ARTICLES
The effects of geomorphic controls on sediment yield in the Andean Rivers of Colombia
Juan D. Restrepo 1, Sergio A. López 2 and Juan C. Restrepo 3
1 Departamento de Geología, Universidad EAFIT, Carrera 49 Nº 7 sur 50-AA, 3300 Medellín, Colombia. E-mail: jdrestre@eafit.edu.co
2 Proyecto GEORED-INGEOMINAS, Diagonal 53 Nº 34-53, Bogotá D.C., Colombia. E-mail: slopez@ingeominas.gov.co
3 Centro de Investigaciones Oceanográficas e Hidrográficas (CIOH), Armada Nacional, Cartagena, Colombia. E-mail: JRestrepo@dimar.mil.co
Received: May 30, 2009 - Accepted: December 15, 2009.
Abstract: This paper examines sediment yield rates and its response to control variables in the principal rivers of Colombian draining into the Caribbean and Pacific coasts. Based on a multi-year dataset of sediment load from six rivers, including Mira, Patía, and San Juan on the Pacific margin, and Magdalena, Atrato, and Sinú, on the Caribbean basin, various morphometric, hydrologic, and climatic variables were estimated in order to understand and predict the variation in sediment yield. A multiple regression model, including two control variables, runoff and relief ratio (the ratio of the maximum height of the drainage basin and the basin length), explains 83% of the variance in sediment yield. This model is efficient (ME = 0.93) and is a valuable tool for predicting total sediment yield from Colombian rivers. These two selected estimators refer to the relative importance of the fluvial transport component in the sediment routing system. Thus, regional-scale variance of sediment yield in the Andean basins of Colombia seems to be explained by the combined influence of tectonics (relief) and surface runoff available for weathering and transport processes. In general, high sediment yielded rivers are high runoff systems with narrow alluvial plains (i.e. Pacific rivers), while low sediment yielded rivers like the Caribbean systems, contain large sections with not significant gradient in their longitudinal profiles. These sections coincide with large floodplains, which all provide sediment storage capacity within the catchments. When considering the three gauged Pacific rivers at their furthest downstream stations, the combined annual sediment load from these rivers into the Pacific Ocean is ~40 Mt yr–1. In contrast, the Magdalena, Atrato and Sinu rivers deliver ~173 Mt yr-1 into the Caribbean. Overall, Andean rivers of Colombia exhibit the highest sediment yields of all medium-large sized rivers of South America due to the interplay of (1) high rates of runoff (1,750-7,300 mm yr-1), (2) steep relief within catchments, (3) low values of discharge variability (Qmax-Qmin), and (4) episodic sediment delivery due to either geologic events or climatic anomalies.
Keywords: Sediment yield; Runoff; Relief; Andes; Colombia.
Resumen: En este estudio se analizan los factores físicos que controlan la producción de sedimentos de los principales ríos andinos de Colombia que drenan hacia las costas Pacífica y Caribe. Con base en datos multi-anuales de caudal y transporte de sedimentos en suspensión en las estaciones de aforo aguas debajo de los ríos San Juan, Patía y Mira, en la margen Pacífica, y Atrato, Sinú y Magdalena, en la cuenca Caribe, además del cálculo de variables hidrológicas, climáticas y morfométricas en cada cuenca de drenaje, se realizaron análisis de regresión potencial y multivariada. Los resultados indican que la escorrentía y el coeficiente de relieve (cociente entre la altura máxima y la longitud total de la cuenca), explican el 83% de la variabilidad regional en la producción de sedimentos. El modelo multivariado compuesto por estos dos estimadores es estadísticamente significativo al 95% de confiabilidad y presenta una eficiencia ME = 0,93. Por lo tanto, la varianza regional en la producción de sedimentos de los principales ríos andinos colombianos parece ser explicada por la interacción entre las condiciones tectónicas (relieve) y la cantidad de escorrentía disponible para los procesos del lavado de suelos y del transporte de sedimentos. En general, los ríos con altas tasas de producción (cuenca Pacífica) son sistemas caracterizados por altos valores de escorrentía y estrechos planos fluviales de inundación. En contraste, los ríos de la margen Caribe, con menores valores de producción de sedimentos, son ríos que tienen gran parte de su perfil topográfico sin gradientes significativos. Estas áreas coinciden con amplios planos de inundación para la captación de sedimentos. Al evaluar las descargas totales de sedimentos, los tres principales ríos de la margen Pacífica aportan al océano un total de ~40 Mt año-1. Los ríos del Caribe, incluyendo Magdalena, Atrato y Sinú, aportan al Mar Caribe ~173 Mt año-1. En conclusión, los ríos andinos de Colombia presentan las tasas más altas de producción de sedimentos de todos los ríos suramericanos que drenan hacia las márgenes Atlántica y Pacífica debido a la interacción de (1) altos niveles de escorrentía (1,750-7,300 mm año-1), (2) relieves pronunciados y reducidos planos de inundación en las cuencas de drenaje, (3) bajos valores de variabilidad de descarga (Qmax-Qmin) y (4) altos aportes de sedimentos durante eventos geológicos de alta energía y anomalías climáticas.
Palabras clave: Producción de sedimentos; Escorrentía; Relieve; Andes; Colombia.
INTRODUCTION
Sediment yield (or sediment production) is the
sediment load normalized for the drainage area and
is the net result of erosion and deposition processes
within a basin. Thus, it is controlled by those
factors that control erosion and sediment delivery,
including local topography, soil properties, climate,
vegetation cover, catchment morphology, drainage
network characteristics, and land use (Walling, 1994;
Hovius, 1998). Knowledge of sediment yield and the
factors controlling it provides useful information
for quantitative models of landscape evolution and
geochemical and sediment mass balance studies,
and for estimating net erosion intensities within
river basins (Pinet and Souriau, 1988; Summerfield
and Hulton, 1994; Walling, 1994; Harrison, 2000).
Measurements of sediment yield are also key
elements for understanding the impacts of past landuse
or climate changes (e.g. Dearing, 1992; Walling,
1997; Verstraeten and Poesen, 2001).
Previous studies have attempted to explain the
global pattern of sediment yield in terms of climatic
factors (Langbein and Schumm, 1958; Fournier,
1960; Douglas, 1967, 1973; Wilson, 1973; Ohmori,
1983; Walling and Webb, 1983), the role of relief and elevation of drainage basins (Ahnert, 1970;
Pinet and Souriau, 1988; Milliman and Syvitski,
1992; Summerfield and Hulton, 1994), vegetation
as controlled by climate (Douglas, 1967; Jansen
and Painter, 1974), and land use (e.g. Trimble,
1975; Dunne, 1979; Verstraeten and Poesen, 2001;
Molina et al., 2008). Other investigations have tried
to explain sediment yield in terms of the combined
effect of morphometric, climatic, and hydrologic
variables of drainage basins. These relations have
often been presented as single or multiple regression
models (e.g. Pinet and Souriau, 1988; Summerfield
and Hulton, 1994; Hovius, 1998; Ludwig and Probst,
1998; Harrison, 2000; Aalto et al., 2006).
The Andes is a tectonically active region
characterized by active volcanism, ongoing uplift,
earthquakes (Fig. 1), and high magnitude mass
movements (Vanacker et al., 2003; Harden, 2006;
Molina et al., 2008). Uplift has caused rivers to incise
and denudation rates to be high. In this region of
steep slopes, mass movements are triggered by wet
conditions and by earthquakes (Aalto et al., 2006;
Harden, 2006). Thus, tropical catchments located in
the cordilleras of the Andes are highly susceptible
to soil erosion due to their topography and erosive
climate, and the occurrence of extreme geologic events (Hess, 1990; Milliman and Syvitski, 1992;
Wunder, 1996; Dadson et al., 2003).
Figure 1. a) Regional tectonic features and major neotectonic structures of the northern Andes, showing the main Andean rivers of Colombia draining into the Pacific and Caribbean coasts. Arrows denote relative motion of tectonic plates (mm yr-1): Caribbean (CAR), Cocos (COC), Nazca (NAZ), South American (SAM), Costa Rica - Panama (CRP) and Coiba (CO). Major structural blocks and fault systems are also shown, including: North Andean Block (NAB), East Frontal Thrusts (EFTS), Algeciras (AFS), Maracaibo (MFS), Oca (OFS). Other features include Ecuador - Colombia Suture Zone (ECSZ), North Panama Deformed Belt (NPDB), South Caribbean Deformed Belt (SCDB), and South Panama Deformed Belt (SPDB) (Modified from Gómez et al., 2008). b) Location and magnitude of earthquakes that occurred in Colombia between 1993 and 2008. Codes indicate
the location of seismic sensors (Courtesy of the Colombian Seismological Network, INGEOMINAS).
Studies analyzing physical and anthropogenic
controls on sediment yield at the scale of smallmedium
sized Andean catchments (101-103 km2),
have shown that sediment yield is controlled by (1) slope movement on hillslopes (Vanacker et al., 2003),
vegetation cover and lithology in the Ecuadorian
basins (Molina et al., 2008), (2) lithology, catchment
slope, climate (Aalto et al., 2006), and landsliding
(Safran et al., 2005) in the Bolivian Andes and
(3) runoff and maximum water discharge in the Magdalena drainage basin (Restrepo and Syvitski,
2006). However, there are no studies analyzing
on a preliminary basis the environmental factors
controlling regional variations in sediment yield in
the main Andean rivers of Colombia. It has been
already demonstrated that the Colombian Pacific
and Caribbean rivers (Fig. 1) are important rivers on
a worldwide basis due to (1) their contribution of
sediment fluxes for the global budgets (Milliman and
Meade 1983; Milliman and Syvitski 1992; Restrepo
and Kjerfve 2000a,b), (2) their high sediment yields,
the largest for the Atlantic and Pacific coasts of
South America (Restrepo and Kjerfve, 2000a), (3)
their high spatial variability of geologic, climatic
and anthropogenic factors within drainage basins
(Restrepo and Syvitski, 2006), and (4) the occurrence
of extreme episodic geologic and climate events that
are important in delivering sediment to the coastal
ocean (Restrepo and Kjerfve, 2000b; Restrepo and
López, 2008).
This paper aims to synthesize and update the
status of the main Andean rivers of Colombia draining
into the Pacific and Caribbean margins (Fig. 1) with
respect to sediment yield. It provides the first attempt
to analyze the main physical variables explaining
most of the spatial variation in observed sediment
yields in the Colombian Andes. This synthesis, which
is based on previous results presented by Restrepo
and Kjerfve (2000a,b), Restrepo (2005), Restrepo et
al. (2006) and Restrepo and Syvitski (2006), shows
new analysis and discussions in terms of: (1) analysis
of environmental factors controlling sediment yield
on a regional scale; (2) discharge variability and
comparisons to other tropical rivers; (3) sediment
load trapping by floodplains; and (4) further
comparisons to other systems in South America and
elsewhere. This information is a significant addition
to the understanding of (1) The denudation of the
continents and the transfer of sediments to new
depositional environments; (2) The spatial variability
of sediment yield through rivers, which provides the
information for better management decisions; (3)
Fluvial fluxes to oceans and the context of Colombian
rivers in the global budgets; (4) The functioning of
coastal ecosystems and the evolution of deltas and
other coastal landforms; and (5) Future scenarios of
sediment flux under different conditions of climate
change and human perturbations (Syvitski et al.,
2003; Morehead et al., 2003, Syvitski and Milliman,
2007).
DATA AND ANALYSIS
Monthly water discharge and sediment load data
were obtained from the Hydrological Institute of
Colombia-IDEAM for the most downstream station
at each river (IDEAM, 2003) (Fig. 1), or from various
studies (Restrepo and Kjerfve, 2000a,b; Restrepo et
al., 2002; Latrubesse et al., 2005; Restrepo, 2005).
Morphometric and climatic variables for the
analyzed catchments (Table 1) were obtained by using
an ARCINFO® database (IDEAM, 2001) and a GIS
software, HidroSig Java® (version 1.8) (HIDROSIG,
2001), which includes all existing hydrological and
meteorological databases of Colombia. An available
30 arc second Digital Elevation Model (DEM) with a
resolution of 1 x 1 km, supplemented by 1:1,000,000
maps (IDEAM, 2001), was used in HidroSig Java® to
calculate morphometric variables such as watershed
boundaries, area, river length, indices of slope,
maximum elevation, relief ratio, topographic profiles,
and also climatic parameters, including mean and
maximum annual precipitation and precipitation
ratios. Definition and derivation of hydrologic,
morphometric, and climatic controlling variables
used in correlation and multivariate analysis were
obtained from Summerfield and Hulton (1994),
Hovius (1998), and Harrison (2000).
Table 1. Summary list of some analyzed environmental variables of major Andean rivers draining into the Pacific and Caribbean coasts of Colombia. River length (L), drainage basin area (A), maximum elevation (H), relief ratio (Hr), average annual precipitation (P), maximum monthly precipitation (Pmax), precipitation peakedness (Ppk), runoff
(?f), runoff coefficient (?f/P), mean annual temperature (T), mean annual water discharge (Qav), monthly maximum
water discharge (Qmax), monthly minimum water discharge (Qmin), water discharge peak (Qpk), suspended sediment
load (Qs), and basin wide sediment yield (Y). The location of each river is shown in figure 1. Definition and derivation of hydrologic, morphometric, and climatic controlling variables used in correlation and multivariate analysis were obtained from Summerfield and Hulton (1994), Hovius (1998) and Harrison (2000).
a Sediment load estimates were obtained from the regression equation of annual suspended sediment load on basin area for orogenic continental rivers of South America and Asia with precipitation rates higher than 700 mm yr-1 (Latrubesse et al., 2005).
To obtain predictive relationships and examine which environmental factors control sediment yield, series of correlation calculations were done using the database from the six rivers (Table 1). Both single and multiple correlations were performed. Pearson correlation coefficients were calculated for some variable pairings for catchment properties and for the neperian logarithm of delta variables. To analyze if a significant component of the regional variation of sediment yield can be explained by a combination of several controls, a step-wise regression was implemented on data listed in Table 1. This multivariate regression approach has been used to analyze global and regional variations in sediment yield, where individual catchments are represented by a single sediment yield value (e.g. Hovius, 1998; Ludwig and Probst, 1998; Harrison, 2000). Here we examine a set of estimator variables, select those that are most efficient at explaining the variance in a response variable, and build them into a model. The predictive efficiency of the regression model was evaluated based on the Root Mean Square Error (RMSE) and the adjusted R-square. In order to compare Colombian rivers with South American and major fluvial systems, we used databases of river properties from Milliman and Syvitski (1992), Ludwig and Probst (1998), Latrubesse et al., (2005), and Syvitski (2005).
RESULTS AND DISCUSSION
Fluvial Fluxes and Sediment Yield
The Pacific Rivers. In general, the drainage basins on
the eastern side of South America are large, whereas
the numerous basins with discharge into the Pacific
are comparatively small because of the crowding of
the drainage basins west of the Andes imposed by
regional geology and tectonics.
The basins of the Pacific coast of Colombia,
measuring 76,365 km2 and extending from latitude
00°36' N to latitude 07°45' N and from longitude
75°51' W to longitude 79°02' W, are characterized
by the presence of active fault systems (Fig. 1),
high precipitation rates, slopes frequently steeper
than 35°, and dense tropical rain forests. These
conditions are favorable for the occurrence of rapid
mass wasting, caused by slope erosion processes and
thus high sediment loads. The basins extend inland
60-150 km and comprise all of Colombia west of the
Cordillera Occidental of the Andes, linking Panama
and Ecuador. They consist of a broad coastal plain
and the western slopes of the Cordilleras. The principal rivers from north to south are the San Juan,
Patía, and Mira (Fig. 1).
The extreme climatic conditions along the Pacific
basins of Colombia are responsible for the high water
discharge into the Pacific. The largest is the San Juan
with a mean discharge of 2,550 m3 s-1. The Patía has
a mean river basin discharge of 1,291 m3 s-1, and
the Mira River contributes an average 868 m3 s-1.
Table 1 shows the main climatic and morphologic
characteristics, and water discharge and sediment
load for the largest Pacific and Caribbean rivers.
Sediment flux to the coastal ocean is conditioned
by geomorphic and tectonic influences (basin area
and relief), geography (temperature, runoff), geology
(lithology, ice cover), and human activities (reservoir
trapping, soil erosion) (e.g. Milliman and Syvitski,
1992; Syvitski and Milliman, 2007). On a global
basis, the warm temperate region has the highest
sediment yield of all the climate zones and accounts
for nearly 2/3 of the global sediment delivery. Also,
river basins draining high mountains (>3,000 m)
contribute 60% of the global sediment flux to the
coastal zone (Syvitski et al., 2005a,b).
The Pacific basins are characterized by small rivers
with high sediment yield (Table 1). The San Juan
occupies a 16,465 km2 basin with the highest mean
sediment load (16 Mt yr-1) and basin-wide sediment
yield (1,150 t km-2 yr-1) on the entire west coast of
South America (Restrepo and Kjerfve, 2000a). The
Patía River, with a drainage area of 23,700 km2, has
a sediment load and basin-wide sediment yield of
14 Mt yr-1 and 972 t km-2 yr-1, respectively. Based on
daily measurements from 1,982 to 2,000 by IDEAM
at Pipiguay, which only covers 4% of the upstream
basin, the upper portion of Mira River has an annual
sediment load of 213,160 t yr-1 and a sediment yield
of 507 t km-2 yr-1. This sediment yield is much lower
than the yield of 856 t km-2 yr-1 estimated by Restrepo
and Kjerfve (2000a), which only included 10 years of
data 1982-1992. To remedy this, sediment load for
the Mira River catchment was estimated from the
regression of sediment load on basin area for tropical
rivers with precipitation rates higher than 700 mm
yr-1 (Latrubesse et al., 2005). The best estimate for the
total sediment load into the Pacific Ocean from the
Mira is 9.7 Mt yr-1. This results in a sediment yield
estimate of 1,018 t km-2 yr-1, very similar to the yields
of 1,150 t km-2 yr-1 and 972 t km-2 yr-1 estimated for
the San Juan and Patía rivers, respectively. The total
measured sediment load into the Pacific from gauged Colombian rivers is ~40 Mt yr-1.
In general, climate determines where tropical
weathering occurs, while tectonics increase erosion
rates and dictate the composition of erosion products
(Stallard, 1988). Catchments with intense tectonic
activity usually have high sediment yields (Meade
1988; Milliman and Syvitski 1992), as in the case of
San Juan and Patía rivers (Fig. 1). Furthermore, the
presence of unstable and cation-rich minerals in the
suspended load and bedload of rivers draining the
Andean basins suggests that rapid erosion is indeed
occurring. Thus, along the slopes of the Pacific basins,
high temperatures, humid conditions, and abundant
vegetation in these high-rainfall basins promote
rapid chemical weathering and high denudation
rates (Restrepo and Kjerfve, 2000a).
Besides climate and weathering factors, other
processes such as landslides lead to slumps that
increase sediment loads. In humid uplands,
landslides are the dominant mass-wasting process
(Hovius et al. 1997, 1998). The Colombian Pacific
basins are characterized by the presence of active
fault systems, high precipitation rates (reaching
as much as 12 m in the Atrato watershed), slopes
frequently steeper than 35°, and dense tropical rain
forests (Restrepo and Kjerfve, 2000a). According to
Hovius et al. (1997), these conditions are favorable to
the occurrence of rapid mass wasting caused mainly
by hillslope erosion processes such as landslides,
slumps, and slides.
The Caribbean Rivers. Caribbean Colombia is
principally drained by the Magdalena and Sinú
Rivers, and also receives the Atrato drainage from
west of the Cordilleras (Fig. 1). The drainage basins are
characterized by high and moderate rainfall patterns
averaging 4,944, 1,750, and 2,050 mm yr-1 in the
Atrato, Sinú, and Magdalena watersheds, respectively
(Table 1). The climate of the region is modulated
by the geographical position of the Intertropical
Convergence Zone (ITCZ). The seasonality of the
position of the ITCZ corresponds to the windy and
dry (December-April) and rainy (August-October)
seasons. The rest of the year is transitional between
these two seasons. In the dry season the ITCZ resides
at its southernmost position (0-5°S). At that time the
northern trade winds dominate the area (Andrade,
1993). During the rainy season the ITCZ moves over
the southwestern Caribbean, decreasing the wind
speed of the southerlies there and promoting the highest rate of precipitation anywhere in the western
hemisphere (Andrade and Barton, 2000).
The Magdalena River is the largest river system
with a length of 1,612 km. The drainage basin area
measures 257,438 km2 and occupies a considerable
part of the Colombian Andes. Daily Magdalena
water discharge measurements from 1942 to 2002
at Calamar indicate an annual discharge of 7,176
m3 s-1. Load measurements during the 1975-1998
period yielded an annual sediment load of 144 Mt
yr-1. Based on this sediment load value, Restrepo and
Kjerfeve (2000b) estimated that the Magdalena River
contributes 9% of the total sediment load discharged
from the east coast of South America. The current
144 Mt yr-1 estimate of sediment load is higher than
the 133 Mt yr-1 value reported by Marín (1992), but
considerably lower than the estimate by Milliman
and Meade (1983) of 220 Mt yr-1. This sediment
load estimate implies a sediment yield of 560 t km-2
yr-1 for the Magdalena, which is more realistic than
the previously reported values of 1,000 t km-2 yr-1
(Meybeck 1976, 1988), 900 t km-2 yr-1 (Milliman
and Meade 1983), and 920 t km-2 yr-1 (Milliman and
Syvitski 1992).
Because there is no measurement of sediment
load of the Atrato River at its lower course, we
estimated sediment flux from the regression of
annual suspended sediment load on basin area for
orogenic continental rivers of South America and
Asia (Latrubesse et al., 2005). The best estimate
for the total sediment load into the Caribbean from
the Atrato is ~25 Mt yr-1. This results in a more
reliable sediment yield estimate of ~700 t km-2 yr-1.
A previous sediment yield value of 315 t km-2 yr-1
(Restrepo and Kjerfve, 2000a), which was based on
sediment load data from an upstream station covering
only 14% of the Atrato catchment, does not reflect
the denudation rates present in this humid drainage
basin of western Colombia. Finally, the annual
discharge and sediment load of the Sinú River are
373 m3 s-1 and 4.2 Mt yr-1, respectively, with a basinwide
sediment yield of 589 t km-2 yr-1 (Table 1). The
total sediment flux into the Caribbean from the three
main Andean rivers of Colombia is ~177 Mt yr-1.
Relation between Sediment Yield and Geomorphic Variables
Univariate regression analyses indicate that the
control variables explaining most of the variation in observed sediment yield are mean annual runoff
(?f) and to a lesser extent maximum elevation (H).
Curvilinear regression of sediment yield on mean
annual runoff and maximum elevation yielded
a coefficient of determination of 0.79 and 0.40,
respectively, both regressions significant at the 95%
level (Fig. 2a, b). Thus, at the regional scale, 79%
of the observed variance in sediment yield can be
explained by runoff. Other estimators such as mean
slope, drainage area, mean annual precipitation,
and maximum water discharge are not significantly
linked to sediment yield.
The relationship between sediment yield and
mean annual runoff (Fig. 2a) has been demonstrated
in many studies examining global sediment yields.
Summerfield and Hulton (1994) showed that for
hydrological and climatic variables, only mean
annual runoff, and to a lesser extent mean annual
precipitation, were strongly associated with denudation
rates. Further, the increase of sediment yield for
precipitation rates higher than 700 mm yr-1 runoff is
a feature that has already been described (Langbein
and Schumm, 1958; Wilson, 1973; Walling and
Webb, 1983; Latrubesse et al., 2005).
Figure 2. Relation of sediment yield versus mean annual
runoff (a) and maximum elevation (b) for the main Andean
rivers of Colombia listed in Table 1. c) Model (eq. 1)
predictions compared to sediment yield (Y) observations
from six analyzed Andean rivers within the Pacific and
Caribbean basins, showing the model efficiency value
(ME). d) Scatter plot of observed Y versus model (eq. 1)
performance.
According to Syvitski and Milliman (2007), when
two basins with similar size, relief, and temperature
are compared, the wetter one will be more able
to erode and transport sediment downstream. In
contrast, drier basins produce less sediment through
mechanical and chemical weathering, and more of
this sediment load will be stored between the source
area and the receiving basin. Similar comparison
can be made in the Andean rivers of Colombia.
High runoff systems are clearly grouped as high
sediment yield rivers while Caribbean systems, that
are comparable in drainage area and elevation (i.e.
Atrato and Sinú), exhibit much lower yields (Fig.
2a).
According to Milliman and Syvitski (1992),
topography and basin area exert the major controls on
sediment yield of most rivers, with climate, geology,
and land use being second-order influences. They
demonstrated a robust correlation between sediment
yield and maximum elevation for mountainous rivers
in North and South America, Asia, and Oceania, and
showed that mountainous rivers have greater loads
and yields than do upland rivers, which in turn have
higher loads and yields than lowland rivers. Other
studies simulating sediment load to the coastal ocean
have shown that sediment flux is conditioned by geomorphic and tectonic influences, including basin
area and relief (Moulder and Syvitski, 1996; Sytitski
et al., 2005a; Syvitski and Milliman, 2007). Our data also indicate that maximum basin elevation shows
some degree of association (Fig. 2b). This is probably
due to the fact that some of the analyzed rivers have
their headwaters at high elevations and others are
lowland rivers (Fig. 3). In fact, high sediment yielded
rivers which descend rapidly from high Cordilleras
to their limited alluvial plains (i.e. Pacific rivers)
have tributary basins (70-90% of the drainage area) in
elevations higher than 1,000 meters. Low sediment
yielded rivers like the Atrato, Magdalena and Sinú
(Table 1), are characterized by large sections with a
very gradual gradient in their longitudinal profiles.
These sections coincide with large wide floodplains,
which all provide sediment storage capacity within
the catchments (Fig. 3c).
Figure 3. a) Digital elevation model (DEM) of the Andean
region of Colombia, showing the Western, Central, and
Eastern Cordilleras, the Pacific and Caribbean basins, the
floodplains (Ap) within the lower course of the Atrato (3),
Sinú (4) and Magdalena (5) rivers. We also show the location
of the Patía (1) and San Juan rivers (2) and their deltas (D).
b) Near infrared Aqua MODIS satellite image during July
24th, 2008. Black indicates water bodies surrounding the
Mompox tectonic depression within the lower course of the
Magdalena River. c) Longitudinal river profiles of systems
that trap sediment through their extended floodplains
(Atrato, Sinú and Magdalena) and rivers along the Pacific
basin that have limited alluvial plains and the highest
sediment yield.
A multiple regression of sediment yield was obtained following the stepwise procedure. The two estimators, which predict sediment yield (Y) for the main Andean catchments draining into the Pacific and Caribbean margins, are runoff (?f) and relief ratio (Hr), such that:
Y = 1530.55 + 2643.36 log (?f) – 70.256 log (Hr) (1)
This multiregression equation accounts for 83%
of the total observed variance in sediment yield from
these drainage basins. The two independent variables
are statistically significant at a = 0.01 (Fig. 2c). One
of the selected estimators, runoff, refers to the relative
importance of the fluvial transport component in
the sediment routing system. According to Hovius
(1998), specific runoff determines to a certain
extent the transport capacity of the fluvial system
and may also refer to the amount of water available
for hillslope erosion. It is relatively well correlated
with mean annual precipitation (P) and maximum
monthly precipitation (Pmax).
Hillslope erosion processes are controlled in
part by topographic variables such as mean modal
elevation, maximum elevation, relief ratio, and
slope angle of the riverbed. The relief ratio and
slope angle of the river are possibly more relevant to
the fluvial transport of sediment (Hovius, 1998). A
high relief ratio corresponds to a more pronounced
topography and thus to higher erosion. Similar
observations have been reported from catchments
in Colorado, USA (Schumm, 1954), for continental
scale basins (Summerfield and Hulton, 1994), and
in the Magdalena drainage basin (Restrepo and
Syvitski, 2006). Thus, the second estimator, relief ratio, represents the potential energy available for
soil erosion.
Summerfield and Hulton (1994) noted the strong
role of relief and runoff in influencing denudation
rates for major basins worldwide. Jansen and Painter
(1974) showed that climate and topography were
the most important controls on sediment yield from
catchments globally, and Restrepo and Syvitski (2006) demonstrated the combined effect of relief
and runoff on sediment yield from tributaries within
the Magdalena drainage basin. Also, our data show
that relief and climate account for a remarkably high
amount of explained variance from main Colombian
rivers given the fact that only natural variables are
included.
Comparison between observed and predicted
sediment yield of Andean rivers indicates that the
model (eq. 1) replicates successfully the spatial
distribution of sediment load and captures for 83% of
the between-river spatial variation (Fig. 2c). To validate
the model, simulated sediment yield was compared
to measured sediment yield for the same model by
calculating model efficiency (ME) relationship. ME
is 0.93 and the agreement indicates that the model
is robust and predicts well sediment yield for the
studied rivers in the Pacific and Caribbean basins.
When evaluating model performance, sediment
yields are well predicted within ~20% bias over one
order of magnitude (Fig. 2d).
Comments on Discharge Variability, Runoff, and Floodplains
Rainforest rivers show high but variable peak discharges during the rainy season. Similar to other world rainforest rivers, Andean rivers of Colombia display two annual flood peaks, in agreement with bimodal distribution of rainy periods in summer (main) and fall (secondary). However, when analyzing discharge variability for world tropical rivers in different hydrologic and morphoclimatic zones (Latrubesse et al., 2005), Colombian rivers show the lowest values of Qmax/Qmean and Qmax/Qmin ratios compared to other rainforest rivers (Fig. 4a). Thus, Andean Pacific and Caribbean rivers are high runoff systems (Fig. 4d) with low discharge variability and more water available for hillslope erosion and sediment transport.
Figure 4. a) Discharge variability for world tropical rivers in different hydrologic and morphoclimatic zones. Colombian rivers show the lowest values of Qmax/Qmean and Qmax/Qmin ratios compared to other rainforest rivers (modified from Latrubesse et al., 2005). (b) Relationship of drainage basin area (km2) and delta area (km2) for the major Colombian and global river deltas. Data for the world's major rivers were obtained from Syvitski (2005). Further comparisons between Colombian and major global rivers in terms of sediment yield (c) and runoff (d) are shown. Note that major South American rivers, including Amazon, Orinoco, and Paraná, are also shown. Data from Milliman and Syvitski (1992), Syvitski (2005), Latrubesse et al. (2005), and this study.
The Pacific and Caribbean drainage basins are
characterized by strong tectonic activity (Fig. 1b),
high rates of runoff, low discharge variability, and
steep slopes that promote high sediment yields.
Even though these rivers drain small basins,
with the exception of the Magdalena, this unique
combination of geomorphic factors promote high
sediment delivery and the construction of extensive
deltas under destructive geologic and oceanographic conditions, including (1) high tectonic activity in the
receiving basins (Fig. 1b), (2) drainage basin divides
close to the ocean (Fig. 3a), (3) narrow continental
shelves, (4) moderate to high marine energy
conditions, (5) increasing trends in relative sea
level, and (6) strong oceanographic manifestations
associated with the ENSO cycle, causing sea-level
rises during El Niño years (Restrepo and López,
2008). Figure 4b shows the trend between drainage
basin area and delta area for the Colombian rivers
determined by log– log regression of basin area on
delta area. It indicates that Colombian deltas fit the
global trend well and are comparable with major world rivers within two orders of magnitude. The
small Pacific rivers, including the Atrato, show a
clear cluster. Although the Atrato discharges into
the Caribbean Sea (Figs. 1, 3), its drainage basin is
located west of the Cordilleras and has the same
drainage basin characteristics as the San Juan and
Patía rivers (e.g., headwaters at high elevations, high
rainfall rates, and high tectonic activity).
It is well recognized that natural lakes, wetlands,
and floodplains are also of prime importance in
predicting river basin delivery (Stallard, 1998;
Vörösmarty et al., 2003). Rivers with lower values
of sediment yield like the Sinú and Magdalena
(Table 1) contain large sections with not significant
gradient in their longitudinal profiles. These sections
coincide with large floodplains, which all provide
sediment storage capacity within the catchments
(Fig. 3c). The lower course of the Magdalena River
has large floodplains known as the Mompox tectonic
depression, a "natural dam" of 6,000 km2 in which
much of the sediment load is stored for long time
periods. The Mompox basin concentrates around
80% of the total number of "ciénagas", depressions
in the Cretaceous-Tertiary bedrock with stagnant or
river-dependent bodies of water that accumulate
sediments (Van der Hammen, 1986). Findings of
Plazas et al. (1988) and Modis images support that this region gets flooded for up to 8-9 months of the
year and acts as natural lakes, trapping significant
amounts of sediment (Kettner et al., in press) (Fig.
3b). According to Restrepo (2008), 14% of the
Magdalena sediment load, approximately 21 Mt yr-1,
is trapped in the Mompox tectonic depression basin
with a sedimentation rate of 2.0 mm yr-1.
Based on monthly values of sediment load for
the Atrato River (1982-1993), Restrepo and Kjerfve
(2000a) estimated that the sediment load of the Atrato
is 11.3 Mt yr-1, and the corresponding sediment yield
315 t km-2 yr-1. As mentioned before, this sediment
load is very low and rather uncertain, considering
that the Atrato partially drains the western slopes, a
region characterized by the presence of active fault
systems (Fig. 1), high precipitation rates (reaching as
much as 12 m within the Atrato watershed), slopes
frequently steeper than 35°, and dense tropical rain
forests. Nevertheless, the sediment yield appears to
be comparatively low because of the large size of the
drainage basin and the extensive low-lying alluvial
flood plains (Fig. 3), with an area of 5,500 km2, where
significant sediment deposition and storage occur.
In contrast, high sediment yielded rivers in the
Pacific margin (Table 1), contain less sections of low
gradient slopes in their longitudinal profiles (i.e.
Mira, Patía) (Fig. 3c). Over a distance less than 75 km, the San Juan River falls abruptly from an elevation of
3,900 m to 100 meters. Likewise, the Patía descends
from its headwaters at 4,580 m elevation to 400 m
over a distance of 150 km. Although the San Juan
shows large wide floodplains in its topographic
profile compared to the lowlands of the Patía and
Mira rivers (Fig. 3c), its lower course is highly
controlled by Tertiary formations and structural
highs. In addition, its flood plain is of limited size,
and the bends of the river are frequently in direct
contact with bedrock valley walls. Thus, the control
exerted by narrow alluvial plains in the Pacific rivers
results in less sediment deposition/storage within
the drainage basins.
FURTHER COMPARISONS
According to Milliman and Syvitski (1992), basin area and morphology exert the major controls on sediment yield, with climate, geology, and land use being second-order influences. They demonstrated a robust correlation between sediment yield and basin area for mountainous rivers in North and South America, Asia, and Oceania. To illustrate the global trend, we replotted sediment yield on basin area on a log-log plot for several mountainous and tropical rivers of Southeast Asia, Oceania, and South America, using the data of Milliman and Syvitski (1992) and Latrubesse et al. (2005) and including our estimates for San Juan, Patía, Mira, Atrato, Magdalena, and Sinú. Figure 5 indicates that the Colombian rivers fit the global trend very well. The scatter reflects local effects caused by morphology, tectonism, rainfall, land use, and other effects.
Figure 5. Variation of sediment yield with basin area for major tropical and mountainous rivers of Asia/Oceania, South America
(Amazon, Orinoco, Paraná, Negro-Brazil) and Colombian rivers draining into the Caribbean Sea (Magdalena, Sinú, and Atrato)
and into de Pacific Ocean (San Juan, Patía and Mira). Line A-B shows the global trend while line C-D indicates the scatter due
to local effects caused by morphology, tectonism, rainfall, land use, and other effects. Data from Milliman and Syvitski (1992),
Latrubesse et al. (2005), and this study.
The Atrato, Sinú, and Magdalena rivers have the
highest sediment yield of the medium-large rivers
along the Caribbean and Atlantic coasts of South
America (Fig. 5). Their yields are almost four times
greater than the yield of the Amazon, 167 t km-2 yr-1,
Orinoco, 158 t km-2 yr-1 (Latrubesse et al., 2005), or
Negro (Argentina), 140 t km-2 yr-1 (Milliman and
Syvitski, 1992), and much greater than the yield of
the Paraná, 43 t km-2 yr-1, Uruguay, 16,4 t km-2 yr-1 (Latrubesse et al., 2005), and São Francisco, 10 t km-2 yr-1 (Milliman and Syvitski, 1992) (Fig. 4c).
In comparing rivers with small basins in high
rainfall areas in Colombia and Asia/Oceania, the San
Juan, Patía, and Atrato rivers are similar, in terms of
water discharge, sediment load, and sediment yields,
to the Purari and Fly rivers in Papua New Guinea. Average annual rainfall ranges from 2,000 to 8,500
mm in the 33,670 km2 catchment of the Purari, which
has a mean discharge of 2,360 m3 s-1 (Pickup, 1983).
The Fly River has a mean discharge of 4,760 m3 s-1
and a sediment yield of 1,087 t km-2 yr-1 (Latrubesse
et al., 2005). Although the San Juan drains a basin
approximately half as large as the basin of the Purari
(33,670 km2) and far smaller than the 64,400 km2
size of the Fly, it has greater water discharge than
the discharge of the Purari and almost the same
magnitude as the discharge of the Fly. In addition,
the Atrato River is comparable to both the Purari and
Fly rivers in terms of water discharge and sediment
yield (Table 1).
Milliman and Syvitski (1992) stated that
mountainous rivers with basin areas of approximately
10,000 km2 in southeast Asia/Oceania have sediment
yields between 140 and 1,700 t km-2 yr-1 and have
higher yields by a factor of two to three than rivers
draining other mountainous areas of the world.
Taking the overall yields for the Pacific basins of
Colombia to be between ~800 and 1,200 t km-2 yr-1,
it is apparent that rivers draining steep slopes in the
western Andes have yields very similar to rivers
draining mountainous terrain and high rainfall areas
in South Asia and Oceania (Fig. 5).
The monsoonal influenced Pacific and Caribbean
rivers of Colombia appear to have the higher runoff
rates of the large world rivers in terms of sediment
load (Fig. 4d) as a result of the combined influence of
(1) the annual shift of the Inter Tropical Convergent
Zone, (2) climatic anomalies due to the El Niño
Sothern Oscillation during its positive phase of La
Niña, (3) low pressure systems from the Caribbean
Sea, (4) orographically driven rainfall within the
Andes cordilleras, and (5) cold fronts from the
Amazon basin.
Our conclusion is that the Andean rivers of
Colombia exhibit the highest sediment yield rates
of all medium-large sized rivers of South America
due to the interplay of (1) high runoff conditions,
(2) steep relief within catchments, (3) low discharge
variability, and (4) episodic sediment delivery due to
either geologic events or climatic anomalies.
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