Physical and chemical characteristics of Lavapés and Capivara rivers, tributaries of Barra Bonita Reservoir (São Paulo – Brazil)

MORETTO¹, E.M. & NOGUEIRA¹, M.G.

¹ Department of Zoology, Institute of Biosciences, UNESP, Rubião Junior, 18618-000, Botucatu, São Paulo, Brazil.

ABSTRACT: A limnological study was carried out during winter (dry period) and summer (rainy period) in the lower stretches of the rivers Capivara and Lavapés, as well in the transitional zone of these rivers mouth into Barra Bonita reservoir (river Tietê, SP). Water quality differences between the rivers and rivers-reservoir system were efficiently detected by variations in the values of conductivity, dissolved oxygen, dissolved and total phosphorus, total nitrogen, nitrite, ammonium and suspended solids. The limnological conditions in the Lavapés are strongly conditioned by the untreated sewage loads received from the city of Botucatu in the river upper stretches. The significative altitudinal difference between the upstream and downstream regions certainly increases the natural capacity of the river self-purification. However, the water observed in the lower stretch is still very rich in nutrient making this river an important contributing factor for the eutrophication of Barra Bonita reservoir. Despite of the siltation, the summer rains promote a dilution on the nutrient concentration. The conservation of native vegetation is critical for both basins, but the problem is even worst for Lavapés. In case of Capivara other important factor of degradation is the drainage of original wetland areas (“várzeas”) for agricultural purposes. Thus, during the rainy season there is a remarkable increase of solids concentrations in both rivers. As a consequence high loads are introduced into the reservoir. In this process the contribution of Capivara is higher as its volume increases more in summer. This is because the Capivara watershed is larger and also receives a higher number of small tributary streams. The precipitation cycle has not the same direct effects in the reservoir main channel. At least in terms of hydrodynamics, as the reservoir depth showed higher dependence on the dam operation. The limnological conditions in the region of the reservoir located in front of the rivers mouths were different from the ones found in the rivers themselves. Therefore, the rivers influence is limited, probably due to the small contribution in terms of volume. Nevertheless, on a long-term scale it can not be neglected their cumulative impact on Barra Bonita reservoir, which is already an eutrophic system.

Key-words: reservoir tributaries, physical and chemical variables, spatial and temporal variation.