EL CONCEPTO DE ZONA TOPOGRÁFICA FLUVIAL

The objectives of the present study were: (i) to propose a new concept of a zone where the relationship between the river and hillslopes is more intense, which is called Fluvial Topographic Zone (FTZ), as well as to describe the method to delineate this zone on maps; and (ii) to compare the Permanent Protection Area (PPA) with the FTZ along the main river (28.8 km) of Bugres river basin (78.2 km 2 ) which is located in Rio Negrinho city, Santa Catarina state, Brazil. The indices TI (Topographic Index), STI (Soil Topographic Index), and sinuosity were selected to verify the applicability of FTZ, because TI and STI are mainly influenced by the hillslopes, meanwhile sinuosity is related to the river. A significant correlation may indicate that such zone comprises a more intense interaction between the river and hillslope. The boundary of FTZ is the nearest contour line to the river channel. This procedure was evaluated for the studied main river by using a map (1:50,000). TI, STI, and the sinuosity of the main river and of the contributing areas were estimated. Subsequently, the correlations of the sinuosity with these two indices were evaluated for the whole basin and considering only the FTZ. It was showed that the sinuosity is more strongly influenced by the topography than by the soil properties and that this influence becomes more significant in FTZ. It implies the importance of FTZ as an area for protection as well as for scientific researches. Based on the Brazilian law, the PPA was delimited along the main river and its area was compared with FTZ. The FTZ and PPA comprise 10% and 3% of the total basin area, respectively. This comparison shows that only in the upper part of the main river the FTZ coincides with the PPA very well and that it gradually increases towards downstream. In the lower part of the basin, the width of FTZ reaches 300 m, while PPA keeps the 30-m width. The obtained results permit to conclude that FTZ comprises river meandering dynamics, which is of extreme importance for the preservation of various species of both flora and fauna of the riparian zone. Furthermore, the variable width of FTZ depending on each river can be considered positive, because it is adaptable to rivers' peculiarities.