SECTION 3
THE UPPER PARAGUAY RIVER
AND PANTANAL OF MATO GROSSO
This section describes the physiographic and hydrologic
characteristics of the Upper
Paraguay River and the adjoining Pantanal of Mato Grosso. It is divided
into four subsec
tions:
1.Geologic setting,
2.Geomorphologic setting,
3.Hydrologic setting, and
4.Ecologic setting.
The information of this section serves as the basis for the
analysis of Section 4.
The
references sources are listed in Appendix 1.
The figures and tables
are included in
Appendices 2 and 3, respectively.
3.1
Geologic Setting
The geology of the Upper Paraguay river and environs is described
in Almeida (1945);
Projeto Bodoquena (1979), Projecto RADAMBRASIL (1982) (Volumes 27 and
28), and
Godoi Filho (1986).
The predominantly Upper Precambrian formations
underlie exten
sive quaternary deposits, with significant rock outcroppings.
Geomorphologic evidence
reveals the presence of substantial tectonic activity in the form of
subsidence and uplift
(DNOS, 1974; Orellana, 1979; Ab'Sáber, 1988).
Table 5 shows the stratigraphic units present in the Upper Paraguay
river.
The Up
per Precambrian is represented by the groups Alto Paraguay, Corumbá, and
Jacadigo,
and their respective formations; the Paleozoic by the Coimbra formation;
the Mesozoic
by the Alkaline Fecho dos Morros formation; and the Cenozoic by the
Xaraiés and Pan
tanal formations. Table 6 lists primary rock types, including
sandstones, siltstones, lime
stones, conglomerates, dolomites, calcareous dolomites, syenites,
trachytes, calcareous
tufa, and travertines. Table 7 shows the general location of rock
outcroppings in the
vicinity of the Upper Paraguay river.
Navigation charts confirm the existence of numerous rock
outcroppings on or along
the Upper Paraguay river (Marinha do Brasil, 1974; and later dates). A
list of these out
croppings is shown in Table 8, and their possible rock types in Table 9.
Four of these
outcroppings are located in the middle of the channel (Passo Simão Nunes
Inferior, Cór
rego Bonfim, Farolete Balduíno, and Passo Mucunã), effectively
functioning as grade
controls.
The rock outcroppings have been mentioned in INTERNAVE
(1990), and rec
ognized for more than a century (Leverger, 1862a; 1862b).
These facts confirm that the Upper Paraguay river bed is
substantially con
trolled by the prevailing geology. While the sediments of the Pantanal
for
mation (quaternary) are the primary surficial geologic/geomorphic feature
of
the landscape, the longitudinal slope of the river is controlled more by
the
rock outcroppings than by the alluvium. As shown in Table 8, there are
thirty-two (32) rock outcroppings within 1270 km of river, an average of
one
every 40 km.
Other outcroppings along the river, particularly that of canga, may
have been less
thoroughly documented. According to Dorr (1945), canga is colluvium or
talus, com
posed in large part of fragments of iron-rich rock, which has been
recemented into a
coherent mass by limonite. In the vicinity of the Morro de Urucum, these
outcroppings
tend to have a linear distribution, suggesting the possibility of a fault
(Dorr, 1945; Al
meida, 1945). Moreover, it is noted that the Internave report mentions
the occurrence
of canga in several places along the river, including Porto Rabicho (km
2740), entrance
to Passo do Conselho (km 2607), and upstream of Passo Piuva Inferior (km
2573) (IN
TERNAVE, 1990).
Table 10 describes three geologic faults in the vicinity of the
Upper Paraguay river
(Projeto RADAMBRASIL, 1982). The Falha da Lagoa, which crosses Gaíba
Lake, may
have a significant effect on the flow of the Upper Paraguay river.
Likewise, the Falha da
Penha, which also crosses Gaíba Lake, is strategically located in close
proximity to the
river (Fig. 5). According to DNOS (1974), it appears that the northern
portion
of the Pantanal has subsided in relation to the Serra do Amolar (Section
3.2).
This would explain the elevated position of the older rocks in the Serra
do Amolar.
The essentially tectonic character of the Upper Paraguay basin has
been admirably
discussed by Freitas (1951). In this regard, Orellana (1979) has stated
that active faults
in a direction contrary to the regional runoff have created local sills
(soleiras) which act
to impede runoff.
More recently, Ab'Sáber (1988) has attributed the
genesis of the Pan
tanal to "a tectonic character dominated by a system of faults which are
geomorphologi
cally contrary."
A historical note of interest regarding rock outcroppings: In
describing his passage
through Lagoa Gaíba more than one hundred years ago, Captain Augusto
Leverger
noted the following (Leverger, 1862a):
3.2
Geomorphologic Setting
The geomorphology of the Upper Paraguay river is described in
Projecto RADAM
BRASIL (1982) and Da Silva (1986). The geomorphic units in the vicinity
of the Upper
Paraguay river are:
1.Serras of Urucum-Amolar,
2.Serrana Province,
3.Depression of the Paraguay River, and
4.Plains and Swamps of Mato Grosso.
The Serras of Urucum-Amolar comprise two groups of residual
hills/mountains
located to the right of the Upper Paraguay river, close to the Bolivian
border. These
serras are important because of their strategic location, on the right
margin, and often
right next to the river. The Serra do Urucum, southeast of Corumbá,
comprises the
morrarias of Urucum, Santa Cruz, São Domingos, Grande, Rabichão, and
Tromba dos
Macacos; further south, about 40-50 km, are the morrarias of Zanetti,
Albuquerque,
Mato Grande, Saiutã, and Pelada.
The Serra do Amolar, about 60-160 km north of Corumbá, comprises
the Serra do
Amolar proper and the morrarias of Insua, Novos Dourados, Santa Tereza,
Castelo, and
others of smaller size. The elevation of these mountains varies from 300
to 900 m, with
the highest point at Morro Grande (1065 m).
The contact between the
Serra of
Urucum-Amolar and adjacent geomorphic units is often sudden, revealing
the presence
of faults of probable Cenozoic age (Projeto RADAMBRASIL, 1982).
The Serrana Province consists of a group of serras (mountain
ranges) of roughly
parallel crest alignment, located east of Cáceres, in a predominantly SWNE direction.
From SW to NE, these serras are: (1) Simão Nunes, (2) Colonia, (3)
Acorizal, (4) Facão,
(5) Primavera, (6) Ponta do Morro, (7) Quilombo, (8) Morro Grande, (9)
Jacobina, (10)
Barreiro Preto, (11) Chapola, (12) Boi Morto, (13) Bocainão, (14)
Campina, and (15)
Retiro (Folha SE.21-V-B, Projeto RADAMBRADIL, Volume 27).
The Serra Simão Nunes, the most southwestern of the group, comes in
direct contact
with the Upper Paraguay river in several locations along 45 km of river,
on the left mar
gin, from Passo Acuri, at Km 2128 (Km 3368 of the Hidrovia), to Passo
Morro Pelado,
at Km 2083 (Km 3323 of the Hidrovia) (Table 20). The elevations in the
Serrana prov
ince vary from 300 to 700 m, the rock formations being those of the Alto
Paraguay
group (Table 5).
The Depression of the Paraguay River comprises extensive plain
surfaces sur
rounding the Serrana province and to a lesser extent, the Serra of
Urucum-Amolar. The
relief is mild, slightly undular, with elevations varying between 150 to
250 m, and under
lying Precambrian and Cambrian formations, which often outcrop through
the quater
nary deposits.
The Plains and Swamps of Mato Grosso (Planícies e Pantanais
Matogros
senses) resemble an amphitheater of roughly semicircular shape, with its
approximate
center at Corumbá, comprising 28 percent of the Upper Paraguay river
basin (Fig. 2).
The plains are a series of mutually coalescing alluvial fans, surrounded
by the Depression
of the Paraguay River almost continuously to the east, and
discontinuously to the north
and south. They comprise an extensive surface of accumulation, of
extremely flat relief,
with elevations varying from 80 to 150 m, and subject to seasonal
flooding by the Upper
Paraguay river and its tributaries.
Valley slopes are about an order of magnitude greater along the
tributaries than along
the mainstem. The tributaries cross the plains in a predominantly eastwest direction,
with a slight orientation towards the center of the amphiteather, with
valley slopes of 12
to 50 cm/km. The high sediment production of the tributaries has forced
the mainstem
Upper Paraguay, which runs predominantly from north to south, to closely
parallel the
western edge of the plains (Schumm, 1977). Moreover, its unusually mild
slope (0.7-6.5
cm/km) causes it to regularly overflow its banks during the wet season.
Sánchez (1978) has identified ten geomorphic subunits within the
Plains and Swamps
of Mato Grosso (Table 11). The Pantanal is a complex mosaic of geomorphic
features or
subunits, with a regional base level at the Upper Paraguay river, along
the western edge
of the plains. In turn, the base level of the Upper Paraguay river is at
its mouth, at the
confluence with the Apa river.
The sequence of events leading to the formation of the Pantanal is
de
scribed in Projeto Bodoquena (1979) and more recently, by Ab'Sáber
(1988).
During the Jurassic period, the prevailing arid climate led to
substantial sedi
ment deposition, mostly in the form of sand dunes. The Cretaceous period
that followed
saw a change to a more humid climate, which transformed the desert into a
flood plain
with numerous lakes and swamps. The end of the Mesozoic era signaled the
end of sedi
mentation and the beginning of slow epirogenetic movements, which were
generally
positive.
The subsidence of the Pantanal region occurred later, probably in
the Pleistocene (Tri
cart, 1982).
A clear evidence of subsidence is the great depth of
the quaternary depos
its, which at Fazenda Piquiri reaches beyond 320 m, and at Fazenda São
Bento, beyond
420 m (320 m below sea level). The maximum depth of the Pantanal
sediments has
been estimated at 500 m at the center, near the apex of the Taquari fan
(Godoi Filho,
1986).
Tectonic uplift to the south strangled the exit of the
depression. In the last 6
million years, under the effects of subsidence and uplift, the Upper
Paraguay river has
been forced to carve an exit through the basal rocks to the south, a
situation which ex
plains its unusually mild slopes.
3.3
Hydrologic Setting
The hydrology of the Upper Paraguay river is described in DNOS
(1974) and EDIBAP
(1979).
The Upper Paraguay river flows from its headwaters in the Serra
de Tapirapuã,
in Mato Grosso, to its mouth at the confluence with the Apa river, in
Mato Grosso do
Sul. At Cáceres, where it meets the first human settlement of
importance, the river
drains an area of 33 860 km2; at the Apa river confluence, downstream of
Porto
Murtinho, it drains an area of 496 000 km2.
The distance along the river, from Cáceres to the Apa river
confluence is
1270 km. Throughout most of this distance, the Upper Paraguay river ad
joins parts of the Pantanal of Mato Grosso, flooding the Pantanal during
the
wet season and draining it during the dry season. Thus, the hydrology of
the
Upper Paraguay river is effectively connected to that of the Pantanal.
This relationship
tends to blur the distinction between surface water and groundwater,
significantly increas
ing the complexity of hydrologic analysis. In fact, the peculiar
geologic and geomorpho
logic setting of the Upper Paraguay river and its neighboring Pantanal
results in a unique
hydrologic behavior, unmatched in the American continent.
The prevailing climate is dry subhumid, grading to wet subhumid
along a narrow strip
paralleling the mountain ranges which delimit the basin to the north,
east, and south,
and humid in limited northernmost areas bordering with the Amazon basin
(EDIBAP,
1979). The spatially averaged mean annual rainfall in the Upper Paraguay
basin ranges
from 1180 mm (Projeto RADAMBRASIL, 1984) to 1380 mm (EDIBAP, 1979), de
pending on the data source. This fact alone would dictate that the mean
runoff coeffi
cient (the ratio of mean annual runoff to mean annual rainfall) should be
close to the
global average, estimated at 0.39 (for peripheral continental areas) by
L'vovich (1979) or
0.46 by Berner and Berner (1987). Instead, the mean runoff coefficient
of the Upper
Paraguay is quite low, varying in the range 0.07-0.10, depending on which
data is used
in the calculation (Section 3.3.4).
The abnormally low runoff coefficient of the Upper Paraguay river
is a di
rect result of its hydrologic interaction with the Pantanal. The latter
functions
as an inmense surface/subsurface reservoir which stores water annually
and multiannu
ally. In the annual time frame, the Pantanal stores water during the wet
season and re
leases it back to the mainstem river during the subsequent dry season.
In the multiannual
timeframe, the Pantanal stores water in a wet year and releases it back
to the mainstem
(primarily as baseflow) in a dry year. In this process, the slowness of
which is com
pounded by the unusually mild relief, large amounts of would-be runoff
are instead
returned to the atmosphere through evaporation and evapotranspiration.
The latter
helps to sustain a thriving ecosystem, characteristically a marsh or
wetland, its biotic pro
ductivity closely linked to the annual flood pulse (Junk et al, 1989).
3.3.1
Hydrography
The hydrography of the Upper Paraguay river, from Cáceres to the
Apa river conflu
ence, is shown in schematic form in Fig. 6. At Cáceres, the Paraguay
river has already
received the contributions of two of its most important right-margin
tributaries: the Sepo
tuba and Cabaçal. From Cáceres, the river flows south towards
Descalvados, a distance
of 139 km along the river. At a point 71 km downstream of Cáceres, the
Paraguay
receives the contribution of its third important right-margin tributary,
the Jauru.
From Descalvados, the river flows first southeast and then turns
south towards Porto
Conceição, a distance of 121 km. At a point 46 km downstream of
Descalvados, the
river branches into two channels: the mainstem Paraguay to the right and
the Bracinho
to the left. This bifurcation marks the beginning of the Pantanal
proper; from this point
on, as far downstream as Amolar, the Paraguay river crosses extensive
areas of lakes
(baías) and adjoining permanently flooded plains. The two branches form
the island of
Taiamã, and rejoin 43 km downstream (measured along the mainstem).
From Porto Conceição, the river flows first south and then turns
southwest towards
Bela Vista do Norte, a distance of 135 km. At a point 40 km downstream
of Porto
Conceição, the river again branches into two channels: the mainstem
Paraguay to the
right and the Caracará to the left. The two branches form the great
island of Caracará
and rejoin further south close to Refugio das Três Bocas. The mainstem
Paraguay flows
southwest towards Bela Vista do Norte, at the base of Morraria da Insua.
The Caracará
branch flows south towards Refugio das Três Bocas, in the vicinity of
Serra do Amolar.
The island of Caracará constitutes a veritable inland delta, with
its apex at
the bifurcation (40 km downstream of Porto Conceição) and its base the
Paraguay
river itself, which turns from Bela Vista do Norte southeast to Refugio
das Três Bocas,
flowing for a distance of 53 km. During extraordinary and exceptional
floods, most of
the island of Caracará is completely submerged.
In the vicinity of the Morraria da Insua and the Serra do Amolar,
the Paraguay river
interacts hydrologically with three large lakes: Uberaba, Gaíba, and
Mandioré. The larger
of the three, Lagoa Uberaba, located north of Morraria da Insua, receives
overflows from
the Paraguay as well as runoff from Corixa Grande, the last significant
right-margin tribu
tary of the Paraguay, and local streams.
Lagoa Gaíba is located between Morraria da Insua and Serra do
Amolar. It consists of
three smaller lakes:
1.Gaíba, surrounded by mountains to the east and west,
2.Gaíba-Mirim, surrounded by mountains to the east, west, and south, and
connected
to Gaíba during extremely high flows, and
3.Pre-Gaíba, a continuation of Gaíba to the northeast.
The link between the Upper Paraguay river and Lagoa Gaíba is the
Riacho da Gaíba.
However, a branch of the Paraguay drains into Pre-Gaíba.
The Riacho da Gaíba is generally as deep as the Paraguay, excluding
the
exit of Lagoa Gaíba, where it is extremely shallow (a depth of 0.1-0.6 m
and
a width of 2000 m). Judging by the unmixed colors of its waters, the
Riacho da Gaíba
appears to drain Lagoa Gaíba to the right (reddish color: iron-rich
dissolved solids),
Lagoa Uberaba to the center (dark color: humic colloids), and the
mainstem Paraguay
river to the left (light brown color: suspended sediments) (Fig. 7). The
link between
Lagoa Gaíba and Lagoa Uberaba is the Canal Pedro II, with a length of
about 100 km.
The direction of the current in the Canal Pedro II is normally from Lagoa
Uberaba to
Gaíba, but it can change seasonally if the flow is considerably reduced
(DNOS, 1974).
From Refugio das Três Bocas, the river flows south towards Amolar,
a distance of 28
km. Shortly before reaching Refugio das Três Bocas, the river branches
into two chan
nels: the Paraguay and the Moquém. Before rejoining the Paraguay, the
Moquém river
branches into the Ingazal, which joins together with the São Jorge,
another branch of
the Paraguay. In turn, the São Jorge rejoins the Paraguay immediately
upstream of
Amolar. These bifurcations reveal the extremely small gradient in this
section of the
river.
From Amolar, the river flows south towards Porto São Francisco, a
distance of 58
km. At a point 46 km downstream of Amolar, the river flows past Riacho
da Mandioré,
the inlet to Lagoa Mandioré. The Paraguay river flows into Lagoa Mandioré
during high
flows, and out during low flows.
From Porto São Francisco, the river flows in a general southwestern
direction towards
Corumbá, a distance of 146 km, and then east to Ladario, a distance of 7
km from Co
rumbá. At a point 16 km downstream of Porto São Francisco, the Paraguay
river
branches again into two channels: the Paraguay and the Paraguay-Mirim.
The latter
crosses the plains and rejoins the Paraguay river 20 km downstream of
Ladario. Be
tween Porto São Francisco and Corumbá, the river can overflow to the
right into the
Lagoa Conceição, Lagoa do Castelo, and Lagoa Cáceres. The connection
between
Lagoa Cáceres, in Bolivia, and the Paraguay river is the Canal Tamengo,
with its mouth
in the vicinity of Corumbá.
From Ladario, the river flows first east and then southeast to
Porto da Manga, a dis
tance of 69 km. At a point 32 km downstream of Ladario, the Paraguay
river receives
the contribution of the Taquari Velho, an ancient channel of the Taquari
river, to the left.
About 2 km upstream of Porto da Manga, the Paraguay river receives the
contribution of
the Taquari river to the left. The Negro river, a tributary of the
Taquari, flows into the
latter right upstream of its confluence with the Paraguay.
From Porto da Manga, the river flows southwest to Porto Esperança,
a distance of 58
km. At a point 24 km downstream of Porto da Manga, the Paraguay river
receives the
contribution of the Miranda river. Together with the Aquidauana river,
its principal tribu
tary, the Miranda river drains extensive areas of Pantanal and Upper
Paraguay river ba
sin to the southeast.
From Porto Esperança, the river continues to flow southwest to
Forte Coimbra, a dis
tance of 67 km. Between 2 and 40 km downstream of Porto Esperança, the
Paraguay
river overflows during floods to feed its Nabileque branch. The latter
crosses the plains
east of the Paraguay in a general southern direction for about 250 km,
eventually rejoin
ing the Paraguay river at a point located 217 km downstream of Forte
Coimbra.
From Forte Coimbra, the river flows in a general southern direction
for 239 km to
wards Barranco Branco; then an additional 51 km to Fecho dos Morros, and
from there,
36 km to Porto Murtinho. From Forte Coimbra to Porto Murtinho, the
Paraguay river
receives the contribution of several left-margin tributaries, including
the Aquidabã,
Branco, Tereré, and Amonguijá rivers, as well as small surface
contributions from the
Paraguayan Chaco on the right margin. At Fecho dos Morros, 36 km
upstream of Porto
Murtinho, the Paraguay river passes through a group of hills, which
effectively constitutes
a grade control. This control has been referred to as a syenite sill
(DNOS, 1974).
From Porto Murtinho, the Upper Paraguay river flows south for
another 63 km to
reach its mouth at the confluence with the Apa river.
Table 12 shows selected hydrologic data at gaging stations along
the Upper Paraguay
river: drainage area, channel length, channel slope, and mean annual
discharge. The fol
lowing observations are made:
·The subbasin drainage areas are not precisely defined in several points,
particularly
between Descalvados and Amolar. This is due to the extreme complexity of
the drain
age patterns, including channel bifurcations and endorheic (closed)
drainages.
·There is a slight reduction in channel slope and mean annual discharge
from Descal
vados to Porto Conceição, as the Paraguay river enters the Pantanal
proper. This is
where the Paraguay river begins to flood the Pantanal.
·There is a marked reduction in channel slope and mean annual discharge
from Porto
Conceição to Bela Vista do Norte, as the Paraguay river flows through and
around
the island of Caracará.
·At Refugio das Três Bocas-Amolar, the channel slope is further reduced
to 1.82
cm/km, while the mean annual discharge at Amolar increases sharply (to
943 m3/s).
This indicates the substantial contributions of surface and subsurface
runoff immedi
ately upstream of this point.
·Downstream of Amolar, the channel slope increases somewhat to Porto
Esperança,
while the mean annual discharge continues to increases gradually all the
way to Porto
Murtinho, close to the basin mouth.
·Downstream of Porto Esperança, the channel slope decreases again,
reaching a value
of 0.83 cm/km in the reach between Fecho dos Morros and Porto Murtinho.
·The mean annual discharge of the Upper Paraguay river at its mouth is
estimated to
be 1565 m3/s, based on measurements at Fecho dos Morros and Porto
Murtinho.
The average channel slope along the Upper Paraguay river, from
Cáceres to Porto
Murtinho, is 3.2 cm/km. As shown in Table 12, the channel slope varies
between 0.83
cm/km (Fecho dos Morros-Porto Murtinho) to 6.54 cm/km (CáceresDescalvados), and
the bed profile alternates between convex and concave.
According to principles of fluvial geomorphology, a river that is
free to
move its bed eventually carves a concave upwards bed profile (Leopold et
al,
1964; Christofolleti, 1980; Leopold, 1994). Thus, the documented convexi
ties in bed elevation of the Upper Paraguay river reveal the presence of
sub
stantial geologic controls. These controls are operating in at least
three reaches:
1.Refugio das Três Bocas-Amolar, with 1.82 cm/km,
2.Ladario-Porto da Manga, with 2.06 cm/km, and
3.Fecho dos Morros-Porto Murtinho, with 0.83 cm/km.
The extent of the geologic control can be assessed by calculating
the size of the hump
at locations where its presence is suspected. For instance, from Table
12, the average
channel slope from Refugio das Três Bocas to Porto São Francisco can be
calculated to
be 3.51 cm/km. Therefore, the hump at Amolar is:
(3.51 - 1.82) cm/km X 28 km
= 47 cm
Likewise, the average channel slope from Ladario to Porto Esperança
is 2.70 cm/km.
Therefore, the hump at Porto da Manga is:
(2.70 - 2.06) cm/km X 69 km
= 44 cm
The extent to which these humps can cause backwater in these
channels of extremely
small slope is evaluated in Section 4.1.1.
A similar calculation at Porto Murtinho is not possible because of
lack of data at the
river's mouth. However, it is noted that the downstream river (the
Middle Paraguay) has
an average slope of 6 cm/km throughout its 797-km length (Anderson et al,
1993).
This much larger slope downstream reveals that there is a substantial
geologic control in
the vicinity of Fecho dos Morros. Significantly, this is precisely the
location of the sy
enite sill that is mentioned in the literature (DNOS, 1974).
3.3.2
Flood Hydrology
The flood regime of the Upper Paraguay river is a result of complex
climatic interac
tions at the various atmospheric scales. The climate is determined
primarily by the ba
sin's geographic location (latitude and continental location) and
secondarily by its topo
graphic relief and surface features. Mean annual rainfall varies from as
high as 1800
mm at Chapada dos Parecis, the northernmost part of the basin, to as low
as 800 mm
in the alluvial fan of the Taquari river, close to the basin center
(Projeto RADAMBRASIL,
1984). Within these limits, mean annual rainfall increases towards the
mountains and
high plains (planaltos) in the basin perimeter and decreases towards the
alluvial plains in
the basin center.
Rainfall is concentrated in the summer months. The wettest threemonth period is
December-February; the driest is June-August. The temporal distribution
of rainfall has a
tendency to vary spatially in a general north-south direction. The
percentage of annual
rainfall in the wettest three-month period is greatest in the north (48
percent at Cáceres),
gradually decreasing toward the south (to 36 percent at Porto Murtinho).
Thus, the
northern portion of the basin is prone to flooding from tributary
streams. On the other
hand, the percentage of annual rainfall in the driest three-month period
is smaller in the
north (3 percent at Cáceres), gradually increasing toward the south (8
percent at Co
rumbá, and 12 percent at Porto Murtinho) (EDIBAP, 1979). This indicates
the possibil
ity of local droughts recurring on an annual basis.
In any stream, the number of flood peaks per year is a good
indication of the extent
to which surface runoff is being diffused (i.e., attenuated) by the
prevailing geomorphic
conditions. If the number of flood peaks per year is high, say more than
10, there is little
runoff diffusion. Conversely, if there is only one flood peak, runoff
diffusion is at its
maximum. The Upper Paraguay tributaries have a number of flood peaks,
following in
tense storms that cover all or portions of their respective drainage
basins. For instance,
the Cuiabá river at Cuiabá has 15 flood peaks per year on the average;
the Taquari river
at Coxim has 18 flood peaks; the Miranda river at Miranda has 12 flood
peaks (DNOS,
1974). A sequence of several flood peaks depicts the local nature of the
floods as well as
the absence of significant attenuation (the channel gradients vary from 7
to 50 cm/km,
Table 3).
Unlike its tributaries, the Upper Paraguay river behaves quite
differently with respect
to the number of flood peaks.
This reflects both its milder gradient
(0.7 to 6.5 cm/km)
and the presence of the Pantanal, which stores and further attenuates the
flood peak.
The overall effect of this hydrologic process is a reduction in the
number of flood peaks.
At Cáceres, close to the entrance to the Pantanal, there is an average of
five flood peaks
per year. However, the number of flood peaks decreases markedly
downstream, to one
at Amolar, one at Ladario, and one to two from Ladario to Forte Coimbra.
Downstream
of Forte Coimbra, the number of peaks increases somewhat due to local
contributions,
but the locally-generated peaks tend to be much smaller than the peak
propagated from
upstream.
The floods in the Upper Paraguay river have been classified as
follows
(DNOS, 1974; Carvalho, 1986):
1.Common floods, which are exceeded 75 percent of the time (return period
< 2-yr),
2.Mean floods, which are exceeded 50 percent of the time (2-yr return
period),
3.Extraordinary floods, which are exceeded 25 percent of the time (4-yr
return pe
riod), and
4.Exceptional floods, which are exceeded 10 percent of the time (10-yr
return pe
riod).
The hydrographic records at Ladario (1900-95) show the strong
attenuating capacity
of the Pantanal upstream of this point. Throughout the 96-yr period of
record, the flood
wave at Ladario has always had a 12-month duration, i.e., one rise and
one recession
per year. The rise begins usually in December and finishes in June; the
recession begins
in June and finishes in December. The occurrence of the flood peak at
Ladario varies
with the flood level; it is accelerated (to early June, May, April, or
late March) during
extraordinary and exceptional floods, slowed down (to late June, July, or
early August)
during a typical flood year (common or mean flood), and again accelerated
(to April or
late March) during multiannual drought periods (Table 13 and Fig. 8).
The latter behav
ior is due to the drought flow being mostly contained within the river
banks.
Prediction of flood flows along the Upper Paraguay river using
mathematical model
ing was attempted by DNOS (1974) and EDIBAP (1979). Under a limited
budget, the
DNOS model continues to be operated to this date by the Companhia de
Pesquisas de
Recursos Minerais (CPRM), with offices in Rio de Janeiro. It is a
difficult undertaking
because of the temporal and spatial variability and complexity of the
hydrologic proc
esses, which includes high channel sinuosity, branching, overflows,
endorheic surface
drainage, and the presence of aquatic macrophytes in the surface waters.
This is com
pounded by the complex nature of the interaction between surface and
subsurface water,
since the Pantanal has a net gain of water in a wet year, and a net loss
in a dry year
(EDIBAP, 1979; Adámoli, 1986).
The speed of propagation of the annual flood wave can be readily
extracted from the
discharge measurements. It has been established that a typical flood
wave takes about
130 days to travel from Cáceres to Porto Murtinho. This represents an
average speed of
propagation of only 0.11 m/s. This extremely low value is due to the
substantial contri
bution of channel overflows to the overall flood wave propagation.
Table 15 shows low flow, mean annual, and peak flood discharges
along the Upper
Paraguay river. Also shown are the ratio of peak flood to low flow
discharge, and peak
flood to mean annual discharge. The low values of these ratios (compared
to other rivers
in similar climatic settings) show that the Upper Paraguay river is very
effective in de
creasing the flood peaks, and correspondingly increasing the low flows.
This is due to
the presence of the Pantanal.
Thus, the Pantanal is the geomorphic
feature
which provides the mechanism for the spreading (i.e., attenuation,
diffusion)
of flood floods and consequently, the increased permanence of low flows.
3.3.3
Low Flows and Droughts
Table 14 shows the minimum seasonal water surface elevation at
Ladario, from 1900
to 1994. The following conclusions can be drawn from these records:
·The minimum seasonal flow occurs usually late during the calendar year,
in the
month of November or December, although occasionally it may be advanced
to Octo
ber, or delayed to January or February.
·The minimum seasonal flow has been recorded at or below zero gage in the
following
years or drought periods:
--In 1910 and 1915.
--Five times during the nine-year drought period 1936-44.
--In 1948.
--Nine times during the ten-year drought period 1964-73.
As shown by the Ladario gage records, the Upper Paraguay river has
a tendency to
ward multiannual droughts with a recurrence of 28-30 years. The lack of
more extensive
data precludes a more thorough analysis. Nevertheless, the tendency is
borne out by the
data and should be acknowledged.
According to the EDIBAP study, the Ladario gage records, from 1900
through 1977,
show a tendency for a decrease in both maximum and minimum seasonal flows
(EDI
BAP, 1979). In light of the wet period being experienced since 1974,
this conclusion is
no longer valid.
3.3.4
Basin Yield
The complexity of the hydrologic setting in the Upper Paraguay
river precludes a de
tailed analysis of basin yield vs annual precipitation. However, an
approximate analysis
based on mean annual precipitation is possible. During a wet year, the
annual precipita
tion P is split in three ways:
1.Runoff R,
2.Vaporization V, which consists of evapotranspiration from vegetation,
evaporation
from water bodies, and evaporation from bare ground; and
3.Change in basin storage DS, which consists of surface, subsurface, and
groundwater
storage.
Deep percolation is usually small or intractable, and can be
neglected on practical
grounds (L'vovich, 1979). Conversely, during a dry year, annual
precipitation plus (a
fraction of) basin storage go into runoff and vaporization.
A simple water balance equation can be formulated as follows:
P = R + V ± DS (3.1)
where DS is positive during a wet year and negative during a dry year
(Adámoli, 1986a).
In an average year, where change in basin storage is reduced to a
minimum, the above
equation reduces to:
P = R + V (3.2)
from which average basin yield can be calculated.
The spatially averaged mean annual rainfall in the Upper Paraguay
basin is 1380 mm
according to EDIBAP (1979), or 1180 mm according to RADAMBRASIL (1984).
This
estimation is based on isohyetal maps for the Brazilian portion of the
basin (71 percent).
Comparable isohyetal maps for the western portion of the basin, in
Eastern Bolivia and
Paraguay's Northern Chaco (29 percent), are not readily available.
The mean annual discharge at the basin mouth (see Table 12) is 1565
m3/s. Accord
ing to DNOS (1974), the mean of six years of annual discharge
measurements (196571) at Porto Murtinho is 1212 m3/s. However, this value appears too low,
since this
was a particularly dry period (see Section 3.3.3). A longer and wetter
period (1969-78)
of measurement at Porto Murtinho (33 values) gives 2188 m3/s (Hidrologia
S. A., un
published data). The basin drainage area at its mouth is 496 000 km2
(Section 2.2).
Given this information, the mean annual runoff coefficient Kr,
i.e., the ratio of runoff
to rainfall, can be calculated as follows:
(1 565 m3/s) (86 400 s/d) (365 d/y) (1000 mm/m)
(1 380 mm/y)
(496 000 km2) (1 000 m/km)2
A similar calculation, assuming a mean annual discharge Qa = 1700
m3/s (for lack of
better data), with P = 1180 mm/yr leads to:
(1 700 m3/s) (86 400 s/d) (365 d/y) (1000 mm/m)
(1 180 mm/y)
(496 000 km2) (1 000 m/km)2
Thus, the mean annual runoff coefficient for the Upper Paraguay
basin can be taken
as Kr = 0.08. This value is interpreted to mean that on an average year,
runoff at the
basin outlet amounts to eight percent of rainfall, with the balance
returned to the atmos
phere as evaporation and evapotranspiration (i.e., the vaporization
coefficient is Kv =
0.92). A similar analysis for the 11-year period 1965-76 showed a runoff
coefficient
varying in the range 7-14 percent, with a mean of 10 percent (EDIBAP,
1979).
The above calculation confirms that the Pantanal functions not only
as an attenuating
mechanism for flood flows (and consequent increases in low flows), but
also as an ab
stracting mechanism for all flows, i.e., as an effective means of storing
the would-be
runoff and converting it instead into evaporation/evapotranspiration.
Throughout mil
lennia, this process has been responsible for sustaining the
extraordinary biotic potential
of the Pantanal (Section 3.4).
By way of comparison, the mean annual runoff at Cáceres, at the
northern entrance
to the Pantanal, is 382 m3/s, and the contributing drainage area is 33
860 km2 (Table
12). The mean annual precipitation of the subbasin varies from 2000 mm
at the head
waters to 1300 mm at Cáceres (EDIBAP, 1979).
This amounts to a runoff
coeffi
cient Kr = 0.22, which is 2.75 times that of the Upper Paraguay river at
its
mouth.
Likewise, the mean annual runoff coefficient of the Paraná river
at Corrientes
(Argentina) has been calculated at Kr = 0.16 for the decade 1962-71, Kr =
0.19 for
1972-81, and Kr = 0.22 for 1982-91 (Ponce, 1994). These values are from
2 to 2.75
times that of the Upper Paraguay at its mouth.
These calculations confirm that the substantial attenuating and
abstracting property of
the Upper Paraguay is due to the presence of the Pantanal, while the mean
annual
runoff coefficients of the Upper Paraguay river at Cáceres, immediately
upstream of the
Pantanal, and at the Paraná river at Corrientes, 964 km downstream of the
Pantanal,
depict more typical subhumid/humid basins.
The annual potential evapotranspiration in the Pantanal varies
spatially in the range
less than 1100 mm to more than 1400 mm, according to the Thornthwaite
method
(Alfonsi and Camargo, 1986). However, measured pan evaporation data at
Fazenda
São João and Fazenda Rio Negro for 1971-72 show values of 1650 mm
(Tarifa, 1986).
The actual evapotranspiration in an average year can be estimated to be:
Ea = 0.92 X 1180 mm = 1086 mm, or (3.5)
Ea = 0.92 X 1380 mm = 1270 mm,
(3.6)
depending on which value of mean annual precipitation is used in the
calculation (Projeto
RADAMBRASIL, 1984; or EDIBAP, 1979).
Thus, it is concluded that the
Upper
Paraguay basin is able to effectively evapotranspire the difference
between rainfall and
runoff in an average year.
The calculation of runoff coefficient is based on surface runoff at
the basin
outlet, and does not include subsurface runoff at the basin outlet, which
may be real but difficult to evaluate directly. The existence of a
certain amount of
subsurface runoff may be postulated on the basis that the mean annual
discharge at
Asunción, on the Middle Paraguay, 542 km downstream of the Apa river
confluence, is
2700 m3/s (INTERNAVE, 1990). The increase of more than 1000 m3/s is
difficult to
explain, particularly since there are no major intervening drainages. For
instance, OEA
(1975) has calculated that the combined contribution of the Apa,
Aquidabán, and Ypané
rivers, which are measured, is about 180 m3/s. Based on areal
comparisons, the contri
bution of the Aguaray-Guazú and other unmeasured tributaries is estimated
to be about
100 m3/s. The subsurface runoff, if it does exist in substantial
amounts, would have the
effect of reducing the vaporization coefficient Kv to a somewhat lower
value, say, around
0.88-0.90, which is still high in comparison to other basins in similar
climatic settings.
3.3.5
Sedimentology
Measurements of suspended sediments, which include gravel, sand,
silt, and clay parti
cles, along the Upper Paraguay river have been scanty. The longest
existing records are
at Cáceres and at Porto Esperança.
The Cáceres data, shown in Table 16, consists of 55 once-monthly
measurements of
sediment discharge taken between March 1977 and February 1982. The Porto
Esper
ança data, shown in Table 17, consists of 52 once-monthly measurements
taken be
tween April 1977 and November 1981.
These measurements and related calculations were carried out by
Hidrologia S.A. for
the now defunct Departamento Nacional de Obras de Saneamento (DNOS). The
methodologies utilized were the Modified Einstein and the FrijlingKalinske methods.
The values shown in Tables 16 and 17 are total sediment discharge,
consisting of:
·bed load, i.e., coarse particles (primarily gravel and sand) transported
by rolling and
sliding along the bed,
·suspended bed material load, i.e., coarse particles transported in
suspension, and
·wash load, i.e., suspended fine particles, the concentration of which
depends on
source availability and not on the flow hydraulics.
These measurements enable the following observations regarding
total sediment trans
port in the Upper Paraguay river:
1.The measured sediment concentration at Cáceres varies between 28
mg/liter and
507 mg/liter, with a mean value of 147 mg/liter, and a standard deviation
122
mg/liter (Table 18). In general, the low values are associated with low
to average
flows, while the high values are associated with high to exceptionally
high flows.
2.The measured sediment concentration at Porto Esperança varies between
12
mg/liter and 897 mg/liter, with a mean value of 176 mg/liter, and a
standard devia
tion 183 mg/liter (Table 18).
In general, the low values are
associated with low to
average flows, while the high values are associated with high to
exceptionally high
flows.
3.The correlation between sediment discharge and water discharge at both
gaging sta
tions is poor. This is partly due to the presence of the wash load, whose
concentra
tion does not depend on the water discharge. Generally, the wash load
concentra
tion is a function of the degree of natural or human-produced watershed
disturbance
upstream of the gaging station.
Table 18 includes a summary of sediment discharge measurements
along the main
tributaries of the Upper Paraguay river: Cuiabá, Piquiri, Taquari,
Aquidauana, and Mi
randa rivers. Based on this limited by significant data, a preliminary
sediment budget
analysis is performed in Section 4.2.
3.4
Ecologic Setting
The ecologic setting of the Upper Paraguay river basin and the
Pantanal of Mato
Grosso is unique in the American continent. The basin is strategically
located contiguous
to four major South American ecosystems, which surround it, exerting
their influence on
it (EDIBAP, 1979, Adámoli, 1986b):
1.The tropical Amazon rainforest to the north and northwest.
2.The subhumid savanna woodlands (cerrados) of Central Brazil to the
northeast, east,
and southeast.
3.The humid Atlantic forest (Floresta Atlántica) to the south.
4.The semiarid scrub forest (Chaco) of Eastern Bolivia and Western
Paraguay to the
west and southwest.
The unusual combination of geology, geomorphology, and hydrology
(see previous
sections) has contributed to the richness and variety of the vegetation
and associated mi
croclimates of the Pantanal. In turn, this helps sustain a diverse
ecosystem, where a
complex assortment of permanent swamps, seasonal swamps, and terra firma
is annually
replenished with ample moisture, sediment, and nutrients. The entire
process hinges on
the high rate of vaporization (estimated at up to 92 percent on an
average year) which
characterizes the hydrologic budget of the Upper Paraguay river.
3.4.1
Flora
In the existing literature, the Pantanal vegetation is often marked
as a single unit and
referred to as the "Pantanal" complex. Actually, the latter is a mosaic
of many different
communities, with frequent abrupt changes often correlated with
topography, and many
ecotones. The Pantanal has no endemic flora of its own; rather, it is
made up of ele
ments from mato (deciduous and semideciduous forests transitional to the
Amazon tropi
cal rainforest and the humid Atlantic forest), campo (open grassland),
cerrado (savanna
woodland), and caatinga (desert scrub).
There are three main vegetation zones in the Pantanal (Veloso,
1947):
1.The aquatic and hydrophylous zone,
2.The hygrophylous zone, and
3.The mesophylous zone.
The hydrophylous zone is permanently flooded. It is characterized
by three vegeta
tion types: (a) aquatics in flowing water (Eichhornia crassipes, Pistia,
Elodea); (b) float
ing aquatics in stagnant water (Eichhornia azurea, Marsiliea, Reussia
subovata); and (c)
aquatics largely rooted in shallow water (Echinodorus spp., Hydrocleis
spp., Limnocha
ris spp., Victoria amazonica).
The hygrophylous zone is divided into: (a) permanently flooded, and
(b) seasonally
flooded swamps, with the latter usually dominated by one species. Plant
communities of
the seasonal swamps include the Thalietum (dominated by Thalia
geniculata), the Cy
peracietum (dominated by Cyperus giganteus), and the Ipomoeætum
(dominated by Ipo
moea fistulosa).
The mesophylous zone coincides with noninundated alluvial soils.
Many floristic asso
ciations occur in this soil type, as well as many transitional areas
(ecotones). Veloso
(1947) classified associations in order of successional development, and
concluded that
the region is in an active state of change toward a more mesic forest.
The most striking aspect of the Pantanal is its curious combination
of mesic and xeric
vegetation growing side by side, a result of its unique combination of
climate and topog
raphy. Toward the center of the Pantanal, close to Corumbá, the climate
is markedly
seasonal, with a clearly defined drought period. However, given the
extremely flat to
pography, a small difference in elevation is all that is needed to make a
great difference
in seasonal soil moisture, particularly when the underlying strata is
coarse alluvium.
Hoehne (1936) has referred to the Pantanal as a mixture of Amazonas
(hylaea) and
Ceará (caatinga), and provided examples of the two types of flora by
contrasting the gi
gantic candelabra cactus (Cereus peruvianus), and other cacti such as
Opuntia stenar
thra, with the aquatic Alismataceae and Victoria.
Prance and Schaller (1982) have noted the strong cerrado element in
the Pantanal.
These cerrados are dominated by species such as Bowdichia virgiloides,
Caryocar bra
siliense, Curatella americana, and Qualea parvifora, which are typical of
the savanna
woodlands (Planaltos) of Central Brazil. Cerrado occurs mainly in the
nonflooded up
land, but also towards the eastern edge of the Pantanal, where the land
is inundated for
only short periods at the height of the flood season. Such wet cerrado
tends to consists
of numerous islands of cerradão (dense cerrado forest) on slightly
elevated areas that are
not flooded. Cerrado tree species which are most resistant to
waterlogging (e.g., Cu
ratella americana and Byrsonima crassifolia) are common near the boundary
cer
rado/campo, and on raised islands of ground in wet campos (Furley and
Ratter, 1988).
The distribution of these islands produces the campos de murundus,
consisting of an
expanse of wet campos dotted with a regular pattern of raised earthmounds
bearing cer
rado trees, shrubs, and often termitaria. The larger earthmounds, or
capões, are circular
or elliptical in shape, of lengths up to 300 m, and sparsely distributed
across the season
ally flooded campos (Ponce and Cunha, 1993).
The sharpness of the campo/cerrado boundary has been documented by
Eiten
(1975). Within 1 m, or even 0.5 m, the change from the shrubs and low
trees of the
cerrado to the grassy layer without woody plants of the campo is
complete. The reason
for this abrupt change appears to be that the cerrado plants cannot
establish themselves
from seed in continually wet soil. In general, the campo occupies a site
with a lower and
more fluctuating water table, whereas the cerrado occupies the higher
ground, where the
soil seldom if ever remains saturated.
In almost all cases, the cerrado
stops suddenly at
the edge of the campo, apparently due to the competition between the two
vegetation
types as whole plant communities. Cerrado species tolerant of
waterlogging are able to
grow in open campos in places where the soil level is only a few
centimeters higher than
elsewhere. The observation that larger islands on Pantanal landscape are
thickly clothed
with cerrado vegetation confirms that groundwater level exerts a precise
control on the
cerrado/campo boundary.
In the Pantanal, the main trend of vegetational variation is highly
corre
lated with soil moisture and topography. The patent lack of trees in the
wet cam
pos is striking, particularly since a wide range of woody species
successfully colonizes
both the interfluves, which are drier than the campos, and the stream
sides (riparian ar
eas, or gallery forests) which are wetter. The absence of tall, woody
species from areas
which are intermediate in their physical characteristics is attributed to
the fluctuating na
ture of the water table and associated soil moisture. Thus, trees are
able to tolerate both
permanently wet (gallery forest) and moist-to-dry (cerrado) environmental
conditions, but
not an extreme alternation of saturation and desiccation (Cole, 1960).
Areas subject to
the latter are successfully colonized by the grassy elements (campos).
In summary, the Pantanal is extremely rich in floristic diversity
and physi
ognomic composition. It floristic diversity is due to its location, in
the middle and
neighboring four great South American ecosystems: the tropical Amazon
rainforest, the
subhumid savannas of Central Brazil, the humid Atlantic forest, and the
semiarid scrub
forest of the Chaco. Its diverse physiognomic composition is due largely
to its variety of
geomorphic/topographic features, which include baías, barreiros,
cordilheiras, vazan
tes, corixos, capões, murundus, and aterros de bugre (Cunha, 1990; Ponce
and
Cunha, 1993). The annual flood pulse replenishes the Pantanal ecosystem
with water,
sediment, and nutrients, assuring its continuance and survival (Junk et
al, 1989).
3.4.2
Fauna
The ecological diversity of the Pantanal ecosystem has conditioned
its suitability as
habitat for a variety of animal species, among which are reptiles,
mammals, birds, fish,
and insects. The Pantanal remains a rich and unique repository for a
variety of wildlife
species, including 658 species of birds, 1132 species of butterflies, and
405 species of
fish (Brown, 1986; Bucher et al, 1993). It also serves as the resting
place for many
species of migratory birds from the Northern Hemisphere.
Terrestrial and amphibious
species inhabiting the Pantanal include (EDIBAP, 1979; Bucher et al,
1993):
·caiman (jacaré, Cayman crocodilus)
·armadillo (tatú bola, Tolypeutes tricinetus)
·bush dog (cachorro do mato vinagre, Speothos venaticus)
·capybara (capivara, Hydrochoerus hydrochaeris)
·crab-eating fox (cachorro do mato, Dusicyom thous)
·giant anteater (tamandúa bandeira, Myrmecophaga trydactyla)
·giant armadillo (tatú canastra, Priodontes giganteus)
·giant otter (ariranha, Pteronura brasiliensis)
·jaguar (onça pintada, Pantera onça)
·maned wolf (lobo guará, Chrysocyon brachyurus)
·marsh deer (cervo do pantanal, Blastocerus dichotomus)
·neotropical river otter (lontra, Lutra longicaudis)
·ocelot (jaguaritica, Felis pardalis)
·pampas deer (veado campeiro, Ozotocerus bezoarticus)
·rhea (ema, Rhea americana),
·tapir (anta, Tapirus terrestris), and
·peccary (porco monteiro, Tayassu pecari)
These species selectively inhabit the campos, capões, cordilheiras,
gallery forests,
and water courses (baías, vazantes, corixos) of the Pantanal. Such
impressive biodiver
sity is due in large measure to the unusual climatic, geologic,
geomorphologic, and hy
drologic setting of the Pantanal. Wildlife management in the Pantanal
has been dis
cussed by Dourojeanni (1980), Paiva (1984), and Alho (1986), among
others. Wildlife
conservation in the Pantanal, particularly with regard to the jaguar
(Pantera onça), has
been discussed by Quigley and Crawshaw (1992).
_ _ _
"...Virando a Norte, tive de circumdar um como promontorio da margem
do Poente da lagoa o qual é terminado por um cabeço pedregoso e co
berto de mato, ao cual devese dar resguardo a fim de evitar as muitas
pedras que o cercam, umas submergidas, outras a flor de agua ou pouco
elevadas..."