This study aims to evaluate the mechanical erosion processes that occur in a tropical river basin, located in the São Paulo state, southeastern Brazil, through the chemical characterization of fine suspended sediments and the transport mechanisms near the river mouth, from March 2009 to September 2010. The chemical characterization indicated the predominance of SiO2, Al2O3, and Fe2O3 and showed no significant seasonal influences on the major element concentrations, expressed as oxides. The concentration variations observed were related to the mobility of chemical species. The evaluation of the rock-alteration degree indicated that the physical weathering was intense in the drainage basin. The fine suspended sediments charge was influenced by the variation discharges throughout the study period. The solid charge estimate of the surface runoff discharge was four times higher in the rainy season than the dry season. The transport of fine suspended sediments at the Sorocaba River mouth was 55.70 t km−2 a−1, corresponding to a specific physical degradation of 37.88 m Ma−1, a value associated with the mechanical erosion rate that corresponds to the soil thickness reduction in the drainage basin.
The mechanical erosion processes that occur in a drainage basin tend to reduce the thickness of the soils and the volume of the bedrock, by which dislocated and degraded particles are exported by the rivers to the sedimentation areas (lakes and oceans). The sediment production, transport, and deposition at the drainage basin scale are principally controlled by natural processes that may be intensified due to human activities, mainly agriculture and urbanization [
Soil erosion and its impacts on agricultural productivity, water quality, and siltation in rivers and reservoirs have been frequently discussed with regard to sustainable management of the services that these ecosystems deliver to the population [
This study aims at evaluating the main aspects of the mechanical erosion processes that occur in the Sorocaba River basin, located in the São Paulo state, southeastern Brazil, through the chemical characterization of fine suspended sediments and mechanisms of river transport close to its mouth, in the Laranjal Paulista city, from March 2009 to September 2010.
The knowledge of mechanical erosion processes that occur in the Sorocaba River basin is needed because the 50 km downstream of the confluence with the Tietê River is located the Barra Bonita Hydroelectric Power Plant Reservoir, and the change in the drainage system river system provided by the dam, from lotic to lentic, favors the processes of sediment deposition, which in turn affects the water storage capacity and the lifetime of the reservoir.
The Sorocaba River, 227 km long, is the most important left bank tributary of the Middle Tietê River basin (Figure
Drainage basin of the Sorocaba River with the location of the sampling station, near to the mouth, of the limnimetric station (4E-001—Entre Rios) and the pluviometric station (E4-019—Iperó). The main soils type, urban areas, and the division between Atlantic Plateau and Peripheral Depression (dotted line) are shown.
The Sorocaba River basin is divided in two geomorphological units. The upstream part corresponds to the
The drainage system of the Sorocaba River basin is dendritic pattern, and drainage density has a potential fragility from medium to high, an important morphometric parameter of analysis in hydrographic basins related to erosion susceptibility [
According to Koppen classification [
Average monthly discharge (
The fine suspended sediments (FSS) transported by rivers are usually characterized as particles smaller than 63
The average FSS concentration normalized by the discharge (
Additionally, river water composite samples were collected from the left and right margins of the river and in the main axis of the current river. 30 L of water samples are stored in polypropylene containers to obtain sufficient FSS mass for chemical characterization. After the settlement of the FSS, the recovered sediments were air-dried and sieved using a 63
The chemical characterization of the FSS showed a predominance of SiO2 (48.60%), Al2O3 (21.05%), and Fe2O3 (8.23%) for the analyzed elements (Table
Major element concentrations (expressed as oxide %) for the FSS sampled at the Sorocaba River basin mouth during the study period.
Sampling date | Discharge | Major element concentrations (expressed as oxide %) | |||||||
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(m3 s−1) | SiO2 | Al2O3 | Fe2O3 | MnO | CaO | MgO | Na2O | K2O | |
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Aug-25-09 | 123.23 | 47.80 | 21.43 | 6.83 | 0.23 | 0.56 | 0.81 | 0.32 | 2.22 |
Sep-22-09 | 99.84 | 43.20 | 20.26 | 6.99 | 0.31 | 0.57 | 0.78 | 0.31 | 2.07 |
Nov-10-09 | 170.01 | 49.17 | 18.74 | 5.97 | 0.16 | 0.51 | 0.88 | 0.33 | 2.03 |
Dec-12-09 | 363.35 | 48.95 | 19.65 | 6.33 | 0.22 | 0.52 | 0.82 | 0.34 | 2.14 |
Jan-15-10 | 279.15 | 51.65 | 21.35 | 7.71 | 0.34 | 0.68 | 1.04 | 0.37 | 2.46 |
Jan-20-10 | 366.47 | 49.71 | 21.72 | 7.03 | 0.21 | 0.54 | 0.97 | 0.28 | 2.17 |
Feb-23-10 | 184.04 | 53.23 | 20.01 | 7.60 | 0.50 | 0.64 | 0.85 | 0.37 | 2.45 |
Mar-30-10 | 151.30 | 56.55 | 21.61 | 6.65 | 0.20 | 0.56 | 0.86 | 0.33 | 2.38 |
Apr-27-10 | 107.64 | 50.48 | 22.78 | 8.37 | 0.48 | 0.67 | 0.87 | 0.34 | 2.59 |
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Average (%) | 205.00 | 50.08 | 20.84 | 7.05 | 0.29 | 0.58 | 0.88 | 0.33 | 2.28 |
SD | 105.13 | 3.69 | 1.25 | 0.74 | 0.12 | 0.06 | 0.08 | 0.03 | 0.20 |
CV (%) | 85.9 | 9.5 | 9.3 | 20.6 | 41.9 | 22.3 | 8.1 | 12.3 | 9.7 |
The average and the standard deviations (SD) are shown for two periods, low water and high water, and the coefficient of variation (CV) for the study period (all samples). The bold font represents the low water period and the light face font represents the high water period.
The coefficients of variation showed a greater dispersion for Mn (41.9%), Ca (22.3%), and Fe (20.6%) and around 10% for other elements. These results reflect an apparent seasonal influence, which may be linked to the different origins of the FSS [
In the Sorocaba River basin, this mobility is estimated considering the ratio between the FSS chemical composition (
The results in Table
Mobility estimation of the elements during the rock-alteration process in the Sorocaba River basin.
Element | Crust | Sorocaba River Basin | |
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% loss (−) or gain (+) | |
Al | 6.93 | 11.14 | 0 |
Fe | 3.59 | 6.40 | 11 |
Mn | 0.07 | 0.31 | 177 |
Na | 1.42 | 0.27 |
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Ca | 4.50 | 0.49 |
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Mg | 1.64 | 0.52 |
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K | 2.44 | 1.88 |
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Si | 27.50 | 22.71 |
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During the rock-alteration process in the drainage basin, the evolution of primary to secondary minerals is associated with a relative enrichment in Al and Fe and depletion in Si. The increase in Al2O3 and Fe2O3 concentrations to the detriment of SiO2 provides an indication of the rock-alteration degree, expressed by the
The
This rock-alteration process was also assessed by the chemical maturity index (ChM), established by Konta [
The ChM index, calculated for the Sorocaba River basin, was significant (11.1) and similar to the one reported by Mortatti and Probst [
The quick surface runoff is directly associated with mechanical erosion processes that occur in a drainage basin. It is responsible for the transport of particulate matter into the river channel throughout the drainage basin. The particulate matter load that reaches the river channel by quick surface runoff is diluted by river waters from the subsurface and hypodermic and underground compartments, whose particulate matter load can be considered negligible [
Estimation of the particulate matter load of quick surface runoff (
Sampling date |
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Mar-10-09 | 51.50 | 16.33 | 43.11 |
Apr-07-09 | 59.30 | 20.67 | 54.24 |
May-12-09 | 46.83 | 25.33 | 67.16 |
Jun-17-09 | 37.47 | 14.33 | 38.46 |
Jul-21-09 | 43.71 | 46.50 | 123.69 |
Aug-25-09 | 123.23 | 54.33 | 139.90 |
Sep-22-09 | 99.84 | 45.33 | 117.21 |
Nov-10-09 | 170.01 | 145.00 | 371.56 |
Dec-12-09 | 363.35 | 87.17 | 221.86 |
Jan-15-10 | 279.15 | 47.67 | 121.54 |
Jan-20-10 | 366.47 | 101.50 | 258.33 |
Feb-23-10 | 184.04 | 37.83 | 96.85 |
Mar-30-10 | 151.30 | 178.00 | 456.84 |
Apr-27-10 | 107.64 | 51.00 | 131.66 |
May-27-10 | 70.21 | 22.67 | 59.25 |
Jun-24-10 | 42.15 | 14.50 | 38.64 |
Jul-27-10 | 54.62 | 28.83 | 75.92 |
Sep-01-10 | 26.56 | 17.67 | 48.60 |
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Average (%) | 126.52 | 73.55 | 188.41 |
SD | 108.62 | 44.96 | 114.67 |
During dry periods, both preceding the rise of the water level (March–July 2009) and after the recession (May–September 2010), the flow-weighted average concentrations calculated for
The FSS concentrations seem to be influenced by the variation of the Sorocaba River discharge during the study as illustrated in Figure
Temporal variability of the FSS concentration (mg L−1) and respective discharges (
The relationship between the suspended sediment concentration (
Logarithmic correlation between the FSS concentration (mg L−1) and the respective discharges (
The
Although the annual hydrological regime, with the seasonal distribution of precipitation and discharge, as well as the land use (predominantly agricultural) of the Sorocaba River is similar to Tietê and Piracicaba Rivers [
The sediment load carried by the river is evaluated in terms of the FSS, assuming that suspended load represents approximately 90% of the total sediment river flow [
The total transport of FSS calculated for the Sorocaba River mouth was 293 × 103 t a−1, which corresponds to a specific transport of 55.70 t km−2 a−1. This value is in accordance with that reported for the Tietê and the Piracicaba Rivers, 59.60 and 55.50 t km−2 a−1, respectively [
The physical degradation degree of a drainage basin, understood as the mechanical erosion rate in meters per million years, can be evaluated as a function of specific transport of FSS (
The mean density of the Sorocaba River basin soils was determined by the classical method reported by Blake and Hartge [
The mechanical erosion rate calculated for the Sorocaba River basin is 37.99 m Ma−1. Compared to other drainage basins with similar areas and land use predominantly agricultural, the physical degradation degree of the Sorocaba River basin was similar to that observed by Bortoletto Junior [
A focus of this study is to derive a better understanding of the mechanical erosion processes that occur in a tropical river basin of southeastern Brazil, where the predominant land use is agricultural, with increased urbanization in the past decade. The FSS load is influenced by the variation in discharge river during the study period, with the same seasonal effect. This situation is emphasized by suspended matter load associated with the quick surface runoff, which was four times higher in the high water period than in dry periods.
However, the FSS chemical characterization for major oxides does not indicate that seasonal influence and the variability of concentrations observed during the study period were related to the mobility of these elements during rock weathering process. The mechanical erosion in the Sorocaba River basin was 55.70 t km−2 a−1, representing a physical degradation rate of 37.88 m Ma−1, in the same magnitude of the other two major rivers that comprise the Middle Tiete basin.
The authors thank the Fundação de Apoio à Pesquisa do Estado de São Paulo (Process 08/57104-4 and 08/09369-9) and the Conselho Nacional de Desenvolvimento Científico e Tecnológico (Process 134169/2009-3) for the financial support granted, as well as the Centro Tecnológico de Hidráulica e Recursos Hídricos of the Departamento de Águas e Energia Elétrica, for providing the time series of daily discharge and rainfall data for the Sorocaba River basin.