"CAUSES AND CONTROL OF SEDIMENTATION IN THE PARAPETI
RIVER BASIN, BOLIVIA, SOUTH AMERICA"
Prepared for the Organization of American States
By Law Environmental-Caribe, San Juan, Puerto Rico
The objective of this project is the analysis and
delineation (using existing maps) of the areas where
sedimentation is produced in the Parapeti River Basin
upstream of San Antonio del Parapeti, Bolivia, South
America, to discuss the causes of sedimentation and define
the areas where sedimentation is deposited. Among the
topics included in this study is the delineation of the
actual uses of the land and a discussion of possible control
measures to minimize erosion from the basin.
This study is based on existing reports and data,
including maps, aerial photographs and satellite images. In
addition, visits were made to the site and people
knowledgeable of the geographic and hydrologic conditions of
the Parapeti River Basin were interviewed.
A list of the reports that were revised as part of this
study is included in the Bibliography.
1.2 Description of the Study Area
The Parapeti River Basin presents variable hydrologic
characteristics. It has a drainage area that consists in
part of mountains with heights exceeding 2,500 meters.
Between mountains there are plains used for
agricultural and residential activities. In the mountains a
large number of rivers and creeks are formed that during the
rainy season have a high potential to cause erosion of
mountains soil. In the valleys the rivers reduce their
slope significantly becoming wider and deeper. Apparently,
during floods part of the sediment is deposited in the flood
plain of the rivers due to the reduction in the velocities
and dragging forces of the run-off water. The majority of
the sediments, however, are carried over to the "Bafados de
The overflows of the Parapeti River supply the "Baiados
del Abapo-Izozog", a vast area of sandy soil that is very
permeable, allowing water to completely infiltrate even
during heavy rainfall events. The sediment load carried out
by the run-off water is deposited in the "Baiados de Izozog"
downstream of the town San Antonio del Parapeti creating a
sand-bank of more than a kilometer wide by 130 kilometers in
The climate in the watershed is dry with an average
annual rainfall of 800 mm. Seventy per cent of the annual
rainfall occurs between the months of December and April.
The greatest amount of rainfall usually occurs during the
month of January and some years in December (Reference 1).
The average rainfall from May to November is notably low.
The average monthly rainfall in July, August and September
is less than 5 mm. During the heavy rainfall events mean
daily flows have been as high as 1400 m3/seg. with peak
instantaneous flows estimated at 2000 m3/seg. (Reference 2).
During the heavy rainfall season waves up to various meters
in height have been reported in the canyons formed by the
The average annual temperature in the study area is
approximately 230 C with the hottest months being November
and December. The lowest temperatures are registered in the
months of June and July with an average of 170C.
Natural vegetation in the area is not dense due to the
lack and variability of rainfall. There are large portions
of the watershed with very little or no vegetation. The
vegetation consists of shrubs, wild grasses and trees.
Among the variety of trees found are the "Cuchi", "Soto",
"Cedro", "Algarrobo", and others typical of the region. The
most common shrubs are the "Piquillin" and "Tartago".
In the valleys the vegetation is dense, but not enough
to protect the soils against erosion during heavy rainfall.
Along the river margins the valleys there exists
accumulation of sandy sediments that reach a width of up to
0.5 km in some sections, specially along the Parapeti River
upstream of San Antonio and up to 2 km. downstream of San
Antonio. Once the Parapeti River leaves the mountain
ridges, its course becomes unstable and is constantly
The geological materials found in some areas of the
mountain ridges is poorly consolidated and easily eroded.
1.3 Available Information
There are various reports containing valuable
information pertaining to the sedimentation phenomenon in
in the Parapeti River Basin. The most complete of these
reports was prepared by Mr. Hugo Benito, P.E., in August of
1987. He concluded that the results of the sediment load
calculations conducted by him, utilizing existing data are
only approximations of the sedimentation problem of the
Parapeti River Watershed.
Mr. Benito recommends the establishment of a sampling
station in Choreti and the collection of reliable data in
this station and in San Antonio, in order to conduct the
necessary analysis to be used in the feasibility study and
final designs of any water control facility proposed for the
At present, additional samples are being analyzed, but
the results are not yet available.
1.3.2 Maps and Satellite Images
The work conducted in this study was realized using
available topographic maps and satellite images. Topographic
maps were provided at a scale of 1:50000 and 1:250000.
Satellite images were provided at a scale of 1:250000.
Aerial photographs of the Parapeti River Basin taken in 1956
were also provided.
1.3.3 Flow and Data Sedimentation
Two hydrometric stations known as Choreti and San
Antonio have been operating in different time periods in the
Parapeti River Basin. Also, flow measurements have been
conducted in different river locations. The Choreti station
was operated between 1942 and 1954 and San Antonio station
has been in operation since 1971. Although there are plenty
of daily flow measurements for both stations, in the study
conducted by Mr. Benito, P.E. it was found that the specific
flow yield for the basin between Choreti and San Antonio
was very high. According to Mr. Benito this responds to a
systematic error in the flow measurements procedure. The
existing data in both stations does not include measurements
of peak instantaneous flows.
The existing sedimentation data is limited to the San
Antonio station. The available data, however, represents
normal conditions in the Parapeti River, and not intense
flooding conditions. In addition, the samples taken were
not integral, but superficial which does not give a clear
idea of the sediment profile carried by the river (reference
2). These two conditions tend to produce sediment loads and
concentrations estimates lower than the real values. In
studies conducted for the Pirai River it was found that
integral sampling procedures produced higher sediment
concentrations than superficial sampling techniques.
Differences of more than 100% between both methods were
It was found that apparently the sediment load
increases significantly between the towns of Camiri and San
Antonio. According to our observations this could be
caused by the sediment contribution of the mountain ridge
between Camiri and San Antonio, which shows a large number
of landsides exposing highly erosive material to the action
of the rain and the runoff. Unfortunately, the available
information does not permit a comparable evaluation of the
sediment load between these two sections of the watershed.
It is paramount, as recommended by Mr. Benito, P.E. that the
Choreti station be re-established as soon as possible, and
to conduct reliable sampling especially during heavy
rainfall events when the higher sediment loads may be
1.3.4 Field Visits
As part of this project an aerial reconnaissance of the
Parapeti River basin was conducted where a lot of
photographs were taken. These illustrate the most important
characteristics of the study area. Some of the photos taken
during the aerial reconnaissance are included on Appendix A.
Map 1 presents the approximate locations depicted by the
photographs. This aerial reconnaissance permitted the
verification of available maps and photographs provided for
the study, and the verification of data obtained during the
interviews. In addition to the Law Caribe personnel,
members of the Hydroelectric and Irrigation project for the
Parapeti River participated in this aerial reconnaissance.
Their comments and observations were very useful for the
evaluations made in this study.
1.3.5 Proposed Projects
There are several dam projects proposed for the
Parapeti River basin which need reliable sediment yield and
annual load data for their feasibility studies and final
designs. According to our observations, it appears that the
natural production of sediment in the basin is very high,
which is evident along the rivers flooded plains. Erosion
of the natural channels is evident in many rivers and
streams which shows a deep and rugged stream bed.
In our opinion, the natural sediment production of the
watershed could impact significantly the design concept to
be utilized for the proposed projects. Unfortunately, this
condition can not be evaluated until reliable data is
obtained to compute sediment yields expected at several
locations in the basin.
At the present, a "pre-feasibility" study is being
prepared for a dam between Camiri and San Antonio (Oquitas),
and a derivation dam upstream of San Antonio. The purpose
of the project is irrigation and hydroelectric production.
The Oquitas dam will have an approximate height of 80 meters
and a normal reservoir storage volume of approximately 1100
1.4 Terms of Reference
This project is part of the project for the Development
of the Chaco Boliviano and its been realized under a
contract signed in September 1987, by Mr. Jesus Alberto
Fernandez representing the Organization of the American
States, and Mr. Alton F. Robertson representing Law Caribe.
The analyses conducted in this project were based upon
information available at the time of the study. The
available data was supplemented with an overflight of the
Parapeti basin in order to have a general overview of
existing conditions within the study area.
2.1 Erosion and Sedimentation-Technical Aspects
The sedimentation process requires three elements:
erosion, transport and deposition. These are natural
geologic processes that have been occurring for millions of
years. Human activities such as agriculture, construction
and mining, alter the natural sedimentation process
increasing and accelerating the erosion-sedimentation
The sedimentation process requires the detachment of
soils by rainfall, soil transport by runoff, and eventual
deposition of sediments due to reduction in runoff velocity
and dragging forces. The soil is detached by the direct
action of the rainfall drops and by the dragging forces of
the runoff water. Areas that have been exposed to human
actions usually are exposed to a greater degree to the
effect of the runoff waters.
Runoff is not produced until the infiltration capacity
of the soils is exceeded. Thereafter, the quantity and size
of the carried sediments will depend on the turbulence and
velocity of the runoff. Sediments are deposited when the
runoff velocities and turbulence decrease significantly.
Sediment deposition is a selective process where bigger and
heavier particles are deposited first and fine particles are
carried out longer distances sometimes requiring long
periods of time to be deposited. Therefore if an estimate
of the sediment loads that will be deposited at a particular
location (i.e. a reservoir) is desired a size distribution
of the sediments must be obtained.
Runoff waters move initially as laminar flow but are
rapidly concentrated in swales, ditches and natural channels
starting a dynamic erosive process that increases as the
discharge flows increase. Excessive sediment loads are
usually produced by high flows corresponding to the rising
limb of the runoff hydrograph. As the runoff discharges
decrease the sediment deposition usually increases due to
the reduction in flow velocities. In this manner sediments
are carried out downstream from their sources in an
intermittent and progressive manner.
There are only a few methods available to estimate
sediment loads from areas exposed to erosion. From these
methods the most advanced and documented is the Universal
Soil Loss Equation (USLE) method developed by the U.S.
Department of Agriculture-Soil Conservation Service (SCS).
The USLE equation can be used to estimate the annual
erosion loads in tons/km2 that could be expected from areas
exposed to the erosion process. The total sediment load at
a particular location is computed utilizing the erosion load
computed by the USLE equation, and reducing it by factors
that take into consideration the size of the Basin, slopes,
hydraulic characteristics of the natural channels, and the
Eventhough this equation can be applied to the Parapeti
River watershed the parameters needed in its solution have
not been developed for this basin. However, using the
information generated by the SCS in the U.S. and the
Caribbean area and utilizing reported values for the
parameters needed in the solution of the equation, the
relative sediment contribution of some of the areas exposed
to erosion effects within the Parapeti basin could be
estimated. The computed values, even though not
representative of the watershed conditions, will give an
estimate of the areas with the highest erosive potential in
the Parapeti River basin.
3.1 Existing Land Use
Based upon the information collected such as maps,
satellite images, aerial photos and the information
collected during the field visit and interviews, the
existing land uses within the Parapeti River basin were
defined. The principal land uses identified in this study
are presented in Map 2.
The existing land uses in the Parapeti River Basin were
divided into 5 groups:
1) Agricultural Land (A)
2) Grazing Land (P)
3) Petroleum Exploration Areas
4) Urban Areas (U)
5) Not developed areas (ND)
At present only a small percentage of the watershed has
been developed. As shown in Map 2 about 84% of the
watershed is in its natural conditions. The mountain ridges
with its abrupt topography and dry climate are not adequate
for agricultural or residential developments. Their only
future use could be for mining and petroleum exploration
Based upon the information provided for this study,
two areas dedicated to petroleum exploration were identified
within the Parapeti River watershed. One of them is being
conducted south-east of Camiri and the other south of the
Piraymiri community. These activities require the
construction of multiple access roads for which erosion
control methods are not implemented. These roads usually
expose large portions of highly sloped terrain to the
effects of rainfall and runoff. Even though these areas
have a tremendous potential for sediment production, at
present they only occupy about 1.5% of the watershed area.
Future mining and petroleum exploration activities could be
a significant sedimentation source if they are conducted
without implementing erosion control methods.
Urban areas within the watershed cover less than 0.5%.
The most important communities in the basin are Camiri, San
Antonio, Charagua, Vaca Guzman, Huacareta and Monteagudo.
In addition to these relatively large communities, there are
quite a few small communities all over the watershed. The
future development of these urban areas could constitute a
significant source of sedimentation if during the
construction of roads and building erosion control measures
are not applied.
The agricultural and grazing areas represent
approximately 14% of the watershed. The majority of the
valleys between the mountains are being cultivated or have
the potential for agricultural development. In Annex A,
photographs of various cultivated valleys are presented.
The principal products in the area are corn and citrus
fruits. At present, the total area being cultivated is very
small compared to the total area of the watershed. The
future development of other areas should be carefully
planned in order to avoid the high sedimentation loads that
usually are associated with these developments.
3.2 Sediment Producing Areas
Based on the information compiled during this study,
the areas with a great potential to produce significant
loads of sedimentation in the Parapeti River Basin have been
identified as follows:
1. Mountain Ridges Slopes whose geological formation
is not consolidated.
2. Areas between mountain ridges with a mild
3. Agricultural areas.
4. Areas where petroleum explorations are being
5. Grazing Areas.
Map 3 presents areas in the Parapeti River Basin where
sediment is produced. These areas can be divided into two
categories according to the phenomenon that causes them.
The first two areas produce sediment due to the natural
process of erosion. These areas could increment the total
production of sediments if preventive methods are not
applied. The most effective way to avoid increment of the
sediment production in these areas is avoid human
development or to develop them only under strict erosion
The last three areas enumerated as sources of
sedimentation are directly related to human developments.
These areas are of paramount importance for the control of
erosion in the basin, because their potential for producing
sediments is enormous if adequate measures are not taken.
3.2.1 Evaluation of Sediment Loads
As previously indicated it is possible to approximate,
using the USLE Equation, the specific sediment production
for each area exposed to the effects of erosion in the
The USLE Equation established that:
A=Annual Loss of Soil in Tons/Acre
R=Rainfall Erosion Factor
L=Factor corresponding to the Length of the Slope
S=Factor corresponding to the Slope in Percentage
C=Cropping Management Factor
P=Factor for Control Measures
Using the "R" value, representative of the erosion
force of rainfall in the mountains, the "K" value for soils
with an intermediate capacity for erosion and assuming that
the Cropping Management Factor (C=1) has not been
implemented in the area and erosion control measures are not
applied (P=1), the relative values of specific sediment
production for different areas were computed as shown on
You can observe from the results of this Table that the
areas with a great potential for erosion are those exposed
to the natural effects of erosion, with the exception of
those areas used for exploration of petroleum. Although,
the values of annual soil erodibility presented in this
table are not representative of the Parapeti River Basin,
they indicate the areas in the basin with a greater
potential for erosion.
3.3 Areas of Sediment Deposition
The deposition of sediments carried by floods generally
occurred in flat areas where the velocity and turbulence of
the runoff decrease significantly and is insufficient to
maintain in suspension the carried sediments. In most
parts, this phenomenon occurred in the abandoned meanders of
the river and in the river margins when the flows exceeded
the normal capacity. In the Parapeti River Basin, the
floods caused by heavy rainfall deposits part of the
sediments along the river margins, but the larger part of
the sediments produced in the basin are carried to the
"Bafados de Izozog." Some of these areas of sediment
Relative Sediment Loads
Parapeti River Basin
1- Erosive Mountain Ridge
2- Areas between mountain Ridges
3- Agricultural Land
4- Petroleum Exploration Areas
5- Pasture Land
A A Erosive
tons/acre/year tons/km2/year Potential
3650 901550 I
1094 270218 II
79 19513 III
2750 679250 I
368 90896 III
Notes:l- Erosive Potential [>II>III
2- A = Annual soil loss
3- A values presented are not representative of real conditions in the
Parapeti River Basin. The only purposes of this analysis is to present
a relative idea of the erosive potential between sedimentation sources
within this watershed.
deposition observed during the field visit are included in
Photos 1,2,3,7,25 and 30 presented in Annex A.
3.4 Actual Causes of Erosion
The erosive patterns in the Parapeti River Basin are
caused by natural patterns as well as by the effects of
human activities. Most of the uninhabited part of the basin
is in its natural state. The geological material that
outcrops at the surface are being exposed, as they have been
for millions of years, to the erosive effects of rains, wind
and gravity. Generally, the natural channels are deep and
unstable canyons which are eroded by the effect of the flows
concentrated in the channel.
On the other hand, approximately 16% of the basin is
exposed to development activities like agriculture, road
construction and mining activities. These activities
although at present occupy a very small portion of the
basin, have the capability of producing a significant amount
4.0 Erosion Control Measures Applicable to the Parapeti
At present, a large portion of the Parapeti River
Watershed is being affected by natural erosive processes not
related to human developments. In these areas the
implementation of structural erosion control measures may
only produce a small reduction in the sedimentation load of
the watershed usually obtained on a very high cost. For
instance, in the study prepared by the German Mission in
1985, a total of more than 100 million dollars were
estimated for structural measures needed to reduce the
sediment load of the Pirai River upper basin. In areas
where the natural production of sediments is high the
approach of implementing basin wide preventive measures is
more effective and less expensive.
On the other hand, part f the watershed is
being utilized and has potenti 1 for future human
developments. In these areas unpl nned development could
cause significant sediment loads tha could be diminished if
proper erosion control methods are i plemented.
The erosion control measures pplicable to the study
area can be divided into four groups
Structural measures include engineering structures with
the purpose of controlling runoff flows, minimize erosion
and sediment interception. The structural methods
considered for this study are:
Flood Control Dams
Gabions or Riprap
Vegetative practices provide vegetative cover to
exposed areas utilizing typical vegetation from the study
area. The purpose of these measures are soil stabilization,
minimization of the erosive effects of runoff, and
increasing the natural beauty of the area treated. The
vegetative practices considered for this study include:
Agro-Forestation measures include integrated methods of
soil conservation and agricultural and forest production.
On the other hand, preventive measures deal with the
planification and control of development activities within
Erosion control Measures
Flood Control Dams
Riprap or Gabions
Soil conservation measures
Exclude grassing from high slope areas
Pasture Protection by livestock rotation
Increase productivity of small slope terrain
Separation of forest and Pasture areas
Determination and delineation of hazard areas
Prohibition of developments in hazard areas
Determination and delineation of conservation
Adapted from references 4 y 8
Code i ....
Applicable Erosion Control Measures
Code For Control Measures
(see Table 2)
Sedinentation source El E2 E3 E4 E5 E6 E7 V1 V2 V3 Al R2 P1 P2 P3 P4 PS PS P7 P8
Erosive Mountain Ridge (SE) X X X X X X X X X X
Area Betmeen Mountain Ridge (RES) X X X X X X X X X X X
Agricultural Land CRA) X X X X
Petroleum Exploration Area (REP) X X X X X X X X X X X X X X
Pasture Area (P) X X X X X X X X
Leyend : X= Applicable
5.0 Conclusions and Recommendations
At present, approximately 84% of the Parapeti River
Basin is undeveloped. Only about 16 % is exposed to human
development activities including agriculture, mining and
construction. In many undeveloped portions of the watershed
the surface geologic material observed is mainly composed by
unconsolidated sandstone, easily erodable. These areas have
a tremendous potential for sediment production which may be
effectively reduced by the implementation of a basinwide
erosion control plan.
For these areas preventive measures as discussed in
section 4, as well as vegetative practices and agro-
forestation measures should be implemented as the first line
of defense against erosion. The structural measures
previously discussed, even though applicable to the study
area, are not recommended due to the high cost usually
associated with them.
For areas presently affected by human developments, it
is recommended to implement, as soon as possible, the
erosion control measures discussed in Section 4. New
developments within the watershed should not be allowed
without the implementation of adequate erosion control
methods that could minimize the impact of the proposed
In order to implement an effective and adequate
basinwide management of the Parapeti River Watershed it is
1. Conduct a detailed study of the major land
use of the watershed to define hazard and
conservation areas and the areas on which
reforestation could be implemented.
2. Develop specific and enforceable laws and
regulations that could be utilized to
implement a basinwide erosion control plan.
3. Develop an education program to teach the
population of the watershed about the
problems associated with sediment
contamination and the measures available to
control erosion and sedimentation.
Based upon the erosion patterns found in the study area
and the existing sedimentation data for watersheds adjacent
to the Parapeti Basin (i.e. Pilcomayo, Grande and Pirai) it
appears that the specific sediment yield as well as the
annual sediment loads estimated for the Parapeti River are a
lot less than those that may be expected. Unfortunately
there is no adequate data available to verify this
condition. We agree with Mr. Hugo Benito that the Choreti
Station must be installed as soon as possible and integrated
samples must be collected in Choreti and San Antonio.
Sedimentation sampling in these two stations must be
conducted for at least one year, including sampling during
heavy rainfall events, before reliable estimates of specific
sediment yields and annual loads could be computed for this
It is our opinion that, the design concept of any water
control structure proposed for this watershed must be
revised if it is found that the sediment loads on the basin
are high. The concept of passing sediments through the
reservoir instead of storing them could be the most
practical approach when sediment loads are expected to be
By Alton F. Robertson and Raul Colon