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Sedimentological characteristics of small rivers with loessic headwaters in the Chaco, South America
Quaternary International 62 (1999) 69}74
Sedimentological characteristics of small rivers with loessic headwaters in the Chaco, South America Oscar Orfeo CECOAL-CONICET, Casilla de Correo 291, 3400 Corrientes, Argentina
Abstract Some sedimentological characteristics of #uvial environments located in the northeastern region of the Chaco}Pampa plain were analysed in order to determine the possible in#uence of the loessic headwaters of local basins. Concentrations of suspended sediments, as well as grain size of the river beds, were compared. Unexpectedly, it was found that the suspended load has an inverse relationship with the hydrometric levels, with the exception of highly saline streams (conductivity values between 4300 and 11,000 lS cm~1). The mean concentration of suspended sediments was 63.1 and 206.3 mg l~1 in #ood and low water periods, respectively. The grain size of bed sediments was relatively homogeneous along the whole hydrological cycle. Sandy}silty bottoms predominate in channels, followed by silty}sandy and clayey}silty bottom types. Sediment transport mainly occurs in accelerating #ow suspension, overcoming the critical erosion velocity. During #ood periods, some parameters exceeded by 70% the values recorded during the low water phase. ( 2000 Elsevier Science Ltd and INQUA. All rights reserved.
1. Introduction The Eastern Chaco, located in the northeastern region of the Chaco}Pampa plain, is formed by the distal areas of the alluvial fans of large rivers (Iriondo, 1993) The study region includes the distal areas of the Pilcomayo and Bermejo Rivers (Fig. 1). The #oods of these allochthonous rivers discharge large volumes of water into a plain landscape crossed by old #uvial belts, generating basically swamp environments. A series of local #uvial networks were developed in the Central and Eastern Chaco, all having similar characteristics. Collectors are small sinuous streams #owing in abandoned channels of large allochthonous rivers. Their upper basins (a few hundreds of square kilometers in each case) are covered by a loess mantle. Such areas are dominated by in"ltration, with little runo! and small slopes, resulting in a very small contribution of sediment to the channels. The middle and lower parts of the basins are developed on hard, impervious clay, paludal in origin. The region is a sunken block, covered by large permanent swamps and dense vegetation. Fluvial geomorphological elements are not clearly organized in hierarchical terms. Fertonani and Prendes (1983) de"ne this type of area as a `NonTypical Hydrological Systema. Finally, the local rivers #ow into the regional collectors, the ParanaH and Paraguay Rivers.
The pattern and geographical location of these small rivers create particular characteristic sediment nature and dynamics, including silty beds and an inverse correlation between water discharge and concentration of suspended solids. These particular systems appeared in the Chaco plain during the middle Holocene (Iriondo, pers. comm.) The landscape is a!ected by alternating seasons of drought and periods of prolonged #ooding, which has resulted in the progressive coalescence of the water bodies and the saturation of the storage capacity of the basins. Climate, topography, structural conditions, and subsurface lithology de"ne the local drainage system (Patin8 o and Orfeo, 1986) and shape the main landscape characteristics (Nei!, 1986).
2. Location of the studied basins The studied area comprises more than 30 local basins along 400 km of the Paraguay}ParanaH collector (253 00@}283 30@ S; 57330@}59330@ W) (Fig. 2). The area covers the distal parts of the Pilcomayo and Bermejo alluvial fans and a sector of the Paraguay}ParanaH belt (Iriondo, 1993). Considering the geomorphological setting of the region, the results obtained in the present investigation can be extrapolated to the entire Eastern Chaco, from 20 to 303 S.
1040-6182/00/$20.00 ( 2000 Elsevier Science Ltd and INQUA. All rights reserved. PII: S 1 0 4 0 - 6 1 8 2 ( 9 9 ) 0 0 0 2 4 - 5
70
O. Orfeo / Quaternary International 62 (1999) 69}74
Fig. 2. Fluvial environments of the study area (numbers correspond to Table 1).
The climate is subtropical, with 800}1200 mm annual rainfall. The rainy season begins in October and ends in March; winters are dry. Mean temperatures vary between 24 and 303C in summer (Schmieder, 1980).
ment dynamics in the #uvial systems. Width, depth and water velocity were measured for characterization of channel size and energy. Bed sediments were sampled near the #uvial collectors Paraguay and ParanaH , using a core-sampler on the deepest zone of each site. In order to determine the grain size, the sieving and pipette techniques were used (Galehouse, 1971; Ingram, 1971; McManus, 1988), employing the Udden-Wentworth grain size scale with the clast terminology modi"cations proposed by Friedman and Sanders (1978). The graphic method of Folk and Ward (1957) was employed in the calculation of some textural parameters. The Visher criteria (1969) were useful to evaluate di!erent mechanisms of bed-sediment transport. A punctual suspended-sediment sampler was used in several vertical pro"les on cross sections located in the same sites. Sediment concentrations were calculated by "ltering using membrane disks (size of pores"0.45 lm).
3. Methods
4. Results
Grain size analysis of bed sediments and concentration of suspended solids were chosen as measures of the sedi-
In high waters, the width and depth of most streams ranged from 20 to 60 m and from 1.2 to 5 m, respectively.
Fig. 1. Some geomorphological units of the Eastern Chaco (adapted from Iriondo, 1993). References: 1: Pilcomayo alluvial fan, 2: Bermejo alluvial fan, 3: #oodplain of the Paraguay}ParanaH river system.
O. Orfeo / Quaternary International 62 (1999) 69}74
71
Table 1 Some physical parameters of the studied environments (see Fig. 2). =: width (m), D: depth (m), <: water velocity (m s~1), C: suspended solid concentration (mg l~1) No.
Sample station
Hydrological stage High waters
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
Pilcomayo river Negro stream El Lobo swamp Morocho swamp He-He Grande stream Malvinas stream Monte Lindo stream TimboH PoraH stream PilagaH stream PatmH swamp San Hilario stream Tohue stream Salado (Formosa) stream Saladillo (Formosa) stream MbiguaH stream Cangui Chico stream Oro river QuiaH stream Del Tres stream Cuatro Diablos swamp GuaycuruH river Tragadero river Salado (Chaco) River Saladito stream Palometa river Hornero swamp Saladillo (Chaco) stream TapengaH river El RaboH n stream Amores river
Low waters
=
D
<
70 30
7.8 2.5 1.2 1.5 2.5
0.20
20 50 60 50 20 6 23 6 8 35 11
18 40 52 22 36
4.0 5.0 1.5 5.5 1.2 2.3
1.27 0.83
0.40
0.5 4.0
27 30
3.0 4.5 3.0 2.0 3.5 0.5 3.0 4.5
37
5.0
C
=
D
20.5 4.5 30.0 8.0 26.5 4.7 3.25
46 22
6 1.75
31.5 96.0 43.0 37.0 22.0 47.0 5.0 11.0 21.0
8
5 7 23
<
1.0 0.5 2.2 0.2 0.8 0.6 1.7
14 6 20 1 10 5 24
2.3
0.8
0.2 1.0
Null 0.32 0.90
0.40 0.70 0.20 0.60
0.73 0.27 0.41 0.63
35.6 30.5 80.0 258.0 103.0
8 14 40 17 21
2.8 1.2 0.9 0.7 0.7
0.30 0.46 Null Null Null
0.38 0.88
36.5 90.0 470.0
1.2 0.8 3.1 3.0
Null 0.56
0.62
14 12 24 20
During low water periods, the width range was 5}24 m and the depth range was 0.2}3 m (Table 1). In a few courses (sites 5, 9, 17), the width and depth values during the #oods exceeded the low water values by 70%. In other cases (sites 22, 23, 25, 28), the percentage of depth variability was similar, but the width diminished between 23 and 65% under low water conditions. The current velocity in high waters shifted mainly from 0.4 to 0.9 m s~1 (with the exception of the site 6, where it reached 1.27 m s~1). During the low water period, most streams showed current velocity values ranging from 0.2 to 0.6 m s~1. Nevertheless, some rivers (sites 23, 27) remained without water movement in this phase. 4.1. Bed sediments A few streams (sites 17, 28) have sandy bottoms while others (sites 11, 13, 18) have mainly pelitic bottoms. However, the majority of the streams showed a nearly
0.30
C 44.5 13.0 77.0 73.3 28.0 133.0 339.0 248.0 301.0 1110.0 235.0 86.0 64.0 70.0 16.0 30.3 40.5 14.6 80.0 30.3 200.6 48.0 26.8 84.0 19.0 635.0 406.0 1316.0
similar proportion of sand, silt and clay fractions, with slight modi"cations in di!erent hydrological stages (Fig. 3). In general, bed sediments are dark in colour owing to the presence of abundant and partially decomposed organic matter, with the exception of the bottoms constituted by coarse sediments. The type of grain size distribution was unimodal in most of the considered courses (Fig. 4A), with the mode located in the very "ne sand fraction (Fig. 4B). Commonly, the histograms are composed of several columns (Fig. 4C) and the modal fractions are poorly marked. When a secondary mode is present, it often belongs to the very "ne silt fraction (Fig. 4D). The second class in abundance is mainly represented by the coarse silt fraction (Fig. 4E). The mean size of the bed sediments is primarily within the silt fraction (dominantly 15}8 lm), followed by the very "ne sand fraction (sites 17, 30) and, exceptionally, the medium sand fraction (Fig. 5). The phi 1 percentile
72
O. Orfeo / Quaternary International 62 (1999) 69}74
Fig. 3. Textural composition of the bed sediments. Fig. 5. Mean-size distribution of the bed sediments.
Fig. 6. Percentile 1 distribution of the bed sediments.
4.2. Suspended sediments
Fig. 4. Grain size distribution of the bed sediments. A: types, B: main mode distribution, C: numbers of fractions with frequency higher than 1% in weight, D: secondary mode distribution, E: second fraction in abundance.
is generally located in the medium sand fraction (250}500 lm) (Fig. 6). In some streams (sites 1, 16), this parameter showed a slight predominance of the "ner classes during the low water period, but the opposite was also observed (sites 25, 27). Only a few streams (sites 17, 28) transport poorly and moderately sorted sediments, respectively. All other rivers have very poorly sorted sediments, in both high and low water phases (Fig. 7).
The concentration of suspended sediments ranged from 3.2 to 1316 mg l~1 (Table 1), with the lower values in the northern basins of the studied area and the larger ones in the southern region. The hydrometric levels show an inverse correlation with the discharge of suspended sediments, with a clear seasonal trend. In this sense, the average concentrations of suspended sediments were 63.1 and 206.3 mg l~1 during high and low waters, respectively. In most of the rivers, the suspended solid concentrations during the low water period increased in di!ering proportions. Some #uvial courses (especially saline streams of the Chaco province) demonstrated the opposite tendency. 4.3. Flow regime and sediment transport It was possible to recognise the lognormal subpopulations in bedload and suspended load (cf. Visher, 1969).
O. Orfeo / Quaternary International 62 (1999) 69}74
73
Fig. 7. Sorting distribution of the bed sediments. Fig. 9. Transport and deposition of the bed sediments. References: 1: suspension in accelerating #ow, 2: suspension in decelerating #ow, 3: bedload in accelerating #ow, E: critical erosion velocity, S: critical sedimentation velocity.
characteristics of the transport agents. Suspension in accelerating #ow dominated in high water stages, whereas suspension in decelerating #ow was more prevalent in low water periods.
5. Discussion
Fig. 8. Generalized cumulative frequency distribution curves of the bed sediments.
The sediment transport was mainly carried out in suspension and by the saltational mode (Fig. 8). The bedload was sparse, as can be observed in Fig. 8. Although cumulative curves comparing the samples of the #ood and the low water phases were similar, the segment belonging to the saltational transport was better de"ned during low water periods (Fig. 8). Water velocity and grain size of sediments were correlated in Fig. 9, in order to evaluate the hydrodynamic
The rainy seasonality of the region can clearly be recognised in the variations of morphometry (width, depth) of the channels. Nevertheless, the relationship between the hydrological phase and the sediment characteristics was not clear because some parameters show variable behaviour. The texture of the bed sediments is relatively homogeneous along the whole hydrological cycle, with mean size mainly ranging between 8 and 63 lm and a sorting coe$cient belonging to the poor and very poor classes. However, the apparent degree of sorting of sediments with a large proportion of "ne particles is enhanced when the dispersion technique is not performed during grain size analysis (Flemming, 1977; Blasi, 1981), shifting the hydrodynamic interpretation. From this point of view, the sediment mixtures have probably been deposited under more #uid conditions. In the studied environments, the maximum size of the bed sediments is not larger than sand size (commonly medium to "ne), but the #ow velocity showed relatively high values, overcoming the critical erosion velocity. In these cases, the usefulness of the percentile one (" 1) of the grain size distribution to evaluate the competence of the streams is limited, because the transported materials re#ect an inherited property of the original sediments (Pettijohn et al., 1972).
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O. Orfeo / Quaternary International 62 (1999) 69}74
Suspended solids were more sensitive to changes in the #ow regime and seem to be a very important tool in environmental assessment, revealing incipient processes of natural or man-made erosion. In general, concentration values of suspended sediments were low compared with the large rivers of the region, and maintained a negative correlation with the hydrometric levels, probably owing to the decrease of the liquid discharge during the low water period. Notwithstanding this, highly saline streams with low current velocity during the low water period have a positive correlation with low water levels. This is interpreted as a consequence of an active ionic interchange among clay particles, since the salinity of a solution determines the coagulation degree in the natural sediments (Gibbs, 1983; Migniot, 1983; Ottmann, 1983), generating #occules of larger size that precipitate easily under gravity. In the studied environments, this situation was found in the range of #uvial electrical conductivity between 4300 and 11,000 lS cm~1. Near the investigated area, a very shallow saline phreatic level has been mentioned by several authors (Bielsa and Fratti, 1981; MorraH s, 1983; Nei!, 1983; Tenchini and Parera, 1985). This leads to important consequences for hydrological and ecological dynamics, and can explain the high salinity in several rivers of the region. This condition is frequent in large plains, where free aquifers are found at shallow depth with the upper level being the base of in"ltration, originating the capillary ascent of water (Tricart, 1983).
6. Conclusions The climatic seasonality of the region generates contrasting characteristics in the landscape. During #oods, some morphometric parameters of the courses exceeded by more than 70% the recorded values in low waters. Water velocity reaches high values, exceeding the critical velocity of erosion (1.27 m3 s~1). Comparing high and low water conditions, the super"cial bottom material of the studied environments did not have large textural variations, probably due to the lithologic and granulometric homogeneity of the loess headwaters. Sandy}silty beds were most frequently observed, followed by the silty}sandy and clayey}silty types, almost always poorly to moderately sorted, with high organic matter content. Suspended sediments have an inverse relationship with hydrometric levels, with the exception of the highly saline rivers, which have decreased solid transport in low waters. The mean concentration of suspended sediments was 63.1 mg l~1 for the period of high waters and 206.3 mg l~1 for the period of low waters. Such an anomalous character suggests that no erosion occurs in the loessic upper parts of the basins during the
rainy season. Hence, a mechanism of in"ltration and seepage can be proposed for the in"lling of channels.
References Bielsa, L., Fratti, R., 1981. DeterminacioH n del comportamiento del sistema natural y modi"cado con obras en temas referentes a calidad de agua. Convenio Bajos Submeridionales, Consejo Federal de Inversiones, Santa Fe, 43pp. Blasi, A., 1981. RelacioH n taman8 o de grano-seleccioH n en sedimentos actuales. Revista AsociacioH n Argentina de MineralogmH a, PetrologmH a y SedimentologmH a 12 (1}2), 1}10. Fertonani, M., Prendes, H., 1983. HidrologmH a en aH reas de llanura, aspectos conceptuales, teoH ricos y metodoloH gicos. Coloquio Internacional sobre HidrologmH a Grandes Llanuras, UNESCO, Actas, I: 119}155. Flemming, B., 1977. The use of relative sorting as a meaningful parameter in the interpretation of depositional processes. Joint Gso/Uct Marine Geology Programme, Technical Report No. 9, pp. 85}93. Folk, R., Ward, W., 1957. Brazos River bar: a study in the signi"cance of grain size parameters. Journal of Sedimentary Petrology 27 (1), 3}26. Friedman, G., Sanders, J., 1978. Principles of Sedimentology. Wiley, New York, 792pp. Galehouse, J., 1971. Sedimentation analysis. In: Carver, R. (Ed.), Procedures in Sedimentary Petrology. Wiley Interscience, New york, pp. 69}94. Gibbs, R., 1983. Coagulation rates of clay minerals and natural sediments. Journal of Sedimentary Petrology 53 (4), 1193}1203. Ingram, R., 1971. Sieve analysis. In: Carver, R. (Ed.), Procedures in Sedimentary Petrology. Wiley Interscience, New york, pp. 49}67. Iriondo, M., 1993. Geomorphology and late Quaternary of the Chaco (South America). Geomorphology 7, 289}303. McManus, J., 1988. Grain size determination and interpretation. In: Tucker, M. (Ed.), Techniques in Sedimentology. Blackwell Science, Oxford, pp. 63}85. Migniot, C., 1983. Les materies en suspension dans les estuaries. International Association of Engineering Geology Bulletin 28, 61}76. MorraH s, H., 1983. InterrelacioH n de las caractermH sticas pedoloH gicas y los procesos hidroloH gicos en los Bajos Submeridionales. Coloquio Internacional sobre HidrologmH a Grandes Llanuras, UNESCO, Actas, III: 1335}1348. Nei!, J., 1983. Consideraciones sobre la vegetacioH n y sus probables modi"caciones en el aH rea que ocuparaH n los futuros embalses de los Bajos Submeridionales. Convenio Bajos Submeridionales, Consejo Federal de Inversiones, Santa Fe, pp. 85}151. Nei!, J., 1986. Sinopsis ecoloH gica de ambientes acuaH ticos permanentes y temporarios del Chaco Oriental. Revista Ambiente Subtropical 1, 5}35. Ottmann, F., 1983. Etude et impact des materies en suspension sur l'amenagement. International Association of Engineering Geology Bulletin 28, 109}120. Patin8 o, C., Orfeo, O., 1986. AproximacioH n al conocimiento del proceso de erosioH n del suelo en el Chaco Oriental. Revista Ambiente Subtropical 1, 47}59. Pettijohn, F., Potter, P., Siever, R., 1972. Sand and Sandstone. Springer, Berlin, 618pp. Schmieder, O., 1980. GeografmH a de AmeH rica Latina. Fondo de Cultura EconoH mica, MeH xico, 654pp. Tenchini, A., Parera, J., 1985. ContribucioH n al conocimiento de la geomorfologmH a y los suelos de las lagunas La Loca y MartmH n GarcmH a (Santa Fe). V3 Panel de GeologmH a del Litoral, Corrientes, Res: 18. Tricart, J., 1983. Algunas re#exiones relacionadas con el mH tem 2.1: DescripcioH n de procesos. Coloquio Internacional sobre HidrologmH a Grandes Llanuras, UNESCO, Actas, I: 433}436. Visher, G., 1969. Grain size distributions and depositional proceses. Journal of Sedimentary Petrology 39 (3), 1074}1106.
Sedimentological characteristics of small rivers with loessic headwaters in the Chaco, South America Oscar Orfeo CECOAL-CONICET, Casilla de Correo 291, 3400 Corrientes, Argentina
Abstract Some sedimentological characteristics of #uvial environments located in the northeastern region of the Chaco}Pampa plain were analysed in order to determine the possible in#uence of the loessic headwaters of local basins. Concentrations of suspended sediments, as well as grain size of the river beds, were compared. Unexpectedly, it was found that the suspended load has an inverse relationship with the hydrometric levels, with the exception of highly saline streams (conductivity values between 4300 and 11,000 lS cm~1). The mean concentration of suspended sediments was 63.1 and 206.3 mg l~1 in #ood and low water periods, respectively. The grain size of bed sediments was relatively homogeneous along the whole hydrological cycle. Sandy}silty bottoms predominate in channels, followed by silty}sandy and clayey}silty bottom types. Sediment transport mainly occurs in accelerating #ow suspension, overcoming the critical erosion velocity. During #ood periods, some parameters exceeded by 70% the values recorded during the low water phase. ( 2000 Elsevier Science Ltd and INQUA. All rights reserved.
1. Introduction The Eastern Chaco, located in the northeastern region of the Chaco}Pampa plain, is formed by the distal areas of the alluvial fans of large rivers (Iriondo, 1993) The study region includes the distal areas of the Pilcomayo and Bermejo Rivers (Fig. 1). The #oods of these allochthonous rivers discharge large volumes of water into a plain landscape crossed by old #uvial belts, generating basically swamp environments. A series of local #uvial networks were developed in the Central and Eastern Chaco, all having similar characteristics. Collectors are small sinuous streams #owing in abandoned channels of large allochthonous rivers. Their upper basins (a few hundreds of square kilometers in each case) are covered by a loess mantle. Such areas are dominated by in"ltration, with little runo! and small slopes, resulting in a very small contribution of sediment to the channels. The middle and lower parts of the basins are developed on hard, impervious clay, paludal in origin. The region is a sunken block, covered by large permanent swamps and dense vegetation. Fluvial geomorphological elements are not clearly organized in hierarchical terms. Fertonani and Prendes (1983) de"ne this type of area as a `NonTypical Hydrological Systema. Finally, the local rivers #ow into the regional collectors, the ParanaH and Paraguay Rivers.
The pattern and geographical location of these small rivers create particular characteristic sediment nature and dynamics, including silty beds and an inverse correlation between water discharge and concentration of suspended solids. These particular systems appeared in the Chaco plain during the middle Holocene (Iriondo, pers. comm.) The landscape is a!ected by alternating seasons of drought and periods of prolonged #ooding, which has resulted in the progressive coalescence of the water bodies and the saturation of the storage capacity of the basins. Climate, topography, structural conditions, and subsurface lithology de"ne the local drainage system (Patin8 o and Orfeo, 1986) and shape the main landscape characteristics (Nei!, 1986).
2. Location of the studied basins The studied area comprises more than 30 local basins along 400 km of the Paraguay}ParanaH collector (253 00@}283 30@ S; 57330@}59330@ W) (Fig. 2). The area covers the distal parts of the Pilcomayo and Bermejo alluvial fans and a sector of the Paraguay}ParanaH belt (Iriondo, 1993). Considering the geomorphological setting of the region, the results obtained in the present investigation can be extrapolated to the entire Eastern Chaco, from 20 to 303 S.
1040-6182/00/$20.00 ( 2000 Elsevier Science Ltd and INQUA. All rights reserved. PII: S 1 0 4 0 - 6 1 8 2 ( 9 9 ) 0 0 0 2 4 - 5
70
O. Orfeo / Quaternary International 62 (1999) 69}74
Fig. 2. Fluvial environments of the study area (numbers correspond to Table 1).
The climate is subtropical, with 800}1200 mm annual rainfall. The rainy season begins in October and ends in March; winters are dry. Mean temperatures vary between 24 and 303C in summer (Schmieder, 1980).
ment dynamics in the #uvial systems. Width, depth and water velocity were measured for characterization of channel size and energy. Bed sediments were sampled near the #uvial collectors Paraguay and ParanaH , using a core-sampler on the deepest zone of each site. In order to determine the grain size, the sieving and pipette techniques were used (Galehouse, 1971; Ingram, 1971; McManus, 1988), employing the Udden-Wentworth grain size scale with the clast terminology modi"cations proposed by Friedman and Sanders (1978). The graphic method of Folk and Ward (1957) was employed in the calculation of some textural parameters. The Visher criteria (1969) were useful to evaluate di!erent mechanisms of bed-sediment transport. A punctual suspended-sediment sampler was used in several vertical pro"les on cross sections located in the same sites. Sediment concentrations were calculated by "ltering using membrane disks (size of pores"0.45 lm).
3. Methods
4. Results
Grain size analysis of bed sediments and concentration of suspended solids were chosen as measures of the sedi-
In high waters, the width and depth of most streams ranged from 20 to 60 m and from 1.2 to 5 m, respectively.
Fig. 1. Some geomorphological units of the Eastern Chaco (adapted from Iriondo, 1993). References: 1: Pilcomayo alluvial fan, 2: Bermejo alluvial fan, 3: #oodplain of the Paraguay}ParanaH river system.
O. Orfeo / Quaternary International 62 (1999) 69}74
71
Table 1 Some physical parameters of the studied environments (see Fig. 2). =: width (m), D: depth (m), <: water velocity (m s~1), C: suspended solid concentration (mg l~1) No.
Sample station
Hydrological stage High waters
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
Pilcomayo river Negro stream El Lobo swamp Morocho swamp He-He Grande stream Malvinas stream Monte Lindo stream TimboH PoraH stream PilagaH stream PatmH swamp San Hilario stream Tohue stream Salado (Formosa) stream Saladillo (Formosa) stream MbiguaH stream Cangui Chico stream Oro river QuiaH stream Del Tres stream Cuatro Diablos swamp GuaycuruH river Tragadero river Salado (Chaco) River Saladito stream Palometa river Hornero swamp Saladillo (Chaco) stream TapengaH river El RaboH n stream Amores river
Low waters
=
D
<
70 30
7.8 2.5 1.2 1.5 2.5
0.20
20 50 60 50 20 6 23 6 8 35 11
18 40 52 22 36
4.0 5.0 1.5 5.5 1.2 2.3
1.27 0.83
0.40
0.5 4.0
27 30
3.0 4.5 3.0 2.0 3.5 0.5 3.0 4.5
37
5.0
C
=
D
20.5 4.5 30.0 8.0 26.5 4.7 3.25
46 22
6 1.75
31.5 96.0 43.0 37.0 22.0 47.0 5.0 11.0 21.0
8
5 7 23
<
1.0 0.5 2.2 0.2 0.8 0.6 1.7
14 6 20 1 10 5 24
2.3
0.8
0.2 1.0
Null 0.32 0.90
0.40 0.70 0.20 0.60
0.73 0.27 0.41 0.63
35.6 30.5 80.0 258.0 103.0
8 14 40 17 21
2.8 1.2 0.9 0.7 0.7
0.30 0.46 Null Null Null
0.38 0.88
36.5 90.0 470.0
1.2 0.8 3.1 3.0
Null 0.56
0.62
14 12 24 20
During low water periods, the width range was 5}24 m and the depth range was 0.2}3 m (Table 1). In a few courses (sites 5, 9, 17), the width and depth values during the #oods exceeded the low water values by 70%. In other cases (sites 22, 23, 25, 28), the percentage of depth variability was similar, but the width diminished between 23 and 65% under low water conditions. The current velocity in high waters shifted mainly from 0.4 to 0.9 m s~1 (with the exception of the site 6, where it reached 1.27 m s~1). During the low water period, most streams showed current velocity values ranging from 0.2 to 0.6 m s~1. Nevertheless, some rivers (sites 23, 27) remained without water movement in this phase. 4.1. Bed sediments A few streams (sites 17, 28) have sandy bottoms while others (sites 11, 13, 18) have mainly pelitic bottoms. However, the majority of the streams showed a nearly
0.30
C 44.5 13.0 77.0 73.3 28.0 133.0 339.0 248.0 301.0 1110.0 235.0 86.0 64.0 70.0 16.0 30.3 40.5 14.6 80.0 30.3 200.6 48.0 26.8 84.0 19.0 635.0 406.0 1316.0
similar proportion of sand, silt and clay fractions, with slight modi"cations in di!erent hydrological stages (Fig. 3). In general, bed sediments are dark in colour owing to the presence of abundant and partially decomposed organic matter, with the exception of the bottoms constituted by coarse sediments. The type of grain size distribution was unimodal in most of the considered courses (Fig. 4A), with the mode located in the very "ne sand fraction (Fig. 4B). Commonly, the histograms are composed of several columns (Fig. 4C) and the modal fractions are poorly marked. When a secondary mode is present, it often belongs to the very "ne silt fraction (Fig. 4D). The second class in abundance is mainly represented by the coarse silt fraction (Fig. 4E). The mean size of the bed sediments is primarily within the silt fraction (dominantly 15}8 lm), followed by the very "ne sand fraction (sites 17, 30) and, exceptionally, the medium sand fraction (Fig. 5). The phi 1 percentile
72
O. Orfeo / Quaternary International 62 (1999) 69}74
Fig. 3. Textural composition of the bed sediments. Fig. 5. Mean-size distribution of the bed sediments.
Fig. 6. Percentile 1 distribution of the bed sediments.
4.2. Suspended sediments
Fig. 4. Grain size distribution of the bed sediments. A: types, B: main mode distribution, C: numbers of fractions with frequency higher than 1% in weight, D: secondary mode distribution, E: second fraction in abundance.
is generally located in the medium sand fraction (250}500 lm) (Fig. 6). In some streams (sites 1, 16), this parameter showed a slight predominance of the "ner classes during the low water period, but the opposite was also observed (sites 25, 27). Only a few streams (sites 17, 28) transport poorly and moderately sorted sediments, respectively. All other rivers have very poorly sorted sediments, in both high and low water phases (Fig. 7).
The concentration of suspended sediments ranged from 3.2 to 1316 mg l~1 (Table 1), with the lower values in the northern basins of the studied area and the larger ones in the southern region. The hydrometric levels show an inverse correlation with the discharge of suspended sediments, with a clear seasonal trend. In this sense, the average concentrations of suspended sediments were 63.1 and 206.3 mg l~1 during high and low waters, respectively. In most of the rivers, the suspended solid concentrations during the low water period increased in di!ering proportions. Some #uvial courses (especially saline streams of the Chaco province) demonstrated the opposite tendency. 4.3. Flow regime and sediment transport It was possible to recognise the lognormal subpopulations in bedload and suspended load (cf. Visher, 1969).
O. Orfeo / Quaternary International 62 (1999) 69}74
73
Fig. 7. Sorting distribution of the bed sediments. Fig. 9. Transport and deposition of the bed sediments. References: 1: suspension in accelerating #ow, 2: suspension in decelerating #ow, 3: bedload in accelerating #ow, E: critical erosion velocity, S: critical sedimentation velocity.
characteristics of the transport agents. Suspension in accelerating #ow dominated in high water stages, whereas suspension in decelerating #ow was more prevalent in low water periods.
5. Discussion
Fig. 8. Generalized cumulative frequency distribution curves of the bed sediments.
The sediment transport was mainly carried out in suspension and by the saltational mode (Fig. 8). The bedload was sparse, as can be observed in Fig. 8. Although cumulative curves comparing the samples of the #ood and the low water phases were similar, the segment belonging to the saltational transport was better de"ned during low water periods (Fig. 8). Water velocity and grain size of sediments were correlated in Fig. 9, in order to evaluate the hydrodynamic
The rainy seasonality of the region can clearly be recognised in the variations of morphometry (width, depth) of the channels. Nevertheless, the relationship between the hydrological phase and the sediment characteristics was not clear because some parameters show variable behaviour. The texture of the bed sediments is relatively homogeneous along the whole hydrological cycle, with mean size mainly ranging between 8 and 63 lm and a sorting coe$cient belonging to the poor and very poor classes. However, the apparent degree of sorting of sediments with a large proportion of "ne particles is enhanced when the dispersion technique is not performed during grain size analysis (Flemming, 1977; Blasi, 1981), shifting the hydrodynamic interpretation. From this point of view, the sediment mixtures have probably been deposited under more #uid conditions. In the studied environments, the maximum size of the bed sediments is not larger than sand size (commonly medium to "ne), but the #ow velocity showed relatively high values, overcoming the critical erosion velocity. In these cases, the usefulness of the percentile one (" 1) of the grain size distribution to evaluate the competence of the streams is limited, because the transported materials re#ect an inherited property of the original sediments (Pettijohn et al., 1972).
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Suspended solids were more sensitive to changes in the #ow regime and seem to be a very important tool in environmental assessment, revealing incipient processes of natural or man-made erosion. In general, concentration values of suspended sediments were low compared with the large rivers of the region, and maintained a negative correlation with the hydrometric levels, probably owing to the decrease of the liquid discharge during the low water period. Notwithstanding this, highly saline streams with low current velocity during the low water period have a positive correlation with low water levels. This is interpreted as a consequence of an active ionic interchange among clay particles, since the salinity of a solution determines the coagulation degree in the natural sediments (Gibbs, 1983; Migniot, 1983; Ottmann, 1983), generating #occules of larger size that precipitate easily under gravity. In the studied environments, this situation was found in the range of #uvial electrical conductivity between 4300 and 11,000 lS cm~1. Near the investigated area, a very shallow saline phreatic level has been mentioned by several authors (Bielsa and Fratti, 1981; MorraH s, 1983; Nei!, 1983; Tenchini and Parera, 1985). This leads to important consequences for hydrological and ecological dynamics, and can explain the high salinity in several rivers of the region. This condition is frequent in large plains, where free aquifers are found at shallow depth with the upper level being the base of in"ltration, originating the capillary ascent of water (Tricart, 1983).
6. Conclusions The climatic seasonality of the region generates contrasting characteristics in the landscape. During #oods, some morphometric parameters of the courses exceeded by more than 70% the recorded values in low waters. Water velocity reaches high values, exceeding the critical velocity of erosion (1.27 m3 s~1). Comparing high and low water conditions, the super"cial bottom material of the studied environments did not have large textural variations, probably due to the lithologic and granulometric homogeneity of the loess headwaters. Sandy}silty beds were most frequently observed, followed by the silty}sandy and clayey}silty types, almost always poorly to moderately sorted, with high organic matter content. Suspended sediments have an inverse relationship with hydrometric levels, with the exception of the highly saline rivers, which have decreased solid transport in low waters. The mean concentration of suspended sediments was 63.1 mg l~1 for the period of high waters and 206.3 mg l~1 for the period of low waters. Such an anomalous character suggests that no erosion occurs in the loessic upper parts of the basins during the
rainy season. Hence, a mechanism of in"ltration and seepage can be proposed for the in"lling of channels.
References Bielsa, L., Fratti, R., 1981. DeterminacioH n del comportamiento del sistema natural y modi"cado con obras en temas referentes a calidad de agua. Convenio Bajos Submeridionales, Consejo Federal de Inversiones, Santa Fe, 43pp. Blasi, A., 1981. RelacioH n taman8 o de grano-seleccioH n en sedimentos actuales. Revista AsociacioH n Argentina de MineralogmH a, PetrologmH a y SedimentologmH a 12 (1}2), 1}10. Fertonani, M., Prendes, H., 1983. HidrologmH a en aH reas de llanura, aspectos conceptuales, teoH ricos y metodoloH gicos. Coloquio Internacional sobre HidrologmH a Grandes Llanuras, UNESCO, Actas, I: 119}155. Flemming, B., 1977. The use of relative sorting as a meaningful parameter in the interpretation of depositional processes. Joint Gso/Uct Marine Geology Programme, Technical Report No. 9, pp. 85}93. Folk, R., Ward, W., 1957. Brazos River bar: a study in the signi"cance of grain size parameters. Journal of Sedimentary Petrology 27 (1), 3}26. Friedman, G., Sanders, J., 1978. Principles of Sedimentology. Wiley, New York, 792pp. Galehouse, J., 1971. Sedimentation analysis. In: Carver, R. (Ed.), Procedures in Sedimentary Petrology. Wiley Interscience, New york, pp. 69}94. Gibbs, R., 1983. Coagulation rates of clay minerals and natural sediments. Journal of Sedimentary Petrology 53 (4), 1193}1203. Ingram, R., 1971. Sieve analysis. In: Carver, R. (Ed.), Procedures in Sedimentary Petrology. Wiley Interscience, New york, pp. 49}67. Iriondo, M., 1993. Geomorphology and late Quaternary of the Chaco (South America). Geomorphology 7, 289}303. McManus, J., 1988. Grain size determination and interpretation. In: Tucker, M. (Ed.), Techniques in Sedimentology. Blackwell Science, Oxford, pp. 63}85. Migniot, C., 1983. Les materies en suspension dans les estuaries. International Association of Engineering Geology Bulletin 28, 61}76. MorraH s, H., 1983. InterrelacioH n de las caractermH sticas pedoloH gicas y los procesos hidroloH gicos en los Bajos Submeridionales. Coloquio Internacional sobre HidrologmH a Grandes Llanuras, UNESCO, Actas, III: 1335}1348. Nei!, J., 1983. Consideraciones sobre la vegetacioH n y sus probables modi"caciones en el aH rea que ocuparaH n los futuros embalses de los Bajos Submeridionales. Convenio Bajos Submeridionales, Consejo Federal de Inversiones, Santa Fe, pp. 85}151. Nei!, J., 1986. Sinopsis ecoloH gica de ambientes acuaH ticos permanentes y temporarios del Chaco Oriental. Revista Ambiente Subtropical 1, 5}35. Ottmann, F., 1983. Etude et impact des materies en suspension sur l'amenagement. International Association of Engineering Geology Bulletin 28, 109}120. Patin8 o, C., Orfeo, O., 1986. AproximacioH n al conocimiento del proceso de erosioH n del suelo en el Chaco Oriental. Revista Ambiente Subtropical 1, 47}59. Pettijohn, F., Potter, P., Siever, R., 1972. Sand and Sandstone. Springer, Berlin, 618pp. Schmieder, O., 1980. GeografmH a de AmeH rica Latina. Fondo de Cultura EconoH mica, MeH xico, 654pp. Tenchini, A., Parera, J., 1985. ContribucioH n al conocimiento de la geomorfologmH a y los suelos de las lagunas La Loca y MartmH n GarcmH a (Santa Fe). V3 Panel de GeologmH a del Litoral, Corrientes, Res: 18. Tricart, J., 1983. Algunas re#exiones relacionadas con el mH tem 2.1: DescripcioH n de procesos. Coloquio Internacional sobre HidrologmH a Grandes Llanuras, UNESCO, Actas, I: 433}436. Visher, G., 1969. Grain size distributions and depositional proceses. Journal of Sedimentary Petrology 39 (3), 1074}1106.