage loss rate of 162,000 cubic meters per year, which 

represents an annual loss of 0.23 percent of the original 

reservoir volume.

A sedimentation survey conducted by the USGS in 1985 

indicated a reservoir volume of 62.44 million cubic meters 

compared to the 2002 volume of 64.08 million cubic meters 

(Figueroa-Alamo, 1991). This apparent increase in reservoir 

volume is unrealistic because no sediment removal activities 

were performed between 1985 and 2002.

A detailed analysis of the 1985 bathymetric survey data 

indicates two possible causes for the apparent unrealistic 

increased storage: (1) that survey used the maximum pool 

elevation volume as the baseline reference instead of the normal 

conservation pool elevation volume as the original storage 

capacity for the calculations, the sediment accumulation and 

storage loss are over-estimated and, (2) the 1985 bathymetric 

survey used only 31 cross sections in the computations 

compared to the 114 cross sections surveyed during 2002 

(fig. 15). From figure 15 it is evident that more than 1,500 

meters of the Río Toa Vaca branch, and more than 500 meters 

of the Quebrada Grande branch were not surveyed, therefore, a 

substantial water volume was unaccounted-for resulting in an 

underestimation of storage capacity for 1985. For these reasons 

the 1985 survey was not included in the analysis of the data in 

this report. Table 2 shows a comparison of the 1972 topographic 

survey with the 2002 bathymetric survey results.
Approximate borrow areaas shown in USBR plans

Rock-source 

Dam

Figure 12. Aerial photograph of the Lago Toa Vaca dam area showing the approximate extent of the borrow area used in construction of the earthf

Unites States Bureau of Reclamation plans.
Pre-impoundment shoreline Impoundment shoreline

Shoreline modification

EXPLANATION

Figure 13. Aerial photograph of the Lago Toa Vaca dam area showing the rock-source area and the resulting new shoreline.
Storage Capacity and Sediment Accumulation 25

Drill marks 

Rock-source area

Figure 14. Front-view perspective of the rock-source area and drill marks visible during the 2002 bathymetric survey 

of Lago Toa Vaca, Puerto Rico.
26 Sedimentation Survey of Lago Toa Vaca, Puerto Rico, June-July 2002

0

0 500 1,000 FEET

200 400 METERS

Dam

Planned survey lines

and identification number

4 for the1985 survey

Río Toa Vaca branch

Quebrada Grande branch

EXPLANATION

66°29 30 ′ ′′ 66°29 00 ′ ′′ 66°28 30 ′ ′′ 66°28 00 ′ ′′ 66°27 30 ′ ′′ 66°27 00 ′ ′′

18°08 00 ′ ′′

18°07 30 ′ ′′

18°07 00 ′ ′′

18°06 30 ′ ′′

18°06 00 ′ ′′

18°05 30 ′ ′′

Figure 15. Planned location of sounding lines for the 1985 bathymetric survey of Lago Toa Vaca, Puerto Rico.
Storage Capacity and Sediment Accumulation 27

The Lago Toa Vaca reservoir has 6 water-intake structures 

used for potable water withdrawals. The tower is located about 

130 meters upstream from the spillway, faces upstream, and is 

oriented in the northeastern direction. These intakes are at invert 

elevations of 121.00, 124.05, 132.59, 141.73, 150.88, 160.02 

meters above mean sea level, respectively. The invert elevation 

refers to the elevation of the lowest portion of the water intake.

The volume of water contained above the elevation of the 

operational 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 2002 bathymetric 

data indicate that the reservoir bottom in the vicinity of the 

water intake tower was at an approximate elevation of 118.90 

meters above mean sea level. This suggests that all of the water 

intakes are operational and that the bed of the reservoir is about 

two meters below the invert elevation of the lowest intake 

structure. Using the lowest operational intake structure invert 

elevation of 121.00 meters above mean sea level, the live 

storage of Lago Toa Vaca is about 62.86 million cubic meters 

and the dead storage is about 1.22 million cubic meters. The 

storage capacity of Lago Toa Vaca at 1-meter elevation 

intervals is listed in table 3.

The longitudinal bottom profiles “stairs-like” shape of the 

1972 pre-impounded topography of Lago Toa Vaca (fig. 10) is 

the result of an artifact of the Arc/Info TIN-generating software. 

The program is instructed to generate the bottom profile from 

the deepest to the shallowest areas of the reservoir. Thus, the 

program starts generating the profile at the dam portion of the 

reservoir and continues upstream towards the river deltas. 

Because the topographic contour intervals of the 1972 map is 10 

meters; there is considerable distance between the contour 

values; therefore, the software maintains the previous value 

until it finds the next corresponding value resulting in a stairslike bottom profile. Therefore, the reservoir bottom profile 

slope and shape is better illustrated by interpolating the 

preceding and subsequent contour values (fig. 10, adjusted 

bottom profile).

Table 2. Comparison of the 1972 topographic survey with the 2002 bathymetric survey of Lago Toa Vaca, Puerto Rico.

[---, undetermined]

Year 1972 2002

Drainage area at dam site, in square kilometers 56.8 56.8

Reservoir surface area, in square kilometers 3.19 3.22

Storage capacity, in million cubic meters 68.94 64.08

Live storage, in million cubic meters 64.01 62.86

Dead storage, in million cubic meters 4.93 1.22

Years since construction 0 30

Sediment accumulated, in million cubic meters 0 4.86

Long-term storage loss, in percent 0 7

Long-term annual loss of capacity, in cubic meters 0 162,000

Long-term annual loss of capacity, in percent 0 0.23

Drainage area runoff, in million cubic meters1

1Represents the historically measured runoff into Lago Toa Vaca.

21.63 21.63

Storage capacity to annual inflow ratio 3.19 2.96

Trapping efficiency, in percent 99 98

Sediment yield, in cubic meters per square kilometer per year --- 3,086

Year that the reservoir would fill with sediment --- 2400
28 Sedimentation Survey of Lago Toa Vaca, Puerto Rico, June-July 2002

Pool elevation Storage capacity 

 164.90 64.08

 163.90 60.91

 162.90 57.84

 161.90 54.87

 160.90 52.00

 159.90 49.23

 158.90 46.58

 157.90 44.04

 156.90 41.60

 155.90 39.25

 154.90 36.99

 153.90 34.81

 152.90 32.71

 151.90 30.70

150.90 28.78

149.90 26.96

148.90 25.22

147.90 23.57

146.90 22.01

145.90 20.54

144.90 19.14

143.90 17.82

142.90 16.55

141.90 15.35

140.90 14.21

139.90 13.12

138.90 12.08

137.90 11.09

136.90 10.15

135.90 9.26

134.90 8.42

133.90 7.63

132.90 6.89

131.90 6.20

130.90 5.54

129.90 4.92

128.90 4.35

127.90 3.82

126.90 3.34

125.90 2.90

124.90 2.50

123.90 2.12

122.90 1.78

121.90 1.48

120.90 1.20

119.90 0.94

118.90 0.70

117.90 0.49

116.90 0.31

115.90 0.16

114.90 0.06

113.90 0.04

112.90 0.02

111.90 0.005

110.90 0.000

Pool elevation Storage capacity 

Table 3. Storage capacity of Lago Toa Vaca, Puerto Rico, June-July 2002. 

[all elevations in meters above mean sea level, all capacities in million cubic meters] 
Sediment Trapping Efficiency and Water Renewal Rates 29

The long-term sediment accumulation after 30 years of 

reservoir impoundment is not uniform. By nature, the 

morphology of a reservoir bottom immediately after 

impoundment is wedge-shaped, very similar to the topographic 

relief of the surrounding hill slopes. Thus, it provides little 

submerged surface area for sediment deposition and sediment 

accumulates rapidly and vertically at a relatively fast rate. After 

years of sediment accumulation, the topographic relief of the 

impounded area reshapes from a wedge to a trapezoidal-shaped 

surface; therefore, sediment accumulation occurs over a larger 

area and the rate at which water depth decreases with time due 

to sediment accumulation is lower. This apparent non-uniform 

long-term reduction of depth is more evident in reservoirs that 

are surveyed on a more frequent basis (Webb and Soler-López, 

1997).

Along the Río Toa Vaca branch, the profile presented on 

figure 10 indicates that a 6-meter thick layer of sediment has 

accumulated from the base of the dam to about 1,000 meters 

upstream. A more uniform thickness ranging from 12 to 14 

meters of sedimentation extends from about 1,500 meters to a 

distance of about 4,500 meters upstream from the dam. A 

sediment layer about 8 meters thick, between a distance of 

5,000 and 6,500 meters, and a layer of about 12 meters thick 

about 6,900 meters upstream from the dam have accumulated. 

In the riverine portion of the Río Toa Vaca tributary, about 4 

meters of sediment have been deposited. The long-term 

sediment deposition rates in these same segments are: 20, 40, 

30, 40, and 13 centimeters per year, respectively, averaging 

about 30 centimeters per year over the entire branch.

On the Quebrada Grande branch, sediment deposition 

patterns are the same as in the Río Toa Vaca branch to about 

3,200 meters upstream from the dam (fig. 10). At about 4,100 

meters upstream from the dam; the total thickness of sediment 

is about 15 meters, and in the riverine portion of the branch, the 

sediment layer is about 5 meters thick. The deposition rates in 

the Quebrada Grande branch averages about 26 centimeters per 

year. At the historical sediment deposition rate of about 20 

centimeters per year near the dam, the reservoir bottom in the 

vicinity of the water intakes could reach the lowest intake in 

about 10 years (by 2010).

Sediment Trapping Efficiency and Water 

Renewal Rates

Heinemann (1981) considered trapping efficiency to be the 

most informative descriptor of a reservoir. This value is the 

proportion of the incoming sediment that is deposited or trapped 

in a pond, reservoir or lake. Trapping efficiency is dependent on 

several parameters, 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 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 (1953) developed a 

curve (fig. 16) 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 Toa Vaca was 

estimated using the relation established by Brune (1953).

Although the Lago Toa Vaca drainage area has a streamgaging station on the Río Toa Vaca tributary with 13 years of 

continuous record (USGS station Río Toa Vaca above Lago 

Toa Vaca, Puerto Rico, number 50110900), the station accounts 

for only 36.8 of the entire 56.8 square-kilometer basin of Lago 

Toa Vaca, which leaves 20 square kilometers ungaged. The 

mean-annual runoff for the 36.8 square-kilometer drainage area 

above the stream-gaging station for the 13 years of record 

(water years 1989 to 2001) is 14.17 million cubic meters. If the 

same ratio of runoff to drainage area is applied to the adjacent 

ungaged Quebrada Grande tributary (20 square kilometers), the 

annual runoff from this portion is 7.46 million cubic meters. 

Therefore, the estimated long-term mean-annual runoff for the 

entire Lago Toa Vaca drainage area is 21.63 million cubic 

meters.

An alternative method of estimating how much rainfall 

becomes runoff into the Lago Toa Vaca drainage area, is to use 

regional correlations developed for drainage basins in Puerto 

Rico (Giusti and López, 1967) to estimate the average ratio of 

runoff to rainfall (runoff/rainfall). For the Toa Vaca Basin, this 

ratio can be estimated as 0.23. Thus, multiplying the meanannual rainfall of 1,778 millimeters (Calvesbert, 1970) of the 

Lago Toa Vaca basin by the runoff/rainfall ratio of 0.23, the 

estimated mean-annual runoff for the Lago Toa Vaca basin is 

409 millimeters. This number multiplied by the 56.8 square 

kilometer drainage area of Lago Toa Vaca, yields an estimated 

mean annual inflow to the reservoir of 23.23 million cubic 

meters, which is in close agreement with the measured 21.63 

million cubic meters obtained by using the USGS streamgaging station data. The measured mean-annual runoff into the 

Lago Toa Vaca of 21.63 million cubic meters was used for the 

trapping efficiency calculations.

With an original (1972) storage capacity of 68.94 million 

cubic meters and a present (2002) storage capacity of 64.08 

million cubic meters, the ratio of storage capacity to inflow is 

3.19 for 1972 and 2.96 for 2002. Using the median curve of 

Brune’s (1953) relation (fig. 16), and the corresponding 

capacity to inflow ratio, the estimated trapping efficiency of 

Lago Toa Vaca was 99 and 98 percent for 1972 and 2002, 

respectively. However, according to Brune’s empirical relation 

the trapping efficiency of a reservoir will decrease as sediment 

fills the reservoir and lowers the storage capacity. Therefore, 

with a long-term storage capacity loss of only 0.23 percent per 

year, the trapping efficiency of Lago Toa Vaca will remain high 

into the forseeable future.
30 Sedimentation Survey of Lago Toa Vaca, Puerto Rico, June-July 2002 TRAPPING EFFICIENCY (PERCENT OF SEDIMENT INFLOW THAT IS RETAINED)

100

90

80

70

60

50

40

30

20

10

0

0.001

0.002

0.003

0.005

0.007

0.01

0.02

0.03

0.05

0.07

0.1

0.2

0.3

0.5

0.7

1.0

2.0

3.0

5.0

7.0

10.0

RATIO OF STORAGE CAPACITY TO ANNUAL INFLOW (C/I)

Primarily highly flocculated and

coarse-grained sediments

Median curve

Primarily colloidal and dispersed

fine-grained sediments

CAPACITY TO INFLOW RATIO

1972 C/I = 3.19

2002 C/I = 2.96

Figure 16. Reservoir trapping efficiency as a function of the ratio between storage capacity and annual water inflow volume.
Sediment Yield and Reservoir Life Expectancy 31

Taking into consideration that the storage capacity of Lago 

Toa Vaca was to receive water from intra-basin transfers, it was 

designed with a storage capacity three times larger than the 

mean-annual runoff of its basin; however, the plans were never 

completed, and therefore, the pool elevation of Lago Toa Vaca 

is persistently low. Based on the estimated mean-annual inflow 

to the reservoir, the Lago Toa Vaca drainage area supplies only 

about 34 percent of the reservoir water volume, and it would 

normally take about three years to renew the totality of the 

water (without taking into account the reservoir water 

withdrawals).

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 (McManus and Duck, 1993). For the period from 

1972 to 2002, the total estimated volume of sediment 

contributed to Lago Toa Vaca from a drainage area of 56.8 

square kilometers was estimated to be 4.96 million cubic 

meters. This volume was estimated by dividing 4.86 million 

cubic meters of sediment accumulation (the volume loss 

between 1972 and 2002) by the estimated trapping efficiency of 

0.98 for 2002. This estimated rate of sediment influx (4.96 

million cubic meters) divided by the years between 1972 and 

2002 (30 years), results in an average volume of about 165,333 

cubic meters of sediment per year. The sediment volume 

deposited in the Lago Toa Vaca (165,333 cubic meters per year) 

divided by the net sediment contributing area of 53.58 square 

kilometers, (the total drainage area of 56.8 square kilometers 

minus the 3.22 square kilometer surface area of the reservoir) 

results in an average basin sediment yield or reservoir storage 

loss of 3,086 cubic meters per square kilometer per year. An 

estimate of the sediment yield from the drainage area of Lago 

Toa Vaca on a mass basis can be obtained by using the sediment 

dry-bulk density of one gram per cubic centimeter reported for 

Lago Yahuecas, a reservoir located about 25 kilometers 

northwest of Lago Toa Vaca (Soler-López and others, 1998). 

Therefore, the Lago Toa Vaca sediment yield on a mass basis is 

3,086 megagrams per square kilometer per year. This drainage 

area-normalized storage capacity loss of Lago Toa Vaca is more 

than twice the average of 1,389 cubic meters per square 

kilometer per year of 14 reservoirs studied in Puerto Rico at 

different locations, soil-type basins, and rainfall magnitudes 

(Soler-López, 2001).

The life expectancy of Lago Toa Vaca, or any other 

reservoir can be estimated by dividing the remaining storage 

capacity by the annual storage capacity loss. Based on the 

average reservoir storage loss between 1972 and 2002, the 

reservoir would be completely silted in about 400 years.

Summary and Conclusions

The Lago Toa Vaca Dam was constructed in 1972 to 

increase the water availability for irrigation in the southern 

coastal plain of Puerto Rico. The southern coastal plain receives 

an average rainfall of about 900 millimeters per year, an amount 

insufficient to sustain the cultivation of crops without irrigation. 

In addition, the reservoir was to be part of a multi-purpose 

project which included intra-basin tunnels to divert flow to the 

Toa Vaca reservoir. However, the mega project was cancelled 

in the 1970’s and currently the reservoir receives only the runoff 

from its drainage basin. At present, public demand for potable 

water has increased in the region and has put additional pressure 

on the limited resource. In order to update data on the water 

storage capacity and the safe yield of Lago Toa Vaca, the U.S. 

Geological Survey conducted a bathymetric survey of the 

reservoir during June-July 2002 in cooperation with the Puerto 

Rico Infrastructure Financing Authority. The bathymetry 

survey was conducted using state-of-the-art geographic 

information system and global positioning system technology. 

The data obtained were analyzed and compared to the 1972 

pre-impoundment topography to determine sedimentation rates, 

sediment location and accumulation, and to predict the useful 

life of the reservoir

Agricultural land-use practices, mining activities, and 

events such as wildfires enhance erosion affecting the Lago Toa 

Vaca storage capacity by increasing sediment accumulation and 

storage loss. The 2002 bathymetric survey of Lago Toa Vaca 

indicates that the reservoir has lost 4.86 of an original 68.94 

million cubic meters of water storage capacity. This represents 

a long-term storage loss of 0.23 percent per year. With a current 

reservoir trapping efficiency of 98 percent, any increase in rural 

development and land disruption within the Lago Toa Vaca 

drainage area could result in an increased sediment yield, which 

is currently 3,086 cubic meters per square kilometer per year, 

about two and one half times higher than the average rate for 

other reservoirs in Puerto Rico. At the current long-term 

sedimentation rate and estimated sediment trapping efficiency, 

Lago Toa Vaca would be completely silted by the year 2400. 

The life expectancy of Lago Toa Vaca is long according to 

recent data, and sediment accumulation in the reservoir does not 

present an immediate threat to the continued use of essential 

reservoir structures.
32 Sedimentation Survey of Lago Toa Vaca, Puerto Rico, June-July 2002

References Cited

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of the American Geophysical Union, v. 34, no. 3, p. 407-418.

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Virgin Islands: U.S. Department of Commerce, 

Environmental Science Services Administration, 29 p.

Environmental Systems Research Institute, Inc., 1992, Surface 

modeling with TIN, Surface analysis and display: 

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Figueroa-Alamo, Carlos, 1991, Sedimentation survey of Lago 

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830.

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Soler-López, L.R., 2001, Sedimentation survey results of the 

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February 22 and 23, 2001, unpaginated CD.

U.S. Department of Agriculture, Soil Conservation Service, 

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watershed, Puerto Rico, p. 2.

Verstraeten, G., and Poesen, J., 2000, Estimated trap efficiency 

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the assessment of sediment yield: Progress in Physical 

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history of Lago Loíza, Puerto Rico, 1953-1994: U. S. 

Geological Survey Water-Resources Investigations Report 

97-4108, 18 p., 9 pls.
District Chief

Caribbean District

U.S. Geological Survey

Water Resources Division

GSA Center, Suite 400-15

651 Federal Drive

Guaynabo, Puerto Rico 00965-5703

Soler-López—Sedimentation Survey of Lago Toa Vaca, Puerto Rico, June-July 2002—SIR 2004-5035