Alterations in the Seco river bed and losses of agricultural land, province of Tucuman, Argentina

Abstract

The province of Tucuman, Argentina has a dense fluvial network, with rivers that have shown, over the last 45 years, complex and dynamic behaviors, as a result of high streamflows. In the upper basins, located in the mountainous zone and piedmont, diverse anthropic activities developed in contrasted reliefs and erosive slopes. The increase in rainfall recorded in the region since 1970 has increased both the erosive and depositional processes in the alluvial plain by modifying the design of the channels, with important consequences on the farmlands. In this paper, the Seco river bed is analyzed thoroughly. The Seco river is born on the eastern slope of the Aconquija mountains, runs through the piedmont and the alluvial plain in a west-east direction, and finally discharges into the Sali river, the main river bed of the fluvial network of the province. The main objective of this work is an analysis of meander morphology along 6.28 km of the Seco river for the period 1972 to 2017. The main parameters analyzed were the sinuosity of the channel, and meander cuts by overflow (chute cutoff) and by tangency (neck cutoff). These processes have accelerated the loss of agricultural land by lateral erosion on the river ravines, which are composed of fluvial-alluvial sediments, the parental materials of the productive soils of alluvial plains. To begin our study, the general characteristics of the Seco river basin have been considered, including geology, geomorphology, climate, soils, and anthropic activities. In addition, specific features of the sub-basin that contributes its waters to the meander section were also taken into account: the increase of the flows due to the change toward the use of surrounding soils, the slopes of the field, the geomorphological region and morphometry. Landsat and Sentinel satellite images, as well as the SRTM Digital Elevation Model (DEM), were used for mapping the channel and crops. The resulting maps are basin and sub-basin watershed contribution to the meanders, slopes, elevation curves, and drainage network. The results show that the sinuosity of the channel increased in the period 1972-2002, had a notable decrease in the period 2002-2011 due to the processes of meander cuts, and increased again in the period 2011-2017, together with the migration of the channel towards the southern quadrant. These changes resulted in the loss of 370.68 ha of productive soils, which represents 7.41 hm3 of sediments removed and transported by the current. The meanders of the Seco river reflect the degradation of its area of contribution, representing the situation of the basins of the Argentine Northwest, in which the increase in the intensity and frequency of the precipitations since 1970 have generated critical conditions, whose effects are more evident in low areas.