Morphologic changes in the Paraná River channel (Argentina) in the light of the climate variability during the 20th century

Geomorphology 70 (2005) 257 – 278 www.elsevier.com/locate/geomorph

Morphologic changes in the Parana´ River channel (Argentina) in the light of the climate variability during the 20th century Mario L. Amslera,T, Carlos G. Ramonella, Horacio A. Toniolob,1 a

Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas, Facultad de Ingenierı´a y Ciencias Hı´dricas (FICH), Universidad Nacional del Litoral, CC 217, 3000 Santa Fe, Argentina b Department of Civil and Environmental Engineering, University of Alaska, Fairbanks, PO Box 75590047, Fairbanks, AK, United States Accepted 24 February 2005 Available online 22 April 2005

Abstract Recent studies regarding the climate variability in South America during the 20th century, revealed the existence of climate cycles that influenced the hydrologic conditions in the Parana´ River basin, one of the largest in the continent. How that variability affected the channel morphology of this river in its middle reach is quantitatively analyzed in this paper. The link between climate, hydrology and channel morphology is obtained through the computation of effective discharge. This discharge implicitly synthesizes the point hydrologic and bed sediment transport changes in an alluvial stream during a relatively long period. The results were obtained studying, with increasing detail, two channel reaches 373 km and 25 km long, respectively. The analysis involved the processing of more than 180 bathymetric charts, satellite images and hydraulic and sedimentologic data recorded in the Parana´ River since the very beginning of the 20th century. A rather detailed description of the treatment made with this information is given in the paper. It is shown that three periods of different effective discharges fairly well correlated with reported climatic fluctuations occurred during the last 100 years, i.e. two periods of high discharges (at the century beginning and from 1970 till present) and another of relatively low discharges between 1930 and 1970. Morphologic parameters of the main channel, such as mean width, thalweg sinuosity, braided index and aspect ratio, increased or decreased in correspondence with those variations. In transitional channels (between meandering and braided) like the Parana´ River, careful study of the thalweg behavior is a key issue, if a proper approach to the dynamic of morphologic processes operating on the whole channel is intended. Finally, on the basis of theoretical (extreme hypotheses approach) and empirical results, it is suggested that the Parana´ River main channel would not be adjusted to the present high values of effective discharge. Thus, larger

T Corresponding author. Fax: +54 342 4575224. E-mail addresses: [email protected] (M.L. Amsler), [email protected] (C.G. Ramonell), [email protected] (H.A. Toniolo). 1 Fax: +1 907 4746087. 0169-555X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2005.02.008

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erosion of banks (channel widening) and increases in the other cited morphologic characteristics will occur if those values persist. D 2005 Elsevier B.V. All rights reserved. Keywords: Climate variability; Effective discharge; Channel morphology; 20th century; Minimum stream power theory; Parana´ River

1. Introduction The influence of the climate variations on river morphology is well known, at least for small and medium-sized fluvial basins. Relatively modest fluctuations in some climate features (e.g., 1–2 8C of the annual mean temperature and lower than 10–20% of the annual mean precipitation) during time scales of a few decades can, through alteration of the flood regime, produce significant changes to the fluvial system (Arnell, 1996; Knighton, 1998). Regarding South America, the climate variability occurred during the last century and its incidence over the hydrologic regimes of some of its large rivers has being studied since several years ago (Garcı´a and Vargas, 1998; Giacosa et al., 2000; Ferna´ ndez and Larran˜ aga, 2002; Krepper and Garcı´a, 2004). Based on the reported results of some of these investigations, this paper shows how those climatic variations were responsible for the morphologic changes observed in the main channel of the Parana´ River, the sixth largest river in the world (Schumm and Winkley, 1994) and the third in South America. It has a mean discharge of about 20,000 m3/s in its middle reach and a basin of 2.3
106 km2 that cover the surface of five countries (Fig. 1). To the authors’ knowledge this subject has not been extensively explored in large rivers. The general objective of linking the influence of the climatic and (consequent) hydrologic variations with the river morphology, was met computing the values for channel forming discharge for several periods of the 20th century. Thus, the idea followed here is that the channel geometry at a given period of time is largely molded by a single discharge. Though this is a rather old idea (introduced by Schaffernak in 1922 according to Garde and Ranga Raju, 1977), many controversial points persist con-

cerning the definition of that single discharge (Knighton, 1998). Notwithstanding, most river engineers and scientists agree that the concept has merit, sustained by an abundant bibliography (Biedenharn et al., 1999). In this paper, the channel forming flow was evaluated through the effective discharge, i.e., the discharge which performs most work, where work is defined in terms of sediment transport (Wolman and Miller, 1960). This is the flow that cumulatively transports the most sediment. Regarding the transported sediment, Biedenharn et al. (1999) point out that in most alluvial streams the major features of channel morphology are principally formed in sediments derived from the bed material load. This statement was properly attended in the effective discharge computations in a stream like the Parana´ River, a large sand bed river. Much evidence associates this discharge with the bbankfull dischargeQ, but a number of references report about rivers with effective streamflows lower than the bankfull one (Richards, 1982; Knighton, 1998). This controversial point has not been completely solved yet. The Parana´ River is shown to conform with the second group of streams. The effective discharge integrates the features of the hydrologic and sedimentologic regimes of a given stream which will be reflected in its channel morphologic characteristics. Thus, climatic variations of certain magnitude will influence the basin hydrology of a fluvial system in such a way that the effective discharges of some rivers will change and perturb channel geometries. The existence of this linkage in the Parana´ River is the thrust of this paper. The writers reached this goal by comparing the nature of the climatic fluctuations that occurred in South America during several periods of the last 100 years, with the changes in the effective discharges and several geometric parameters of the

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Fig. 1. The Parana´ River basin.

Parana´ River channel computed for the same periods. The main purpose was to get a first answer, i.e., an outline as clear as possible, to the following question: what did occur during the 20th century in connection with the previous topics? Hence, the study do not deepen in the equilibrium–disequilibrium analysis of the Parana´ River channel at each period, a key issue in the computation of the

effective discharge, nor in the complex responses and times required for them when the channel forming discharge changes. In spite of these constraints, some speculations concerning these topics are advanced in the paper, in the light of the results obtained by applying the bextremal hypothesesQ approach to the particular conditions prevailing in the Parana´ River.

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2. Climate variability in the Parana´ River basin during the past century 2.1. General features According to the climatic classification of Kfeppen, three main types of climates influence most of the Parana´ River basin north to south: the tropical savanna between ~138S and ~228S; a temperate rainy climate with a dry winter between ~158S and ~288S, and a temperate, rainy climate moist all seasons, characterizing essentially the south-east region of the basin (Garcı´a, 1994). Regarding precipitation, the principal climatic component in this paper, its pattern is complex in the Parana´ River basin because of its extensive surface topographic variability and the geographic position. Notwithstanding, the local perturbations can be accommodated within the previous regional climatic context. In this regard, the annual precipitation (1961–1990 series) over the basin exceeds 1200 mm, with maximums of more than 2250 mm in the Iguazu´ River central basin (Fig. 1). The absolute minimums of about 500 mm occur on the upper basins of the Pilcomayo and Bermejo Rivers. More than 45% of the annual precipitation concentrates during the summer period (December, January, February) with an average of 480 mm and maximum mean precipitations between 700 and 800 mm on the north and northeast of the basin. Winter (June, July, August) is the dry season with an average precipitation of 130 mm and minimums of 60 mm on the north and Andean regions of the basin (Paoli et al., 2000). This general pattern and the distribution of precipitation govern the Parana´ River regime described below, in such a way that its configuration is homogeneous from the upper reaches till the mouth with the only logical downstream differences in the mean values and time displacements of maximums and minimums (Garcı´a and Vargas, 1996).

described above varied along the past century. Analyzing time series of streamflows and precipitations, they showed that precipitations had the same behavior in the upper and lower basin. According to their results the streamflow and precipitations series at Posadas and Parana´, respectively, have the same tendency changes around 1970–1971. The variations occurred in the Parana´ River behavior at Posadas, are shown in Fig. 2. It is seen that the mean discharge of the 1971–1993 period has an approximate value of 14,500 m3/s, i.e., 20% higher than the corresponding discharge of the 1931–1992 period. Regarding these differences, Garcı´a et al. (2002) show by means of a single Fourier harmonic analysis, that the streamflows variation coefficient of the 1971/1994 period exceeded more than 30% that of 1931/1970, while those of precipitations were 10% to 23% larger (14% in average). The results of this analysis, are shown in Fig. 3. It is clearly seen the change beginning in 1970 and the bdryQ period of nearly 40 years prevailing on the basin in the midst of the 20th century. Considering these facts, Garcı´a et al. (2002) hypothesize that the precipitation changes began in 1970–1971 and the incidence on streamflows, are an indication that the climatic system of the basin has changed. They also present evidence that a certain increment of discharge has an anthropogenic component, consequence of a large scale change in agricultural practices on the Brazilian area of the basin, began in 1968. They

2.2. Climate variability Garcı´a and Vargas (1998) and Garcı´a et al. (2002) studied how the general climatic picture

Fig. 2. Monthly mean flows of the Parana´ River at Posadas (see Fig. 1), for the defined periods (reproduced from Garcı´a and Vargas, 1998).

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Fig. 3. Hydrogram, the first three harmonics and the series tendency curve of annual mean flows at Posadas (see Fig. 1), Upper Parana´ River (reproduced from Garcı´a et al., 2002).

conclude that the Parana´ River flows could diminish but, on average, would not reach the historical values previous to the change in cultural activities unless climatic fluctuation of a large magnitude occurred.

3. Main features of the Parana´ River channel in its middle reach According to the climatic picture described above, the high water stage of the Parana´ River occurs in summer, usually during February and March. For such high river levels, Giacosa et al. (2000) reported mean monthly flows of 22,500 m3/s at Corrientes section, by considering the 1961–1999 discharge series. For low water stages, a mean value of nearly 15,000 m3/s was recorded that occurred during August and September. The extreme discharges ratio is nearly 8, a rather low value as in other large rivers of the world (maximum discharge: 60,000 m3/s; minimum discharge: 8000 m3/s). Along its middle reach the river flows on a wide and complex floodplain, 13 to 40 km in width, inundated completely during the extraordinary floods. The river valley is filled with at least 30 m of fluvial sands deposited over Tertiary claystones and weak sandstones. In some areas, e.g. La Paz and Diamante (km 757 and 530, respectively; Fig. 4), the left bank of the main stream is composed by Tertiary sediments. Based on indirect evidences gathered at some deep points of the river along that reach (e.g., absence of

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sand dunes in the echograms), it is believed that Tertiary sediments are also forming the bed in such local sites. The role of neotectonism influencing certain features of the river, such as changes in valley widths and longitudinal water surface slopes has been claimed in some previous works (e.g., Iriondo, 1988). Nevertheless, this is a topic where more studies are needed, because other explanations are feasible and can account for the same features. From a morphological point of view, the Parana´ main channel is a succession of enlargements with narrower, shorter and deeper sectors between them (Fig. 4). Average river widths (without island widths) are 2000–3000 m (mean depths of 5–8 m) at enlargements, and 600–1200 m (mean depths of 15– 25 m) at constrictions. Channel sand bars and islands concentrate at enlargements and resemble a braided channel pattern, but a conspicuous difference with a typical braided river exists, because the Parana´ River has a welldefined sinuous thalweg that concentrates up to 60% of the river discharge at a given section (Toniolo, 1999). Because of this feature, the values for channel sinuosity (less than 1.3) and the bed material load/ total load ratio (c17%), the Parana´ was classified (Ramonell et al., 2002) as a bbed load channelQ with transitional planform characteristics, i.e., braided with sinuous (or meandering) thalweg pattern (pattern-type 4 of Schumm, 1985). The annual mean total sediment transport, computed with data of the last ten years, amounts 145.106 t/year from which 83% is washload (silts and clays) and the rest, bed material load (Amsler and Drago, in press). Uniform medium and fine sands are predominant in the channel bed along the whole middle reach. Drago and Amsler (1998) showed that the average sizes of these sands are correlated with the mean slopes of the water surface. These slopes ranges between 7.105 and 3.105 in the middle reach.

4. Methodological aspects 4.1. General criteria The methodological criteria follow those suggested by Winkley and Schumm (1994) and Thorne

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Fig. 4. Middle and lower reaches of the Parana´ River (km 1240-mouth) and air view of the typical planform pattern.

and Baghirathan (1994), for geomorphic studies in alluvial streams. The problem was approached from a general to a particular point of view beginning with an inspection of the morphologic behavior observed at a long reach of the Parana´ River channel between km 853 and 480 of the sailing route (Fig. 4). The results obtained from this examination were further extended by means of a more detailed analysis at a reach 25 km long between Villa Urquiza and Bajada Grande (Fig. 5).

As was already stated, the link between the changes of river morphology and the reported climate variations during the last century was the effective discharge whose computation is described next. 4.2. Computation of effective discharge This discharge in the Parana´ River was obtained by means of the Schaffernak procedure (Garde and Ranga Raju, 1977). This method has been used

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per the suggestion of Biedenharn et al. (1999) for the calculation of effective discharge. The nine selected periods and the corresponding Q m values are shown in Table 1. These values were obtained by means of rating curves fit at the Aguas Corrientes section (Fig. 5) and with the 93 years record of daily levels at the Parana´ Port gauge (H pp). These records reveal three stages of the hydrologic behavior of the Parana´ River in Table 1:

Fig. 5. The Villa Urquiza–Bajada Grande reach.

without major changes since its introduction in 1922. Essentially, it defines the effective discharge as the modal value of the product of streamflow frequency and sediment transport. The application was made at the following reaches of the Parana´ River: – Between Villa Urquiza (km 619) and Parana´ (km 601). Though the total river discharge does not flow through this reach (nearly 15% is derived along a system of important secondary channels formed by the Colastine´ and Leyes rivers; Fig. 4), the quantity and quality of cartographic, hydraulic and sedimentologic data obtained by Federal Agencies and scientific Institutes during nearly 100 years at this sector of the main channel, was a decisive factor to select it. – In front of Corrientes City (km 1208) at the very beginning of the middle reach (Fig. 4). Through this section flows the total discharge of the Parana´ River which maintains these values without major change till its mouth (Giacosa et al., 2000). 4.2.1. The effective discharges between Villa Urquiza and Parana´ To attain a first general picture about the streamflows behavior along the 20th century, the mean discharges, Q m, were computed at different periods during 93 years beginning in January of 1904. The time periods (excepting the last one), were selected

1) The first two periods occur when the mean discharge was larger than 13,000 m3/s with a slight increase during 1921–1931. 2) A period of 38 years between 1932 and 1969 has minimum and remarkable stable mean values fluctuating around 12,500 m3/s. 3) In the last 30 years of the century, an increase of the mean values occur from 14,700 m3/s in the 1970–1980 period to more than 16,000 m3/s recorded in the two final series. Based on these results, the effective discharge ( Q ef) was computed first for the period of mean discharge stability between 1932 and 1969 with the detailed data available in the Villa Urquiza–Bajada Grande reach (DHGyA, 1983). The 1932–1969 series of daily discharges was divided first into 3000 m3/s constant intervals starting from 8000 m3/s. This class interval was selected after a trial and error test to obtain a relatively continuous flow frequency distribution and a smooth sediment load histogram with a well-

Table 1 Mean water level and mean discharges of the Parana´ River during the 20th Century, Villa Urquiza—Parana´ City reach Period 01-01-1904 01-01-1921 01-01-1932 01-01-1942 01-01-1949 01-01-1960 01-01-1970 01-01-1981 01-01-1991 a

to to to to to to to to to

12-31-1920 12-31-1931 12-31-1941 12-31-1948 12-31-1959 12-31-1969 12-31-1980 12-31-1990 12-31-1995

At Parana´ Port gauge.

Mean water levela (m)

Mean discharge (m3/s)

2.57 2.83 2.38 2.21 2.42 2.41 2.95 3.76 3.62

13,075 13,879 12,505 12,048 12,625 12,603 14,713 16,435 16,019

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defined peak (Biedenharn et al., 1999). The following 6 intervals resulted: Interval 1 2 3 4 5 6

Q [m3/s] b 8000 8000–11,000 11,001–14,000 14,001–17,000 17,001–20,000 N20,000

The discharge frequencies at each interval were computed as the ratio between the number of discharge data in the interval and the whole number of data. To obtain the total bed sediment transport, G s, the Engelund–Hansen formula (Vanoni, 1975) was used, because it predicts reliable rats of sediment transport in the Parana´ River as was shown by Amsler and Prendes (2000) and Alarco´n et al. (2003). The G s values were computed for the discharges representing each interval estimated by the arithmetic mean of the interval discharges. Finally, the product of the interval frequencies times the corresponding rats of sediment transport, were made. The results of this last computation can be seen in Fig. 6 that clearly shows an effective discharge value of 12,560 m3/s for the period. This discharge occurs for a 2.40 m water level in the Parana´ Port gage and is similar to the mean discharge of that period (see Table 1). Thus, the effective discharges for the immediately previous and subsequent periods to the analyzed one, were computed with the rating curves available at the reach. For the more recent periods (1981–1990 and 1991–1995), the complete Schaffernak procedure

Fig. 6. Results of the dominant discharge computation for the 1932– 1968 period at the Villa Urquiza–Bajada Grande reach.

Table 2 Mean and effective discharges of the Parana´ River during the 20th Century, Villa Urquiza—Parana´ City reach Period

Mean Effective Water level Water level of discharge discharge of mean effective (m3/s) (m3/s) dischargea (m) dischargea (m)

01-01-1904 to 12-31-1931 01-01-1932 to 12-31-1969 01-01-1970 to 12-31-1980 01-01-1981 to 12-31-1990 01-01-1991 to 12-31-1995

13,477

13,600

2.70

2.74

12,445

12,560

2.35

2.40

14,713

15,146

2.95

3.08

16,435

16,460

3.76

3.77

16,019

16,311

3.62

3.72

Bankfull discharge (01-01-1981 to 12-31-1995): 17,140 m3 /s (water level at Parana´ Port gauge: 4.00 m). a At Parana´ Port gauge.

was again applied. The results are presented in Table 2. The corresponding water levels in the Parana´ Port gage together with the mean discharges, are also included. The values of the bbankfull dischargeQ that occurs at an approximate water level of 4 m at that gage is another reference parameter given in the table. 4.2.2. The effective discharges in front of Corrientes City Water level series, recorded in front of Corrientes City from 1904 to the present together with more than 100 streamflow measurements performed by several Argentina Agencies since 1971 until 1997 at that section, were available to make this computation. The sedimentologic data were obtained from Drago and Amsler (1998). With this set of data, it was possible to fit a rating curve and to compute hydraulic and sedimentologic variables such as: mean stream velocities, u; average depth, h, and width, B; median distribution of the sizes of bed sediment, d 50, and slopes, S. The whole water level series was divided into the following time periods: 1 1904–1920 2 1921–1931 3 1932–1969 4 1970–1980 5 1981–1990

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These periods were separated in 3000 m3/s class intervals based on the same criteria described earlier. The Schaffernak method was then applied with the respective hydraulic and sedimentologic variables. The 1932–1969 period division is presented next: 3

Interval 1 2 3 4 5 6 7 8 9 10 11

Q [m /s] b 5000 5001–8000 8001–11,000 11,001–14,000 14,001–17,000 17,001–20,000 20,001–23,000 23,001–26,000 26,001–29,000 29,001–32,000 N 32,001

The effective discharge computation sequence for this case is showed in Table 3. A similar sequence was applied for the other periods. The results can be seen in Table 4. 4.3. Determination of the variables for channel geometry 4.3.1. The reach between Esquina and Pto. Gaboto The Parana´ River channel geometry at this reach was studied by means of bathymetric charts recorded in 1906, 1932–1934, 1969– 1972 and 1986–1989, and satellite images of 1994 (see Appendix A for a brief description of

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Table 4 Mean and effective discharges of the Parana´ River at Corrientes section (km 1208) Period

Mean Effective Water level of discharge discharge effective (m3/s) (m3/s) dischargea (m)

01-01-1904 01-01-1921 01-01-1932 01-01-1970 01-01-1981

to to to to to

12-31-1920 12-31-1931 12-31-1969 12-31-1980 12-31-1990

16,185 18,106 15,464 16,860 21,490

18,489 21,550 15,424 18,315 24,467

3.88 4.61 3.10 3.84 5.43

Bankfull discharge (01-01-1981 to 12-31-1990): 27,330 m3 /s (water level at Corrientes Port gauge: ca. 5.90 m). a At Corrientes Port gauge.

these records) supplemented with measurements reported in FICH (1993, 1997a,b). The morphologic parameters were obtained considering the succession of channel enlargements and constrictions already described (Fig. 4), including the transitional areas between them, i.e., confluences and expansions. The enlargements and constrictions limits were first marked out on the charts to standardize the information analysis in each period. Based on the experience acquired with the map studies of the Parana´ River, a bconstrictionQ was defined as a narrow channel area compared with the upstream and downstream widths and with nearly parallel banks (or 0 m level isobates) along all its length. The benlargementQ was set down with a similar simple criteria. Moreover, viewing the channel features recorded on the bathymetric charts it was

Table 3 Sequence of the effective discharge computation in front of Corrientes City (km 1208) of Parana´ River, 1932 to 1969 period (13,380 days) ¯ (m3/s) Q

Wla (m)

A (m2)

h¯ (m)

I
105

u¯ (m/s)

s0 (kg/m2)

gs (kg/s m)

B (m)

Days

Frequency

Gs (kg/s)

Freq.
G s

n

4420 7137 9513 12,500 15,424 18,444 21,438 24,374 27,327 30,442 34,355

0.59 5.3 1.36 2.29 3.10 3.87 4.59 5.24 5.87 6.45 7.05

10,040 11,328 12,388 13,684 14,938 16,233 17,528 18,813 20,127 20,999 21,886

7.46 8.26 8.92 9.71 10.46 11.23 11.99 12.73 13.49 14.07 14.66

1.81 2.47 2.69 2.69 2.57 2.42 2.34 2.34 2.46 3.25 4.05

0.44 0.63 0.77 0.91 1.03 1.14 1.22 1.30 1.36 1.55 1.69

0.135 0.204 0.240 0.261 0.268 0.272 0.280 0.298 0.331 0.457 0.594

0.016 0.060 0.113 0.182 0.243 0.300 0.364 0.448 0.576 1.220 2.151

1346 1371 1389 1409 1428 1446 1462 1477 1492 1492 1492

74 1019 2397 2922 2587 1843 1370 948 381 183 156

0.005 0.073 0.173 0.211 0.186 0.133 0.099 0.068 0.027 0.013 0.011

21.23 82.26 157.45 256.91 346.61 434.29 531.81 661.30 859.53 1821.03 3210.52

0.112 6.038 27.191 54.079 64.609 57.666 52.490 45.167 23.594 24.008 36.086

0.037 0.032 0.029 0.026 0.023 0.022 0.021 0.020 0.021 0.021 0.023

a

Wl: water level (at Corrientes Port gauge).

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possible to trace the bounds of the bexpansionQ, the transitional areas between constrictions and enlargements. It was defined as the sector downstream of a constriction where banks (or 0 m level isobates) diverge regularly along the current direction until that condition changes, or until the site where a sand bar (morphologically developed at least until the local 0 m level isobate) appears. It is well known that the stream diverges in these channel areas and, as in the Parana´ River case, flows downstream along the enlargement essentially concentrated in two or more deep strips (regarding the bars developed until the 0 m isobate) or just in a single one if the bars develop laterally close to the channel banks. Respecting the flow confluence areas, it was not possible to describe them clearly with a single planform feature in all the available charts. Thus, the enlargements were defined simply as the river sector between a given expansion and the constriction downstream. This simplified criteria proved to be suitable for performing a consistent and handy analysis of the charts within the context of the study

objectives. Naturally it must not be conceived as summarizing all the possible processes for flow diversion and sand bar formation that take place at river widenings. The three channel morphologic units were also identified on the 1994 satellite images. The limits between successive expansions/enlargements were here considered where the divergence of the regular banks disappeared. With these criteria it was possible to resolve the separation in nearly all the cases. The thalweg track was also traced on the bathymetric charts by means of a continuous and smooth line following the deeper channel areas along the current direction between both ends of a given map. It was typical to find this line between parallel isobates corresponding to a certain depth (e.g. 10 m or more) at constrictions while those at expansions acquired a bYQ plan pattern with the barmsQ directed towards the downstream enlargement. One of these arms was shorter than the other for a given depth. All the isobates related with the longer barmQ generally remained well defined downstream through only one

Fig. 7. Base line and cross-sections considered in the Villa Urquiza–Bajada Grande reach analysis.

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of the two canals normally existing at the enlargement. This one was linked with the thalweg track. The shorter shallower arms (depths smaller than the 2–3 m isobates in nearly all the cases), were also traced. These second-order canals are typical morphologic elements of the channel expansions and enlargements but they do not always exist in each of those areas (only two exceptions were found along the studied reach). Cross-sections equally spaced were traced on all the charts (including the 1994 images) at a rate of three sections at constrictions and expansions, and 6 (up to 12) at enlargements depending on the lengths.

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Finally, on the base of the previous treatment the following parameters were measured: Right length, L r , of all sectors. Thalweg track, L t , and second-order canals, L i, lengths at each sector. Partial widths (referred to the 0 m level in the local gauges), B 0 partial, of the main channel, sand bars (with/without islands) and secondary channels at each cross section. Partial top widths of the main channel, islands and secondary channels, B m partial, at each cross section. Cross-section average depth, h 0, referred to the 0 m level in the local gauges.

Fig. 8. Morphological changes at the Villa Urquiza–Bajada Grande reach during the 20th century (banks and bars/islands contours drawn at the 0 m level isobate).

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Cross-section maximum depth, h ma´x, referred to the 0 m level in the local gauges. With these parameters, it was possible to compute: Channel width referred to the 0 m level in the local gauges, B 0. Channel top width, B m. Thalweg sinuosity, P t = L t/L r. BraidedP index (Mosley, 1981), PE, expressed here as PE = L i/L t. (*) Total sinuosity P P (Robertson-Rintoul and Richards, 1993), P = L i/L r. (*) P (*) In the computation of PE, and P, L t was included in the sum of the L i value.

To digitize the maps, a base line was traced according to the reach morphology. This line was placed along the left bank because of its larger stability with its origin upstream of Villa Urquiza and its downstream end near Bajada Grande. Perpendicular to the base line 43 cross-sections were traced, 500 m equally spaced (the exception was section 20 coincident with the base line break point, Fig. 7).

Toniolo et al. (1999) showed that it is possible to define the thalweg geometric dimensions, B t and h t, by means of the Eqs. (1)–(4) (see Results). The B t and h t values were computed with these equations in the first four considered periods at each cross-section where the channel width, B 0, and the mean depth, h 0, values were available. The procedure to obtain B t and h t for the morphologic condition of 1994, was based in relationships between B m vs. B t, and B 0 vs. B m fitted with available flow measurements and depths data recorded along the sailing line reported by FICH (1993, 1997a,b). Measuring B m at each cross-section on the 1994 chart, the corresponding B t and B 0 were determined. The h t values were assumed to be similar to the sailing line depths. With this parameters and Eqs. (1)–(4), the h 0 values at each morphologic unit (constrictions, etc.) on the 1994 chart, were also computed. 4.3.2. The reach between Villa Urquiza and Bajada Grande It has a typical planform pattern: two enlargements with a constriction between them (Fig. 5). The detailed analysis of its geometry was performed by means of bathymetric charts performed in the following years, each representing nine morphologic scenarios along the last century: 1905, 1920/21, 1934/35, 1949/50, 1960/61, 1970/72, 1988/89, 1996/ 97, 2000. See Appendix A for more details about these charts.

Fig. 9. Morphological changes of three selected cross-sections (see Fig. 5) at the Villa Urquiza–Bajada Grande reach (years are representative of periods with different dominant discharges during the 20th century).

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Table 5 Braiding and thalweg sinuosity parameters of measured reaches of the Parana´ River between Esquina (km 853) and Pto. Gaboto (km 480) P Year Measured channel Braiding parameter (PE) Total sinuosity ( P) Thalweg sinuosity ( P t) length (km) Maximum Mean Minimum Maximum Mean Minimum Maximum Mean Minimum 1905/1906 1932/1934 1969/1972 1986/1989

99.30 59.05 70.35 106.95

2.64 2.80 2.89 2.88

2.17 2.32 1.98 2.04

1.17 1.50 1.00 1.67

3.17 3.66 3.77 3.15

Figs. 8 and 9 show plain views of the reach along the 20th century and the changes that occurred in some of its cross-sections, respectively.

5. Results 5.1. The effective discharge in the middle reach of the Parana´ River Tables 2 and 4 show that effective discharge in the Parana´ River did not maintain a constant value along the last 100 years. The values were on the order of the mean discharge but always lower than the bankfull discharge though the difference with this one became smaller during the last period, i.e., between 1981 and 1995. The variations in effective discharge are a consequence of the climatic phenomena that occurred over the whole fluvial system and led the morphologic changes observed in the Parana´ River channel during the 20th century. The features and scope of this linkage are discussed below.

2.56 3.23 2.39 2.53

1.81 2.51 1.10 2.14

1.55 2.44 1.37 1.68

1.21 1.42 1.20 1.25

1.02 1.02 1.07 1.04

5.2. Changes in channel morphology 5.2.1. Esquina–Pto. Gaboto reach P The mean braided parameters (PE and P) and thalweg sinuosities, measured at enlargements in the first four periods considered in this reach, are presented in Table 5. In Table 6, the mean thalweg sinuosities at channel constrictions, expansions and enlargements for the same periods, and the total sinuosity computed with weighting coefficients that account for the influence of the lengths of the morphologic units at a given period, are included. The last sinuosity values differ from those of Table 5 though the tendency is identical. The variations of the thalweg sinuosity and braiding parameters, with the effective discharge in the middle reach (Table 4), are sketched in Figs. 10, 11 and 12. 5.2.2. Villa Urquiza–Bajada Grande reach The values for thalweg sinuosity, the channel average widths, depths (mean and maximum) and volumes between Villa Urquiza and Bajada Grande

Table 6 Total and partiala thalweg sinuosities of measured reaches of the Parana´ River between Esquina (km 853) and Pto. Gaboto (km 480) Year

Morphological unit

Numbers of measured units

Measured channel length (km)

Weighting coefficientb

Mean thalweg sinuosity

Thalweg sinuosity

1905/1906

Constriction Expansion Enlargement Constriction Expansion Enlargement Constriction Expansion Enlargement Constriction Expansion Enlargement

10 9 10 4 3 6 6 4 8 8 8 8

23.45 18.80 99.30 6.92 6.70 59.05 13.30 6.95 70.35 18.30 14.55 72.65

0.17 0.13 0.70 0.13 0.13 0.74 0.18 0.11 0.71 0.16 0.14 0.70

1.04 1.14 1.18 1.09 1.17 1.47 1.01 1.02 1.20 1.07 1.20 1.29

1.15

1932/1934

1969/1972

1986/1989

a b

Discriminated according to channel morphological units. Weighting coefficients account for the length influence of each morphologic unit.

1.38

1.14

1.24

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Fig. 10. Changes of the thalweg sinuosity ( P t) with the effective discharge during the 20th century (Esquina–Pto. Gaboto reach).

P Fig. 12. Changes of the total sinuosity ( P) with the effective discharge during the 20th century (Esquina–Pto. Gaboto reach).

are given in Table 7 for the nine morphologic episodes studied in the 20th century. The evolution of those geometric parameters is represented in Fig. 13a,b (see also Fig. 9).

computed with the following empirical expressions: Enlargements Bt =h0 ¼ 1:1346ðB0 =h0 Þ0:8124 r2 ¼ 0:774

ð1Þ

5.3. Relationships between the thalweg and the channel aspect ratios

Bt =ht ¼ 1:0599ðB0 =h0 Þ0:6799 r2 ¼ 0:799

ð2Þ

As was stated previously, Ramonell et al. (2002) showed that the channel pattern of the Parana´ River is braided with a sinuous (or meandering) thalweg. By studying the shifting modes of the thalweg, they revealed how the flow concentrated in it governs the main geomorphic processes occurring in the whole channel. Thus, the definition of the thalweg geometric characteristics is essential to understand the adjustment mechanisms of the river. Toniolo et al. (1999) showed that its width, B t, and depth, h t, can be

Bt =h0 ¼ 0:1381ðB0 =h0 Þ1:1629 r2 ¼ 0:865

ð3Þ

Bt =ht ¼ 0:0974ðB0 =h0 Þ1:0685 r2 ¼ 0:819

ð4Þ

Fig. 11. Changes of the braiding index (PE) with the effective discharge during the 20th century (Esquina–Pto. Gaboto reach).

Confluences, expansions, constrictions

These equations were devised through a procedure based on 65 selected streamflow measurements performed at 16 cross-sections of the Parana´ main Table 7 Geometric parameters of the Parana´ main channel between Villa Urquiza and Bajada Grande Year

Mean width (m)

Mean depth (m)

Mean maximum depth (m)

Channel volume (Hm3)

Thalweg sinuosity

1905 1920 1934 1949 1960 1972 1988 1997 2000

1,818 1,718 1,462 1,120 1,313 1,407 1,527 1,520 1,536

6.79 6.45 7.40 9.52 8.97 7.59 7.48 8.15 7.99

13.64 13.36 13.18 16.82 15.55 15.09 13.09 15.55 14.73

174.29 153.87 150.92 127.44 136.62 141.22 136.54 – 157.15

1.20 1.09 1.12 1.12 1.09 1.12 1.15 – 1.18

The thalweg sinuosity, P t, values were computed between sections 6 and 40 (Fig. 7). Depths and widths were computed between sections 18 and 39. Channel volumes were computed between sections 7 and 39.

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271

Fig. 13. (a) Changes of the channel volume (at the 0 m level isobate) during the 20th century at the Villa Urquiza–Bajada Grande reach. (b) Changes of the mean width and mean maximum depth (at the 0 m level isobate) during the 20th century at the same reach.

channel during a period of 83 years of the 20th century (Toniolo, 1999). All the streamflow data corresponded to river stages near to those of the effective discharges. Knowing the channel width, B 0, and mean depth, h 0, the thalweg dimensions are easily obtained at enlargement and node zones. In Table 8, the mean values of B t, h t and the aspect ratio, B t/h t, of the Parana´ River thalweg are given for each of the five periods studied at the Esquina–Pto. Gaboto reach. 5.4. Extremal hypotheses results obtained with the morphologic dimensions of the thalweg Amsler and Ramonell (2002), studied the degree of adjustment of the channel geometry

on the Parana´ River (in the Villa Urquiza–Bajada Grande reach) to the present effective discharge condition by applying the minimum stream power theory (Langbein and Leopold, 1964; Yang, 1971, between others). They used the classical equation: x¼

cQef S B0

ð5Þ

where: x, specific stream power, and c, water specific weight (the rest of the symbols were defined previously). Considering the influence of the flow concentrated along the thalweg strip on the morphologic processes in the Parana´ River, it is possible to gain insight of the channel geometric changes by means of the minimum

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Table 8 Synthesis of the thalweg dimensions in the Parana´ River between Esquina (km 853) and Pto. Gaboto (km 480) Constrictions

Enlargements

Expansions

River channel mean values

B t (m)

h t (m)

B t/h t

B t (m)

h t (m)

B t/h t

B t (m)

h t (m)

B t/h t

B t (m)

h t (m)

B t/h t

1905/1906 Mean Deviation Variation coeff.

396 163 0.41

17.85 3.20 0.18

26 14 0.57

573 164 0.29

12.11 2.54 0.21

50 15 0.30

538 164 0.32

14.40 3.58 0.23

42 18 0.49

539

13.36

45

1932/1934 Mean Deviation Variation coeff.

431 141 0.33

14.88 1.97 0.13

31 13 0.43

496 137 0.28

12.14 3.20 0.26

46 17 0.38

455 120 0.26

13.64 1.28 0.09

33 8 0.23

482

12.70

42

1969/1972 Mean Deviation Variation coeff.

236 90 0.38

21.64 4.68 0.22

13 7 0.55

555 130 0.23

12.37 2.97 0.24

50 18 0.35

417 130 0.33

14.09 2.43 0.22

32 13 0.47

483

14.22

41

1986/1989 Mean Deviation Variation coeff.

370 160 0.43

15.24 4.02 0.26

29 18 0.62

462 128 0.28

12.04 3.19 0.26

42 16 0.37

473 133 0.29

13.04 2.19 0.23

39 14 0.41

449

12.69

40

1994 Mean Deviation Variation coeff.

254 75 0.30

16.41 6.66 0.41

20 12 0.62

564 107 0.19

10.04 3.29 0.33

65 31 0.47

320 79 0.25

11.44 3.42 0.30

32 18 0.57

491

11.07

55

stream power hypothesis but applied in the conditions prevailing in the thalweg sector. Eq. (5) put as a function of the thalweg hydraulic and geometric features stands: xt ¼

cQt S Bt Pt

ð6Þ

where the subscript, btQ, refers to the Eq. (5) parameters but of the thalweg flow being P t (= L t/L; L t, thalweg length in the reach; L, straight reach length) its sinuosity (Table 7). See that S t = S/P t. The discharge, Q t, flowing along the strip of width, B t, and depth, h t, may be obtained with the following equation (Toniolo, 1999): Qt ¼ 6:105 Q2  1:4567Q þ 13; 557

ð7Þ

r2 ¼ 0:908 Eq. (7) is valid at the channel enlargements and nodes. The B t values in Eq. (6) were computed weighting the results obtained with Eqs. (1)–(4) taking

account of the enlargements and node importance in the total length of the studied reach. The results from Eq. (6) are presented in Table 9. Another outcome related with the extreme hypotheses approach in the Parana´ River, was obtained from the Chang’s (1985) relationships when applied to predict the thalweg width (B t) and depth (h t). Taking into account the transitional character of the Parana´ River (braided with sinuous thalweg), it would lie on the zones 2–3 of the Chang’s diagram (Raudkivi, 1990). For rivers in the zone 3, Raudkivi suggests the following equations to compute B and h:  pffiffiffi0:84 B ¼ 278Q0:93 S= d

ð8Þ

h h ¼  0:112  0:0379lnQ  pffiffiffii  0:0743ln S= d Q0:45 ðd in mmÞ

ð9Þ

M.L. Amsler et al. / Geomorphology 70 (2005) 257–278 Table 9 Unit stream power in the thalweg area between Villa Urquiza and Bajada Grande Period Q t (m3/s) B t (m) I
105 (m/m) P t

x t (N m/s/m2)

1905 1920 1934 1949 1960 1972 1988 1997 2000

2.84 3.44 3.63 4.38 3.87 4.15 4.25 – 4.18

4840 4840 4725 4725 4725 5260 5835 5760 5760

675 615 550 455 530 540 585 570 570

4.85 4.85 4.82 4.82 4.82 4.87 4.99 4.98 4.98

1.20 1.09 1.12 1.12 1.09 1.12 1.15 – 1.18

These expressions were applied for the five periods considered in the Esquina–Diamante reach. The following data were used: S = 2
105 = 5
105 d = 0.300 mm (Drago and Amsler, 1998)

Q t : Eq. (7) Q ef : Table 4

The results are shown next: Period Q ef [m3/s] Q t [m3/s] B t0 [m] B tc [m] h t0 [m] h tc [m]

1906 18,490 7,135 539 200–430 13.4 16.9–13.2

1933 21,550 10,030 482 274–591 12.7 18.8–14.5

1970 15,424 5,363 483 153–330 14.2 15.3–12.1

1988 18,315 7,004 449 196–423 12.7 16.7–13.1

1994 24,467 13,834 491 369–798 11.1 20.9–15.9

B t0 : thalweg width obtained in Table 8. h t0 : thalweg depth obtained in Table 8.

Chang’s method predicts geometric dimensions of the thalweg stream in the order of the observed ones. Eight of 10 measured B t and h t values fall within (or are close to) the computed range values which are dependent from the selected slope, S (maximum differences: 43% in h tc for 1994 and 32% in B tc for 1970). The predictions made with the gross dimensions of the channel are not shown here, but it is easily verifiable that they are not good.

6. Discussion The climatic variations that occurred over South America during the 20th century affected the hydrologic conditions over the Parana´ River basin, one of the largest of this continent. Those variations were related to the morphologic changes observed in the main channel by analyzing the values of effective

273

discharge computed for several periods of the last 100 years. This discharge, an integral feature of the river behavior (hydrologic, hydraulic and sedimentologic), controls the size and shape of channels that are dynamically stable. Though the channel stability of the Parana´ River cannot be asserted at all, the association between climate, effective discharge and the fluctuations of channel geometry, the main purpose of the paper, was successfully displayed. The morphometric variations at both studied reaches (Esquina–Pto. Gaboto and Villa Urquiza– Bajada Grande) show clearly two situations: – One characterizing a period of nearly 40 years between 1930 and 1970. – The other defining the first 30 years of the century together with the period between 1970 and the present. According to the results presented in Tables 5–8 and Figs. 10–13, the main morphology of the channel of the Parana´ River during the midst of the 20th century can be characterized by: i. Minimum thalweg widths, sinuosities and aspect ratios B t/h t. ii. Minimum braiding parameters indicating a tendency of the stream to concentrate in a single branch. This relates closely to the low P t values, nearly equal to those of a straight channel. iii. Decreasing channel average widths, B 0, and volumes, with minimums around 1949. The mean and, specifically the maximum depths, increased. This increment is observed in many large alluvial streams when the river maintains low water levels during long periods (minimum effective discharges, in this case), because the flow bconcentratesQ in the thalweg strip bself-dredgingQ its maximum depth areas. The benefits for navigation are obvious. This phenomenon was known for years on the Parana´ River (Repossini, 1912). With Eq. (7) it is possible to verify that the flow concentration in the thalweg area really occurs for decreasing discharges and water levels smaller than bankfull (H pp c 3.50–4.00 m, Q c 17,100 m3/s; Table 2). According to this

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equation, with a discharge around 6600 m3/s in the Villa Urquiza–Bajada Grande reach, flow would be completely concentrated in the thalweg strip. With this condition the available rating curve predicts an approximate water level of 0.10 m at the Parana´ Port gage. The morphologic picture of the river at the beginning of the century and since 1970 until the present shows an opposite tendency in all the parameters that describe the channel geometry. Regarding the channel volumes in the Villa Urquiza–Bajada Grande reach, they varied between 174 Hm3 and 150 Hm3 from 1905 to 1934, but attained a similar value of 157 Hm3 at present. Larger mean widths and lower depths than during the other periods were also recorded in that first 30 years of the 20th century. After 1970, the average widths increased consistently but the maximum depths did not have significantly different values with respect to the previous bdryQ period (excepting in 1988 when abnormal very low water levels changed this tendency). The previous comments indicate that the morphologic changes in the Parana´ River channel compared with the values of effective discharge at each period (Tables 2 and 4), are closely related. These discharges, in turn, reflect the humid and dry climatic cycles that occurred all over the Parana´ River basin during the

last century. The morphologic variables (dependent) had been changing according to the climatic fluctuations that controlled the increments/decrements of the main variable that governs the morphologic processes occurring in the channel of alluvial streams similar to the one studied. The values of the geometric variables show that the channel volume of the Parana´ River varies essentially through changes in its width instead of its bed, which agree with the concept advanced by Schumm (1971) regarding how the bbed loadQ channels adjust their dimensions (see Table 7 and Fig. 13a,b). Particularly during the last 30 years, effective discharges larger than those at the beginning of the century were recorded, but with morphologic parameters that do not have the values of that period. Thus, it is suggested that the Parana´ River would not be in equilibrium with the present Q ef, i.e., it would be in an eroding unbalanced state trying to adjust to the new situation by increasing its width rather than deepening its bed. The evidence already presented support this statement. See also Fig. 14 reproduced from Amsler and Ramonell (2002), where the extreme and 1996 positions reached by the Parana´ River right bank in front of Parana´ City during the 20th century, are shown. Regarding these adjustments to a certain Q ef, the situation at the beginning of the 20th century when the channel had the maximum recorded dimensions

Fig. 14. Extreme and 1996 positions reached by the Parana´ River right bank (at the 0 m level isobate) during the 20th century between km 604 and 584 (reproduced from Amsler and Ramonell, 2002).

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but with effective discharges were not as large as those at present. The question arises: was the big channel molded by those Q ef or by others with a similar value to those of the present, or had even greater values occurred during the last decades of the XIXth century?. Moreover, would not this first period also be an unbalanced, aggrading time (relaxation time in this case) of adjusting the channel morphology to a situation of decreasing discharges? If so, when did this time finish? Anyway, not enough hydrologic and sedimentologic data exist to answer these questions, so they remain a matter for investigation. Concerning these topics, Amsler and Ramonell (2002) found at the Villa Urquiza–Bajada Grande reach that x tended to a minimum of 4.0 N m/s/m2 by the end of the bdryQ period during the 1960s with minimum Q ef. They applied, as was stated earlier, Eq. (5) with the gross channel dimensions. Observe in Table 9 that by the same period, x t, tended to the same value in the thalweg strip. Does it mean that by that time the Parana´ River channel reached an approximate stable form? If so, this minimum power value is characteristic of the equilibrium state to which the Parana´ River should tend with negligible changes in h t and h 0. Combining Eqs. (1), (2) and (6), it would be possible to estimate a B t (and also B 0) value of the Villa Urquiza–Bajada Grande reach and a sinuosity, P t, to which the thalweg should tend to adjust the channel morphology to the present conditions of effective discharge. See also in Table 9 the minimum x t values that occurred in the first decades of the century, a fact that would be consistent with the suggestion made above about the possibility of an aggrading stage of the Parana´ River channel during those years. Finally, the reasonable good results of the Chang’s approach concerning the geometric dimensions of the thalweg irrespective of the assumption of rivers in regime implicit in the method, would suggest the importance of studying the flow dynamics of the thalweg in transitional alluvial courses (between meandering and braided), to understand properly the morphological processes affecting the complete channel. Ramonell et al. (2002) and Amsler and Ramonell (2002) reported detailed qualitative evidence that support this suggestion.

275

7. Conclusions (1) The main purpose of this study concerned the influence of climatic fluctuations on the morphologic behavior of the Parana´ River channel reported for South America during the 20th century. Climatic variations were linked with changes of several geometric parameters measured at two reaches of the main channel. The link between climate and morphology was the effective discharge computed for different periods of the last 100 years. (2) Through this linkage, two situations were identified: ! bHumidQ climate periods at the beginning of the century and especially after 1970 until the present were reflected in high values of effective discharges. These discharges had an impact on the channel morphology by promoting an increase in widths, braided indexes, thalweg sinuosities, width/depth ratios and channel volumes. ! A bdryQ climate period occurred in the midst of the century between 1930 and 1970 when all the previous parameters showed the opposite tendencies. (3) Within this general context, a series of observations can be made: ! Evidence arose suggesting that the channel morphology of the Parana´ River did not reach a true dynamic equilibrium at each of the above climatic scenarios except, perhaps, on the end of the bdryQ period. Notwithstanding, the tendencies described in 2 were clearly in evidence as depicted in Figs. 10–13. ! The extremal hypotheses approach, in spite of not being widely accepted, proved to be an useful tool to gain some insight about river behavior. The application of this theory revealed minimums values for specific stream powers at the end of the bdryQ period, after 40 years of a nearly constant effective discharge. Moreover, it afforded quantitative support to the suggestion that the first 30 years of the last century was part of a period (of unknown extension yet), during which the river was adjusting its morphology to a situation of

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decreasing effective discharges, i.e., to an aggrading condition. ! Because of the lack of channel stability, together with complexities related to climate variations, any prediction concerning river future behavior of the river remain rather speculative. In this regard, Robertson et al. (2001) remark the potential importance of decade-scale climatic fluctuations on the Parana´ River discharges based on the correlation found between the El Nin˜o Southern Oscillation (ENSO) periods, and the summerseason monthly streamflows at Corrientes. The important consequence is that these may be, to some extent, predicted at a decade-scale. In considering this statement, however, the conclusions of Garcı´a et al. (2002) about the anthropogenic component in the precipitation changes began in 1970–1971 and its incidence on the future discharges of the Parana´ River, must be properly addressed. ! The importance of the stream dynamics concentrated in the thalweg sector was clearly revealed in all the morphologic processes molding the complete channel geometry. It would be a key issue to understand the behavior of a transitional planform course (between meandering and braided) like the Parana´ River. Synthesizing, a general picture was obtained of the morphologic changes that occurred during the last century in the main channel of the Parana´ River along its middle reach that combined climatic variations and the effective discharge concept. On the other hand, reasonable founded suggestions were advanced to explain certain features of those changes that may contribute to design suitable investigations towards answering the unsolved questions.

responsible of the hydrographic measurements at the FICH, is also acknowledged. Special thanks deserve the group of undergraduate students that assist the authors in the FICH’s Sedimentology Laboratory. Without their help it would have been nearly impossible to process the enormous volume of river data synthesized in this paper. The suggestions made by the paper reviewers that undoubtedly improved it, are gratefully acknowledged. This study was made within the framework of the project: bCharacterization of the thalweg shiftings at the middle reach of the Parana´ RiverQ, granted by the Universidad Nacional del Litoral and the Secretarı´a de Ciencia, Tecnologı´a e Innovacio´n Productiva, from Argentina.

Appendix A A brief summary of the main details concerning the cartographic material used in the two studied reaches in the Parana´ River is given next. Esquina–Pto. Gaboto reach Period 1904–1906 1932–1934 1969–1972 1986–1989 1993–1996

Number of bathymetric chartsa 26 41 40 52 2b

Scale 1:5000–1:20000 1:5000–1:10000 1:5000 1:5000 1:5000–1:10000

a

The gross of the charts have isobates separated 1 m traced on the base of cross-sections recorded each with a 100–200 m separation along the channel. b Charts were recorded during 1996 and had similar characteristics to those of the other periods. Only depth data recorded along the sailing line during 1993 and 1995 were available in digital support.

Period 1994

Number of satellite images 12 3 9

Scale 1:100000 1:50000 1:250000

Acknowledgments Villa Urquiza–Bajada Grande reach The authors are deeply grateful to the Distrito Parana´ Medio of the Direccio´n Nacional de Vı´as Navegables from Argentina, who kindly supplied the gross of the Parana´ River bathymetric charts, essential to make this study. Regarding this aspect, the collaboration of the field surveys specialists

Period 1905 1920–1921 1934–1935 1949–1950 1960–1961

Number of charts 2 2 2 4 3

Period 1970–1972 1988–1989 1996–1997 2000

Number of charts 4 3 3 1

M.L. Amsler et al. / Geomorphology 70 (2005) 257–278

From the 24 charts, 21 were originally scaled at 1:5000, 2 at 1:10000 and 1 at 1:20000. The charts have isobates separated 1 m traced on the base of cross-sections recorded each with a 200 m separation along the channel.

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