.U.S. Geological Survey 

U.S. Department of the Interior 

Prepared in cooperation with the 

PUERTO RICO AQUEDUCT AND SEWER AUTHORITY 

Sedimentation Survey of Lago Caonillas, 

Puerto Rico, February 2000 

Water-Resources Investigations 

Report 01-4043 
U.S. Department of the Interior 

U.S. Geological Survey 

Sedimentation Survey of Lago Caonillas, Puerto Rico, 

February 2000 

By Luis R. Soler-L6pez 

Water-Resources Investigations Report 01-4043 

In cooperation with the I 

PUERTO RICO AQUEDUCT AND SEWER AUTHORIT~ 

San Juan, Puerto Rico: 2001 
U.S. DEPARTMENT OF THE INTERIOR 

GALE A. NORTON, Secretary 

U.S. GEOLOGICAL SURVEY 

Charles G. Groat, Director 

Use of trade names in this report is for identification purposes only and does not imply endorsement by the 

U.S. Government. 

For additional information write to: 

District Chief 

U.S. Geological Survey 

GSA Center, Suite 400-15 

651 Federal Drive 

Guaynabo, Puerto Rico 00965 

Copies of this report can be purchased from: 

U.S. Geological Survey 

Branch of Information Services 

Box 25286 

Denver, CO 80225 
CONTENTS 

Abstract .............................................................................................................................................................. .. 

Sumario ............................................................................................................................................................... . 

Introduction.......................................................................................................................................................... 2 

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

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

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

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

Storage Capacity and Sediment Accumulation.................................................................................................... 20 

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

Sediment Yield .. . . . . . . . . . . .. .. .. . .. . .. .. . .. . . . . . . . . . . . . . .. . . . . . . . .. . . . . .. .. . .. . . . . . .. . . . . .. . .. . .. . . . . . . . . . . . . . .. . . . . . .. .. . . . . . . . . . . . . . .. . . .. .. .. .. .. . . . . . . . . . .. . . . . 24 

References ........................................................................................................................................................ '.... 25 

PLATE 

[Plate is in pocket] 

1. Lago Caonillas, Puerto Rico, Bathymetry, February 2000 
FIGURES 

1.-5. Map showing 

1. Location of Lago Caonillas in the Rio Grande de Arecibo basin, Puerto Rico......................................... 4 

2. Planned cross-section locations for the February 2000 bathymetric survey of Lago Caonillas, 

Puerto Rico . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . 6 

3. Actual track lines of the February 2000 bathymetric survey of La go Caonillas, Puerto Rico . . . . . . . . . .. . . . . . . . 8 

4. Reference longitudinal distances along the different branches of Lago Caonillas, Puerto Rico............... 9 

5. Selected cross-section locations for the February 2000 bathymetric survey of Lago Caonillas, 

Puerto Rico . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . 10 

6.-9. Graphs showing 

6. Selected cross sections generated from the TIN surface model of Lago Caonillas, Puerto Rico, 

for 1995 and 2000................................................................................................................................... 11 

7. Longitudinal profiles along the thalweg of the different branches of Lago Caonillas, Puerto Rico 

for 1995 and 2000. .. . ..... .. . .. .. .. . .. . .. .. . . .. . . .. ... ... .... .. . ... .. .. .. . .. .. . . . . .. . .. ... .. . . .. . ... ...... ... . ...... ... .. . ... ..... .. .. . .. . ... . . . .... 18 

8. Storage capacity curves for Lago Caonillas, Puerto Rico, for 1995 and 2000 .......................................... 19 

9. Storage capacity to inflow relation established by Brune.......................................................................... 23 

TABLES 

1. Principal characteristics of Lago Caonillas and Caonillas dam as of 2000 .................................................... 3 

2. Storage capacity of Lago Caonillas, Puerto Rico, for February 2000, at 1-meter elevation intervals........... 20 

3. Comparison of the 1990, 1995, and 2000 sedimentation surveys of Lago Caonillas, Puerto Rico............... 21 
CONVERSION FACTORS, DATUMS, and ACRONYMS 

Multiply 

centimeter 

meter 

kilometer 

square meter 

square kilometer 

square kilometer 

cubic centimeter 

cubic meter 

cubic meter 

million cubic meters 

By 

Length 

0.03281 

3.281 

0.6214 

Area 

10.76 

0.3861 

247.1 

Volume 

To obtain 

foot 

foot 

mile 

square foot 

square mile 

acre 

0.06102 cubic inch 

35.31 cubic foot 

0.0008107 acre-foot 

81 0. 7 acre-foot 

Volume per unit time (includes flow) 

cubic meter per second 

cubic meter per second 

cubic meter per second 

35.3 1 cubic feet per second 

15,850 gallon per minute 

22.83 million gallons per day 

Mass per area (includes sediment yield) 

Datums 

gram per cubic centimeter 

kilogram per square kilometer 

megagram per square kilometer 

megagram per year 

62.43 pound per cubic foot 

0.002855 ton per square mile 

2.855 ton per square mile 

1.1 02 ton per year 

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 

GIS Geographic Information System 

PRASA Puerto Rico Aqueduct and Sewer Authority 

TIN Triangulated Irregular Network 

USGS U.S. Geological Survey 
~ .. 

~ 

~ ... • . <i\ i.~ _._ 

[· -
Sedimentation Survey of Lago Caonillas, Puerto Rico, 

February 2000 

By Luis R. Soler-lopez 

Abstract 

Lago Caonillas, a reservoir owned by the 

Puerto Rico Electric Power Authority and located 

in the central part of Puerto Rico, is one of the two 

reservoirs (the other being Lago Dos Bocas) 

proposed to supply water for the Puerto Rico 

Aqueduct and Sewer Authority project called the 

Superaqueduct. The reservoir was impounded in 

1948 and originally provided about 55 .66 million 

cubic meters of water for hydroelectric power 

generation. Sediment derived from the reservoir 

basin has been transported and deposited in the 

reservoir bottom, substantially decreasing the 

water storage capacity over time. Successive 

bathymetric surveys indicated that in 1990 the 

storage capacity was 49.25 million cubic meters, 

decreasing to 48.80 million cubic meters in 1995 

and to 42.27 million cubic meters in 2000. This 

represents an overall storage loss of about 

11.5 percent by 1990, 12.3 percent by 1995 and 

24.1 percent by 2000. The long-term sedimentation 

rate of the reservoir was about 153,000 cubic 

meters per year in 1990, remaining almost constant 

at about 146,000 cubic meters per year in 1995, but 

nearly doubling to 258,000 cubic meters per year in 

2000. The two-fold increase in sedimentation rate, 

and consequently, the reservoir storage capacity 

loss, can be attributed to Hurricane Hortense in 

September 1996 and Hurricane Georges in 

September 1998. Twenty-four percent of the 

original storage capacity of Lago Caonillas has 

been lost to sediment accumulation. About 49 

percent of the reservoir sediment was deposited in 

the last five years, demonstrating the impact of 

these major storms on the reservoir. 

Based on the ratio of storage capacity to 

inflow rate, the estimated trapping efficiency of 

Lago Caonillas is about 93 percent for 2000. The 

sediment yield of the Lago Caonillas net sedimentcontributing drainage area (total drainage area 

minus the reservoir surface area) of 218.74 square 

kilometers, is about 1 ,266 megagrams per square 

kilometer per year. This represents an increase of 

about 69 percent in the material transport and 

deposition process of the Lago Caonillas basin 

between 1990 and 2000. The life expectancy of 

Lago Caonillas was more than 300 years in 1995; 

however, at the storm-accelerated sedimentation 

rate, the life expectancy has decreased to about 

164 years. This implies that the reservoir could be 

filled with sediments by the year 2164 if major 

hurricanes continue to pass through Puerto Rico 

regularly (every 2 to 4 years). 

Sumario 

Lago Caonillas, un embalse perteneciente a Ia 

Autoridad de Energfa Electrica de Puerto Rico y 

localizado en Ia region central de Puerto Rico, es uno 

de los dos embalses que se han propuesto para suplir 

agua al proyecto del Superacueducto de Ia Autoridad 

de Acueductos y Alcantarillados de Puerto Rico (el 

otro embalse propuesto es Lago Dos Bocas). El 

embalse comenz6 a operar en 1948 y originalmente 

provefa unos 55.66 millones de metros cubicos de 

agua para generar energfa hidroelectrica. Con el paso 

de los afios, el fondo del embalse se ha ido llenando de 

sedimento proveniente de Ia cuenca del embalse, lo 

cual ha disminuido sustancialmente su capacidad de 

almacenaje de agua. Estudios de batimetrfa sucesivos 

indicaron que en 1990 Ia capacidad de almacenaje del 
embalse era de 49.25 millones de metros cubicos, Ia 

cual disminuy6 a 48.80 millones de metros cubicos en 

1995, y a 42.27 millones de metros cubicos en el 2000. 

Esto representa una perdida total de alrededor de 11.5 

por ciento para 1990, 12.3 por ciento para 1995 y 24.1 

por ciento para el 2000. La tasa de sedimentaci6n del 

embalse a largo plazo era de alrededor de 153,000 

metros cubicos al afio en 1990, Ia cual permaneci6 casi 

constante a alrededor de 146,000 metros cubicos al 

afio en 1995, pero casi duplicada a 258,000 metros 

cubicos para el 2000. El aumento simultaneo de 

sedimentaci6n y, por consiguiente, Ia perdida de 

capacidad de almacenaje del embalse pueden 

atribuirse al paso del huracan Hortense en septiembre 

de 1 996 y al paso del huracan Georges en septiembre 

de 1998. Veinticuatro por ciento de Ia capacidad de 

almacenaje original del Lago Caonillas se ha perdido 

debido a Ia acumulaci6n de sedimento. Alrededor del 

49 por ciento del sedimento del embalse se ha 

depositado en los ultimos cinco afios, lo cual 

demuestra el impacto de estas dos grandes tormentas 

en el embalse. 

Basados en Ia relaci6n entre capacidad de 

almacenaje y afluente, Ia eficiencia estimada del Lago 

Caonillas para atrapar sedimento es de un 93 por 

ciento para el 2000. El rendimiento de sedimento del 

area neta de aportaci6n de sedimento de 218.74 

kil6metros cuadrados del Lago Caonillas (area total de 

drenaje menos el area de superficie del embalse) es de 

alrededor de 1,266 megagramos por kil6metro 

cuadrado al afio. Esto representa un aumento de un 69 

por ciento en el proceso de transporte y acumulaci6n 

de sedimento en Ia cuenca del Lago Caonillas entre 

1990 y 2000. La expectativa de vida del Lago 

Caonillas era de mas de 300 afios en 1995; sin 

embargo, al incrementar Ia tasa de sedimentaci6n por 

el paso de las tormentas, I a expectati va de vida se ha 

reducido a alrededor de 164 afios. Esto implica que el 

sedimento pudiera llenar Ia totalidad del embalse para 

el afio 2164 si huracanes como estos continuaran 

afectando a Puerto Rico con regularidad ( cada 2 a 4 

afios). 

INTRODUCTION 

As originally designed, Lago Caonillas 

generates hydroelectric power at a facility downstream 

of the Caonillas dam. The water used to generate 

power is released into the Rfo Caonillas delta of Lago 

Dos Bocas, and is then available for additional power 

generation at the Lago Dos Bocas dam. Lago 

Caonillas is proposed to be a supplemental part of the 

Puerto Rico Aqueduct and Sewer Authority (PRASA) 

project called the Superaqueduct. Plans are to capture 

water released during hydroelectric power generation 

at the Lago Dos Bocas dam in a retention pond 

constructed adjacent to the Rfo Grande de Arecibo 

below the dam. The captured water will then be used 

to supply potable water for north coast residents of the 

island of Puerto Rico. The project's success, however, 

depends on the combined available storage capacity of 

the Caonillas and Dos Bocas reservoirs. 

Bathymetric surveys of Lago Caonillas in 1990 

and 1995 had been used to estimate the storage 

capacity and rates of storage loss (Webb and SolerL6pez, 1996). Recent decreases in storage capacity of 

both reservoirs, however, have constrained the original 

project plans. The passage of Hurricane Hortense 

during September 1996 brought torrential rains and 

extensive land erosion. Two years later, during 

September 1998, Hurricane Georges made landfall on 

the island of Puerto Rico, and the rainfall and sediment 

erosion and transport were even greater than that 

associated with Hurricane Hortense. During October 

1999, a sedimentation survey of Lago Dos Bocas was 

performed in cooperation with PRASA to determine 

the impact of these storms on Lago Dos Bocas. 

Because Lago Caonillas is the larger reservoir in terms 

of storage capacity for the Superaqueduct project, it 

was necessary to conduct a sedimentation survey to 

assess the impact of Hurricanes Hortense and Georges 

on Lago Caonillas as well. 

During February 8 to 11, 2000, the U.S. 

Geological Survey (USGS), in cooperation with 

PRASA, conducted a bathymetric survey of Lago 

Caonillas to determine the storage capacity, the 

sedimentation rate of the reservoir, the amount and 
location of deposited material, and to estimate the 

trapping efficiency of Lago Caonillas and the sediment 

yield of the reservoir drainage area. A differential 

global positioning system (DGPS) coupled to a depth 

sounder was used to collect geographic positions and 

water depths. The data were stored in digital form and 

then transferred into a Geographic Information System 

(GIS) for processing and analysis. The GIS was then 

used to calculate the existing storage capacity, the 

volume and distribution of accumulated sediment, and 

to determine the sedimentation rate of the reservoir as 

of February 2000. 

DAM, RESERVOIR, AND DRAINAGE BASIN 

CHARACTERISTICS 

Lago Caonillas is located approximately 7 

kilometers east of the town of Utuado and about 10 

kilometers northwest of the town of Jayuya (fig. 1 ). 

The reservoir dam was completed in 1948 and is the 

principal unit of the Caonillas Hydroelectric project. 

The dam is a concrete gravity structure with a total 

length of 248.41 meters, a maximum height of 71.63 

meters, and a maximum base width of 59.44 meters. 

Nonoverflow sections on each abutment have a total 

length of 183.79 meters. (Sheda and Legas, 1968). 

The principal characteristics of Lago Caonillas and 

Caonillas dam are listed in table 1. 

Table 1. Principal characteristics of La go Caonillas and Caonillas dam as of 2000 (modified from Noll, 

1953 and Sheda and Legas, 1968) 

[Elevations in meters above mean sea level] 

Total length of dam at top (spillway and nonoverflow sections) .................. ..................................... 248.41 meters 

Length of spillway section ...... .............. ... ................ .............................. .............................................. 61 .0 meters 

Elevation of crest of spillway ............................................................................................................ 251.76 meters 

Maximum width at base ...................................................................................................................... 59.44 meters 

Crown elevation of penstocks ......................................................................................................... 213.06 meters 

Installed power-generating capacity .............................................................................................. 22,000 kilowatts 

Maximum flood-level storage ........................... ..................................... ......................... 71.4 million cubic meters 

Design discharge over the spillway at a head of 7.93 meters 

(elevation 259.69 meters) ......................... ....... ........... ............................................. 3,030 cubic meters per second 

Surcharge storage (flood control) 1 

•••••.•.•.•..•••.••...••••.••.••••••••••.•...•••.•..••••.•••••.•...•...•.••...... 22.6 million cubic meters 

Drainage area at damsite 2 ............................................................................................... 221.44 square kilometers 

Maximum height of dam .......................... .. ..... ...................... ................. ............. ........ ........................ 71.63 meters 

Maximum depth during the February 2000 survey 3 ........................... ................................................. ... .40 meters 

February 2000 reservoir surface area .................................................................................. 2.70 square kilometers 

1 Assumes that the capacity between elevations of 251.76 and 259.69 meters has not changed since dam construction. 

2 1ncludes 126.65 square kilometers of the natural Caonillas drainage basin and 94.79 square kilometers that is diverted 

from the Rio Grande de Arecibo drainage basin (Noll, 1953, p. 10). 

3 Below spillway elevation of 251.76 meters above mean sea level. 
ATLANTIC 

.. >-·•·/ 

.r ··"' / ... .?.-/ '·-....., ,'; 

~/ '\ 

( / \_/'" 

0 1 2 3 4 5 KILOMETERS I I I I II II I 

0 2 3 4 5 MILES 

OCEAN 

PUERTO RICO 

II FLORIDA 

EXPLANATION 

50026140-&- Lake-elevation station 

and number 

50026025£. Surface-water station 

and number 

II Town 

r Tunnel flow direction 

Cafio tiburones 

flow direction 

- ··- Drainage basin 

boundary 

Figure 1. Location of Lago Caonillas in the Rio Grande de Arecibo basin, Puerto Rico. 
Although flash boards were originally installed 

at the dam to increase the storage capacity of the 

reservoir, they were removed after a short period of 

time. The original storage capacity of the reservoir 

was reported to be 61.86 million cubic meters at the 

maximum pool elevation of 252.98 meters above 

mean sea level with flash boards (Noll, 1953). In 1995, 

however, the original storage capacity was recomputed 

using GIS technology and yielded a capacity of 55.66 

million cubic meters at the spillway elevation of 

251.76 meters above mean sea level (Webb and SolerL6pez, 1996). The recomputed original storage 

capacity of 55.66 million cubic meters is used as the 

starting point for calculations of storage loss in this 

report. 

The predominant soil types in the Lago 

Caonillas drainage basin are those within the PellejasLirios-Ingenio association of the Arecibo area. The 

Pellejas soil type constitutes about 85 to 90 percent of 

the total soil coverage. These soils are generally deep, 

steep, and excessively drained with slopes ranging 

from 40 to 60 percent. Typically, the surface layer is a 

clay loam about 15 centimeters thick. The subsoil is a 

sandy clay loam about 25 centimeters thick. The 

substratum is sandy loam to a depth of about 18 meters 

or more. Infiltration is moderate in the upper layers 

and high in the lower layer. The available water 

capacity is moderate and the runoff is very rapid, thus 

making this soil association somewhat unstable and 

suited for erosion by runoff (Acevedo, 1982). 

Vegetation is abundant with pasture, fallow fields, 

forest, and rangeland. Agricultural practices within the 

basin have declined over time, and only small private 

lands are currently being farmed. 

METHOD OF SURVEY 

The sedimentation survey of Lago Caonillas 

involved planning, data collection, processing, and 

analysis. A GIS (Arc/Info) was used to plan the 

cross-section locations for the survey and for analysis 

of the bathymetric data. Cross-section locations were 

established at a spacing of 50 meters starting at the 

dam and continuing upstream along the different 

branches of the reservoir (fig. 2). Position and depth 

data were acquired using a DGPS coupled to a depth 

sounder. A total of 21 ,545 data points with x, y 

(geographic location) and z (depth) coordinates were 

collected over the entire reservoir. Using the positional 

and depth data, a contour map representing the 

reservoir bottom surface was constructed. The contour 

map was converted into a triangulated irregular 

network (TIN) surface model of the reservoir using the 

GIS. For this study, GIS algorithms (Environmental 

Systems Research Institute, 1992) were used to 

calculate the volume of the tridimensional surface 

model represented by the 2000 contour map. From the 

TIN, the storage capacity and sedimentation rate were 

calculated. The same methodology was used 

previously to calculate reservoir volumes for 1990 and 

1995. The algorithms used for all volume calculations 

are mathematical equations intrinsic to the Arc/Info 

program. 

Field Techniques 

The bathymetric data were collected during 

February 8 to 11, 2000. The reservoir pool elevation 

was continuously monitored at the USGS lake-level 

station Lago Caonillas at damsite near Utuado, Puerto 

Rico (number 50026140, fig. 1 ). Water was 

continuously flowing over the spillway structure (just 

a film) during the survey, therefore, depth data did not 

have to be corrected to represent depths below 

spillway elevation. The Bathymetric/Land Survey 

System (BLASS) developed by Specialty Devices, 

Inc., was used for the bathymetric survey. The system 

consists of two Novate I DGPS receivers for horizontal 

positioning of the survey boat. One unit was located at 

a master or reference station with known coordinates 

and the other was installed on the survey boat and used 

as the mobile unit. 

The benchmark "LOOKCAON2" 

(18°17'17.771"N., 66°39'29.206"W.) established by 

the USGS in 1998, was used to locate the reference 

station on the north side of the Lago Caonillas dam, on 

a hill with an unobstructed view of the entire reservoir. 
66°40'00" 

18°17'00" 

0 100 300 500 METERS 

0 500 1,000 1,500 FEET 

EXPLANATION 

/ " 

/ 

/ 

Planned cross section 

and identification number 

Dam 

97 

99 

101 

Figure 2. Planned cross-section locations for the February 2000 bathymetric survey of La go Caonillas, 

Puerto Rico. 
The DGPS unit at the reference station sent correction 

signals to the DGPS unit on the survey boat every 5 

seconds, to maintain the horizontal position precision 

of the survey boat to within 2 meters of the true 

geographic position while navigating along the 

planned survey lines. When the correction signal of 

the reference station was lost because elevated land 

surfaces obstructed the view, a signal repeater was 

installed at a convenient location to regain correction 

signals. Reservoir depths were measured using a 

BLASS-MSU-IDS Intelligent Depth Sounder that 

collects depth data with an accuracy of 2 centimeters. 

The depth sounder was calibrated at water depths of 

2, 6, 10, and 16 meters. The bathymetric survey 

software HYPACK was used to collect data and to 

navigate. HYPACK received and recorded water depth 

and geographic position every second while in survey 

mode. The data were stored in a computer hard disk 

and were later transferred into the GIS. Plans were to 

collect data at 155 pre-established cross sections to 

match most of the 1995 survey lines. Sediment 

accumulation in the riverine sections of the reservoir, 

however, limited data collection to 139 cross sections. 

Figure 3 shows the actual track lines where 

bathymetric data were collected for the February 

2000 bathymetric survey of Lago Caonillas. The 

longitudinal distances from the dam upstream to 

the different branches of the reservoir are shown in 

figure 4. 

Data Processing 

Initial editing and verification of the positional 

and depth data were performed within the HYPACK 

program. Positions were corrected to eliminate 

anomalous spikes or jumps, which can occur if the 

signal transmission from the satellites or reference 

station are obstructed by local topographic features or 

disrupted by electromagnetic interference. In these 

cases, the erroneous positions were interpolated 

between the correct anterior and posterior positions. 

Depth data were also corrected to eliminate erroneous 

depth readings. Errors in depth readings can be 

generated by floating debris or insufficient signal gain 

of the fathometer. In these cases, the incorrect depth 

was interpolated between the correct anterior and 

posterior depth readings. The edited data were then 

transferred into the GIS, where data points were color 

coded according to different depth values. Then, data 

points having the same color were connected manually 

by drawing a line between the points, and a reservoir 

bathymetric contour map was generated (plate 1 ). The 

contour map of the reservoir was used to generate a 

TIN surface model of the reservoir. From the TIN 

surface model, selected cross sections (locations 

shown on figure 5) representing the reservoir bottom 

from shore to shore (fig. 6) and longitudinal profiles 

along the different branches of the reservoir (fig. 7) 

were generated. These cross sections and longitudinal 

profiles were compared with the 1995 data to show the 

extent and location of sediment accumulation. 

The Arc/Info program was customized by 

creating an Arc-Macro Language (a series of 

sub-commands within a master command) to calculate 

the reservoir water volume at 1-meter datum 

increments, and capacity curves for 1995 and 2000 

were generated (fig. 8). The reservoir surface model 

was divided into three distinct segments, and the 

volumes of those individual segments for 1995 and 

2000 were calculated. The volumes comparison is 

discussed later in this report. 
66°40'00" 

18°17'00" ,-----------------r---------------,.--------, 

0 100 300 500 METERS 

o ' '5bo '1 ,boo' ' 1.5oo FEET 

EXPLANATION 

Track lines where bathymetric 

data were collected 

/ Dam 

Figure 3. Actual track lines of the February 2000 bathymetric survey of La go Caonillas, Puerto Rico. 
Dam 

// 

1,000 / 

/ 

f 

1,250 ~ 0 '<-, 

1.() 

,...._ 

,.... 

0 100 300 500 METERS 

0 500 1,000 1,500 FEET 

EXPLANATION 

Distance from the dam, 

in meters 

/Dam 

5,000 

5,250 

Figure 4. Reference longitudinal distances along the different branches of Lago Caonillas, Puerto Rico. 
66°40'00" 

18°17'00".------------------.--------------------.-----, 

Dam 

EXPLANATION 

'\ / Model cross section and 

/ identification number 

/Dam 

11 

0 100 300 500 METERS 

6 I 

1

5601 

I 1 .ood I 1 ,5oo FEET 

Figure 5. Selected cross-section locations for the February 2000 bathymetric survey of La go Caonillas, 

Puerto Rico. 
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> w 

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/ SECTION 1 / 10 / 

I 

'/ 

20 ~ 

I 

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30 

40 

50 

0 

10 ~ 

' ' '\ 

20 ' ' '\ 

\ 

30 

40 -------------- SECTION 2 

50 

0 

10 SECTION 3 

20 

30 

40 ------- --2000 --..___ ______ --- 1995 

50 

0 100 200 300 400 500 600 

Vertical exaggeration X 3.75 

DISTANCE FROM THE LEFT BANK, IN METERS 

Figure 6. Selected cross sections generated from the TIN surface model of La go Caonillas, Puerto Rico, 

for 1995 and 2000. Refer to figure 5 for cross-section locations. 
...J 

w 

> w 

...J 

c( 

w 

en 

z 

c( 

w 

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> 0 

r:c 

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......... __ 

" " \ 

\ 

--------------.../ 

SECTION 4 

/ 

I 

I 

I 

SECTION 5 

/ 

'/ 

I 

/ 

SECTION 6 

--2000 

--- 1995 

/, 

5o~~~~~~~~~~~~~~~~~~_L~~~~~~~~~~~~~~~~~~ 

0 100 200 300 400 500 600 

Vertical exaggeration X 3.75 

DISTANCE FROM THE LEFT BANK, IN METERS 

Figure 6. Selected cross sections generated from the TIN surface model of La go Caonillas, Puerto Rico, for 

1995 and 2000-Continued. Refer to figure 5 for cross-section locations. 
..J 

w 

> w 

..J 

<( 

w 

rn 

z 

<( 

w 

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m 

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0 

10 

20 

30 

40 

50 

0 

10 

20 

30 

40 

50 

0 

10 

20 

30 

40 

50 

0 

I 

\ 

, _________ I / 

/ 

/ 

I 

_ -........ 

~-----------------------~ 

100 200 300 400 

SECTION 7 

/ --- SECTION 8 

SECTION 9 

--2000 

--- 1995 

500 600 

Vertical exaggeration X 3. 75 

DISTANCE FROM THE LEFT BANK, IN METERS 

Figure 6. Selected cross sections generated from the TIN surface model of La go Caonillas, Puerto Rico, 

for 1995 and 2000-Continued. Refer to figure 5 for cross-section locations. 
10 fQ~ I I~ -y I 

-~~ ~~ - -- ,.,..- ......_ _______ _.,.. 

I I 

-

SECTION 10 

20 f- -

30 f- -

40 f- -

so~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 

0 100 200 300 

2 

3 

4 

I 

I 

I 

.....__\ I 

\ I 

\ I 

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\ I 

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SECTION 11 

s~~~~~~~~~~~~ 

0 100 200 

400 500 

I 

1 \ I 

\ I .._ ___ / 

SECTION 12 

2 

3 

4 --2000 

--- 1995 

s~~~~~~~~~~~~ 

0 100 200 

Vertical exaggeration X 3. 75 

DISTANCE FROM THE LEFT BANK, IN METERS 

600 

Figure 6. Selected cross sections generated from the TIN surface model of Lago Caonillas, Puerto Rico, for 1995 

and 2000-Continued. Refer to figure 5 for cross-section locations. 
.J 

w 

> w 

.J 

<C 

w 

t/) 

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0 

SECTION 13 

10 

20 -------

30 

40 

SECTION 14 

0 

SECTION 15 

10 

20 

30 

40 --2000 

--- 1995 

50 

0 100 200 300 400 500 600 

Vertical exaggeration X 3.75 

DISTANCE FROM THE LEFT BANK, IN METERS 

Figure 6. Selected cross sections generated from the TIN surface model of La go Caonillas, Puerto Rico, 

for 1995 and 2000-Continued. Refer to figure 5 for cross-section locations. 
...J 

w 

> w 

...J 

<( 

w 

en 

z 

<( 

w 

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w 

> 0 

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0 

o~~~~~~~~~~~~~~~/~~~~~~~~~~~~~~~~~~~~~~ 

/ 

SECTION 16 

10 

20 

30 

40 

o~~~~~~~~~~~~~~~=-~/~~~~~~~~~~~~~~~~~~~~ 

10 

' ----------------" I 

20 

I 

I 

/ 

/ SECTION 17 

~ 30 

> w 

...J 

w 

~ 

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en 

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g 10 

SECTION 18 

w 

r::a 

en 

0::: 

w 

1-

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z 

J: 

ID. 

w 

c 

--2000 

--- 1995 

so~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 

0 100 200 300 400 500 600 

Vertical exaggeration X 3. 75 

DISTANCE FROM THE LEFT BANK, IN METERS 

Figure 6. Selected cross sections generated from the TIN surface model of Lago Caonillas, Puerto Rico, for 

1995 and 2000-Continued. Refer to figure 5 for cross-section locations. 
..J 

w 

[ij 

..J 

~ 0 

z 

c( 

w 

:E 

w 

~ m 

c( 

~ w 

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10 

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20 

30 

40 

01"""'<... ....... I I I . I 

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10r20r30r40r-

-~ 

I 

I 

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I 

SECTION 19 

SECTION 20 

SECTION 21 -

-

-

--2000 -

--- 1995 

so~~~~~~~~~~~~~~~~~~~~~~~~·~~~~~~~~~~~~~~~~~ 

0 100 200 300 400 500 600 

Vertical exaggeration X 3. 75 

DISTANCE FROM THE LEFT BANK, IN METERS 

Figure 6. Selected cross sections generated from the TIN surface model of Lago Caonillas, Puerto Rico, for 

1995 and 2000-Continued. Refer to figure 5 for cross-section locations. 
..J 

w 

> w 

..J 

<( ~ 10 I I 

~ 20 I 

w 

:!E 

w 30 

f) 

m ~ 1 

___ r~------~~----~~-~-~/ Rio Caonillas branch 

________ / ____ _ <( 

tn ffi 50 

1-

w 

:!E 

~ 0 

"'"" 

~ 10 LL 

0 

z 

0 

w 

~ ..J 

w 

Southern branch 

~ 

~ 5o~~~-L-L~~~~L_~~~-L_L~~~~L_~~~_L_L~~~~L_~~~_L_L~~~~L_~~~-L-L-L~~~L_L_~~~~ 

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ii: 

tn 

== 0 

o~-.-.-.-.~~------~~------~~--.-.-~-.~-.-.,.-.-.-.-~-.-.-.-.,.-.-.-.-~-.-.-.-.~.-.-.-.-.-~-.-.~ 

..J 

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al 

10 

~ 20 

w 

1-

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~ 40~- -- 1- ---

fu 50 

c 0 

j 

~ 

"/" 

2,000 2,500 3,000 3,500 4,000 4,600 5,000 

DISTANCE FROM THE DAM, IN METERS 

Figure 7. Longitudinal profiles along the thalweg of the different branches of La go Caonillas, Puerto Rico, for 1995 and 2000. 

Eastern branch 

--2000 

--· 1995 

5,500 6,000 

Vertical scale X 25 
255 

Spillway elevation = 251.76 meters above mean sea level 

/ 

250 / / / / / / / / / 

245 / 

/ 

/ 

/ 

/ 

..J / 

w / 

> 240 / 

w / ..J / 

< / 

w / 

U) / 

z I 

< 235 I 

w I 

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> I 

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> w I 

..J I 

w I 

I 

215 I 

I 

I 

I 

210 --2000 

--- 1995 

205 

0 10 20 30 40 50 60 

VOLUME, IN MILLION CUBIC METERS 

Figure 8. Storage capacity curves for La go Caonillas, Puerto Rico, for 1995 and 2000. 
STORAGE CAPACITY AND SEDIMENT 

ACCUMULATION 

The bathymetric surveys and GIS analysis 

indicate that the storage capacity of Lago Caonillas 

has decreased considerably, about 12.6 percent over 

the last 10 years and 24.1 percent since construction. 

In 1990, the storage capacity was 49.25 million cubic 

meters, decreasing to 48.80 million cubic meters in 

1995 and to 42.27 million cubic meters in 2000. The 

capacity loss of 0.45 million cubic meters from 1990 

to 1995 was minimal. On the other hand, of the 13.39 

million cubic meters of sediment that has accumulated 

since dam construction, 6.53 million cubic meters or 

49 percent of the total accumulation occurred between 

1995 and 2000. Probably most of this recent sediment 

accumulation between 1995 and 2000 actually 

occurred between 1996 and 1998. Table 2 shows the 

February 2000, storage capacity of Lago Caonillas at 

1-meter stage intervals. 

The accelerated loss of storage capacity in the 

last 5 years can be attributed primarily to two major 

storms: Hurricane Hortense in September 1996 and 

Hurricane Georges in September 1998. These storms 

brought intense rainfall, so that water and sediment 

discharge into Lago Caonillas was high. Peak 

discharges measured at the Rfo Caonillas at Paso 

Palmas, Puerto Rico, gaging station (number 

50026025, fig. 1) were 626 and 1,020 cubic meters per 

second for Hurricanes Hortense and Georges, 

respectively. Both peak discharges are historical highs 

for the period of record at the station (from October 

1995 to present day). Runoff entering the reservoir 

from the other tributaries was not measured. Historical 

peak suspended-sediment loads entering the reservoir, 

measured at station Rfo Caonillas at Paso Palmas were 

449,000 tonnes on September 10, 1996, and 864,000 

tonnes on September 22, 1998 (associated with 

Hurricanes Hortense and Georges, respectively; Dfaz 

and others, 1998 ). 

Table 2. Storage capacity of La go Caonillas, Puerto 

Rico, for February 2000, at 1-meter elevation intervals 

Elevation, in meters above 

mean sea level 

251.76 

250.76 

249.76 

248.76 

247.76 

246.76 

245.76 

244.76 

243.76 

242.76 

241.76 

240.76 

239.76 

238.76 

237.76 

236.76 

235.76 

234.76 

233.76 

232.76 

231.76 

230.76 

229.76 

228.76 

227.76 

226.76 

225.76 

224.76 

223.76 

222.76 

221.76 

220.76 

219.76 

218.76 

217.76 

216.76 

215.76 

214.76 

213.76 

212.76 

211.76 

Volume, in million 

cubic meters 

42.27 

39.67 

37.25 

35.02 

32.87 

30.83 

28.87 

27.00 

25.19 

23.48 

21.84 

20.32 

18.88 

17.55 

16.28 

15.10 

13.97 

12.90 

11.87 

10.91 

9.98 

9.12 

8.30 

7.52 

6.78 

6.08 

5.41 

4.77 

4.15 

3.55 

2.98 

2.45 

1.94 

1.51 

1.12 

0.80 

0.51 

0.27 

0.07 

0.02 

0.00 
Although several sedimentation surveys of the 

reservoir have been conducted in the past, for the 

purpose of this report (the impact of two major storms 

on the reservoir) only the results of the latest three 

bathymetric surveys are presented. Table 3 

summarizes the results of the 1990, 1995, and 2000 

sedimentation surveys of Lago Caonillas. 

Sediment accumulation in the reservoir is not 

uniform. More than half of the sediment accumulation 

between 1995 and 2000 occurred along the Rio 

Caonillas and southern branches of the reservoir. On 

the Rio Caonillas branch, a total of about 3.23 million 

cubic meters accumulated since 1995, according to 

GIS calculations. The volume for the Rio Caonillas 

branch was calculated for the portion of the reservoir 

upstream of selected cross section number 7. On the 

southern branch of Lago Caonillas, a total of about 

0.44 million cubic meters have accumulated in the 

same period of time. The volume was calculated for 

the portion of the reservoir upstream of selected cross 

section number 17. The third segment consisted of the 

remaining portion of the reservoir downstream of 

selected cross sections 7 and 17. 

The combined sediment accumulation in both 

upstream branches is 3.67 million cubic meters. This 

represents about 56 percent of the total sediment 

accumulation of 6.53 million cubic meters in the 

reservoir since 1995. The storage loss of the reservoir 

downstream of selected cross sections 7 and 17 is 

about 2.86 million cubic meters or 44 percent of the 

total loss. 

Table 3. Comparison of the 1990, 1995 (Webb and Soler-L6pez, 1996), and 2000 sedimentation surveys 

of Lago Caonillas, Puerto Rico 

[ ---, no data available or undetermined] 

Total capacity, in millions of cubic meters 

Live storage, in millions of cubic meters 2 

Dead storage, in millions of cubic meters 3 

Years since construction 

Sediment accumulated, in millions of cubic meters 

Storage loss, in percent 

Annual loss of capacity, in percent 

Intersurvey sedimentation rate, in cubic meters per year 

Long-term sedimentation rate, in cubic meters per year 

Year the reservoir is projected to fill 

Sediment yield, in megagrams per square kilometer per year 4 

1990 

42 

6.41 

11.5 

0.27 

152,619 

2313 

7505 

1995 

48.80 

48.35 

0.45 

47 

6.86 

12.3 

0.26 

90,000 

145,957 

2329 

7176 

1 From Webb and Soler-L6pez (1996). Original capacity in 1948 was 55.66 millions of cubic meters. 

2 Above penstock crown elevation of 213.06 meters above mean sea level 

3 Below penstock crown elevation of 213.06 meters above mean sea level 

4 Assuming a dry bulk density of 1 gram per cubic centimeter and excluding the reservoir surface 

area of 2.70 square kilometers. 

5 Adjusted by the trapping efficiency of the reservoir for 1990. 

6 Adjusted by the trapping efficiency of the reservoir for 1995. 

7 Adjusted by the trapping efficiency of the reservoir for 2000. 

2000 

42.27 

42.24 

0.03 

52 

13.39 

24.1 

0.46 

1,306,000 

257,500 

2164 

1,2667 
On the Rfo Caonillas branch, a layer averaging 

about 3.5 meters in thickness has accumulated since 

1995 (fig. 6, sections 8 to 12 and 20 to 21 ). On the 

southern branch an average of about 4 meters in 

thickness accumulated in the same period of time 

(fig. 6, sections 17 to 19). Some of the cross sections 

presented in figure 6 show what could be bank 

slumping and material depositional patterns near the 

reservoir shore or may be artifact differences in 

surveys resolution. The soils comprising the Lago 

Caonillas shoreline were subjected to extraordinary 

hydraulic and aerodynamic forces during the storms 

which could have eroded shoreline material. Lago 

Caonillas also is subjected to seasonal changes in pool 

elevation that exposes the banks to weathering. Scour 

and depositional features are apparent in some areas 

near the shoreline. During major flood events such as 

the ones generated by hurricanes, the reservoir may 

behave like a natural river channel with areas of very 

high water velocities and erosion along the outside of 

meanders and areas of low water velocities and 

deposition inside of meanders. Another explanation 

for this apparent scour and depositional pattern is that 

the 1995 survey was conducted at low pool elevation, 

and the data near the reservoir shoreline (from depths 

between 22 meters and 0 meter) were generated from 

aerial photographs taken by Caribbean Aerial Surveys 

Inc. in 1990. Depth data generated from aerial 

photography are not as reliable as bathymetric 

surveys, and some discrepancies are expected, but are 

considered to be relatively small compared to the 

volume of sediment influx between 1995 and 2000. 

Sediment accumulation in the vicinity of the 

penstock structure at the dam has reached an elevation 

of 211.76 meters above mean sea level, according to 

the 2000 data. The crown elevation (the elevation of 

the upper part of the structure) is at 213.06 meters 

above mean sea level. This means that the reservoir 

bottom is about 1.3 meters from reaching the elevation 

of the penstock. In terms of dead storage (the volume 

below the elevation of the penstock structure, used 

to accommodate sediments without disabling the 

structure), the capacity was reduced from 0.45 million 

cubic meter in 1995 to 0.03 million cubic meter in 

2000. In a future flood event, the penstock could 

become inoperable by the influx of sediment if the 

structure is not operated on a regular basis. The 

sluiceway structure (at an elevation of 202.69 meters 

above mean sea level) is currently buried under about 

9 meters of sediment. 

Lago Caonillas is currently an effective 

sediment trap, such that insignificant quantities of 

sediments pass from Lago Caonillas reservoir to Lago 

Dos Bocas reservoir (Soler-L6pez and Webb, 1998). 

Additional moderate to severe flood events, however, 

will disable the penstock structure if it is not operated 

regularly. Most importantly, because dead storage has 

been reduced to almost zero, considerable amounts of 

sediments will begin to pass to Lago Dos Bocas 

through the power-generating structure, which could 

exacerbate the rapidly declining storage capacity of 

Lago Dos Bocas (Soler-L6pez, 2000). 

TRAPPING EFFICIENCY 

Heinemann ( 1981) suggested that the single 

most informative descriptor of a reservoir is its 

trapping efficiency. 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. They include the 

particle size distribution, which controls the trapping 

efficiency in relation to retention time (for example, 

the average time that the incoming runoff remains in 

the reservoir). Coarser material will have a higher 

settling velocity, and will require less time for 

deposition. Very fine material, on the other hand, 

requires more time to be deposited. The trapping 

efficiency is also determined by the characteristics of 

the inflow hydro graph in terms of high- and base-flow 

runoffs and frequencies. Small runoffs tend to have 

low rates of sediment influx, while large runoffs tend 

to have high rates of sediment influx. Also, the shape 

of the reservoir and outlet structures influence the 

retention time, and hence, the trapping efficiency 

(Verstraeten and Poesen, 2000). 

Many empirical studies showing the relation 

between reservoir storage capacity, water inflow, and 

trapping efficiency have been conducted, of which 

Brune's (1953) is the most widely used and accepted. 

Brune developed a curve that estimates the trapping 
efficiency of a reservoir based on the relation of 

storage capacity to annual inflow (fig. 9). The trapping 

efficiency of Lago Caonillas was estimated using the 

relation established by Brune. 

Although a stream gaging station measures the 

inflow to Lago Caonillas from the Rfo Caonillas 

(USGS station number 50026025), this station was 

installed in October 1995 and does not have sufficient 

period of record to estimate the long-term flow 

characteristics of the basin. Thus, the neighboring 

basin of Rfo Saliente was used to estimate the average 

annual runoff for the Lago Caonillas drainage area. 

The Rfo Saliente basin has somewhat similar 

topography, land use, and slopes, and the runoff/ 

rainfall ratio is considered to be similar to the Lago 

Caonillas basin. The USGS surface-water station Rfo 

Saliente at Coabey near Jayuya, Puerto Rico (number 

100 

1-

z 90 w 

(.) 

0:: 

w 

D. 80 

~ 

c 

w 70 D. 

D. 

~ 60 1-

1-

z 

w 

:E 50 

c 

50025155) has an average annual runoff of 1.12 

meters for a period from 1989 to 1998. (Dfaz and 

others, 1998). The average annual rainfall for the Rfo 

Saliente basin is 2.29 meters (Calvesbert, 1970), thus 

the runoff/rainfall ratio equals 0.49. The Lago 

Caonillas drainage area has the same average annual 

rainfall of 2.29 meters (Calvesbert, 1970). Multiplying 

the average annual rainfall of 2.29 by the runoff/ 

rainfall ratio of 0.49 gives the estimated runoff of 

1.12 meters per year for the Lago Caonillas basin. 

Multiplying this value by the Lago Caonillas drainage 

area gives an estimated annual inflow value of 248.01 

million cubic meters. Using the median curve of Brune 

(fig. 9) the estimated trapping efficiency of Lago 

Caonillas was about 94, 93, 93, and 93 percent during 

1948, 1990, 1995, and 2000, respectively, giving an 

average long-term trapping efficiency of 93 percent 

for Lago Caonillas. 

Primarily highly flocculated and 

coarse-grained sediments 

w 

~ ~-- Primarily colloidal and dispersed 

>- 40 

(.) 

z 

w 

0 30 

u:: 

LL. 

w 

" 20 z 

a: 

D. 

~ 10 

1-

0 

0 

0 

0 

N (t) lC) I'- 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

N M 0 0 

0 0 

fine-grained sediments 

lC) I'- ..... 

0 0 0 

0 0 

RATIO OF CAPACITY TO ANNUAL INFLOW 

Figure 9. Storage capacity to inflow relation established by Brune (1953). 

0 0 

C\i c-? 
SEDIMENT YIELD 

Sediment yield has been defined by the 

American Society of Civil Engineers as the total 

sediment outflow from a catchment or drainage basin, 

measurable at a point of reference and for a specified 

period of time per unit of surface area (McManus and 

Duck, 1993). For Lago Caonillas, the total amount of 

sediment that has entered the reservoir (14.40 million 

cubic meters) was estimated by dividing the 

accumulated sediment (13.39 million cubic meters) by 

the long-term trapping efficiency (0.93). To determine 

the average annual rate of sediment influx, 14.40 

million cubic meters was divided by the age of the 

reservoir (52 years), to give 276,923 cubic meters per 

year. The sediment yield of the Lago Caonillas 

drainage basin was calculated by dividing 276,923 

cubic meters per year by 218.74 square kilometers (the 

drainage area minus the reservoir surface area) 

resulting in a yield of 1 ,266 cubic meters per square 

kilometer per year. Assuming a sediment dry-bulk 

density of one gram per cubic centimeter, the 

estimated sediment yield of the Lago Caonillas 

drainage area is about 1,266 megagrams per square 

kilometer per year for 2000. 

Using the same calculations, the average annual 

sediment yield of Lago Caonillas was 750 megagrams 

per square kilometer per year in 1990, decreasing to 

717 megagrams per square kilometer per year in 1995 

and almost doubling to I ,266 megagrams per square 

kilometer per year in 2000. This represents a 69 

percent increase in erosion rate within the Lago 

Caonillas basin since 1990. The true sediment yield of 

the basin, however, is likely higher than estimated 

because the calculations do not account for eroded 

material resulting from Hurricanes Hortense and 

Georges, as well as other rainfall events which is 

temporarily stored in river channels upstream from the 

reservoir. High flows such as those tend to flush 

downstream previously eroded material, but also 

deposit additional material in the river beds upstream 

of the reservoir. This temporarily stored material has 

the potential to reduce the storage capacity of the 

reservoir further when transported and deposited into 

the reservoir during future floods. 

The life expectancy of Lago Caonillas did not 

seem to be a pressing concern according to the 

previous surveys of 1990 and 1995 (table 3); however, 

the 2000 survey indicates that large storm events such 

as Hurricanes Hortense and Georges can substantially 

reduce the life expectancy of the reservoir. These 

storm events induced erosion and transport of large 

volumes of material that is naturally weathered or 

made available by human activity. Although these 

flood events do not regularly occur, such events will 

likely affect Puerto Rico in the future. If the stormaccelerated sedimentation rate recorded between 1995 

and 2000 continues, Lago Caonillas has a useful life of 

about 164 more years. However, the actual life 

expectancy of Lago Caonillas could be somewhat 

shorter or longer depending on the future rainfall 

frequencies and magnitudes. 
REFERENCES 

Acevedo, G., 1982, Survey of Arecibo area, northern 

Puerto Rico: U. S. Department of Agriculture, 

Soil Conservation Service, Washington D.C., 

p. 32, 1 pl. 

Brune, G.M., 1953, Trap efficiency of reservoirs: 

Transactions of the American Geophysical Union, 

v. 34,no. 3,p.407-418. 

Calves bert, R.J., 1970, Climate of Puerto Rico and the 

U.S. Virgin Islands: U.S. Department of 

Commerce, Environmental Science Services 

Administration, 29 p. 

Dfaz, P.L., Aquino, Zaida, Figueroa-Alamo, Carlos, 

Vachier, R.J., and Sanchez, A.V., 1998, Water 

Resources Data, Puerto Rico and the U.S. Virgin 

Islands, Water Year 1998: U.S. Geological Survey 

Water Data Report PR-98-1, 629 p. 

Environmental Systems Research Institute, Inc., 1992, 

Surface modeling with TIN, surface analysis and 

display: Redlands, Cal., 8 chapters. 

Heinemann, H. G., 1981, New sediment trap efficiency 

curve for small reservoirs: Water Resources 

Bulletin, v. 7, p. 825-830. 

McManus, J., and Duck, R.W., eds., 1993, 

Geomorphology and sedimentology of lakes and 

reservoirs: Chapter 6, Reservoir sedimentation 

rates in the Southern Pennine Region, UK: John 

Wiley & Sons, New York, p. 73-92. 

Noll, J.J., 1953, The silting of Caonillas reservoir, 

Puerto Rico: U.S. Department of Agriculture 

Publication SCS-TP-119, 22 p. 

Sheda, H.E., and Legas, James, 1968, Condition of 

Caonillas Dam, Puerto Rico, in Condition of 

concrete dams in Puerto Rico: U.S. Department 

of the Interior, Bureau of Reclamation, section 4, 

18 p., 13 pis. 

Soler-L6pez, L.R., 2000, Sedimentation survey of 

Lago Dos Bocas, Puerto Rico, October 1999: 

U.S. Geological Survey Water-Resources 

Investigations Report 00-4234, 18 p. 

Soler-L6pez, L.R., and Webb, R.M.T., 1998, 

Sedimentation Survey of Lago Dos Bocas, Puerto 

Rico, June 1997: U.S. Geological Survey WaterResources Investigations Report 98-4188, 14 p., 

2 pis. 

Soler-L6pez, L.R., Webb, R.M.T., and CarrasquilloNieves, R.A., 2000, Sedimentation survey of 

Lago La Plata, Puerto Rico, October 1998: U.S. 

Geological Survey Water-Resources 

Investigations Report 00-4045, 23 p. 

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

efficiency of small reservoirs and ponds: Methods 

and implications for the assessment of sediment 

yield, Progress in Physical Geography, June 2000, 

v. 24,no. 2,p. 219-251. 

Webb, R.M.T., and Soler-L6pez, L.R., 1996, 

Sedimentation survey of Lago Caonillas, Puerto 

Rico, February 1995: U.S. Geological Survey 

Water-Resources Investigations Report 96-4153, 

27 p., I pl.