Evaluación e interacción de las influencias naturales y antrópicas sobre la cuenca del río Negro, Argentina.

Abstract

The Patagonian region is an area of great extension and importance for the population and economic development of the country, whose productivity is closely linked to the source of water resources. It has numerous river basins of snowy source in the Argentinean Andes, which provide ecosystem services to the region, making their conservation and good management necessary. One of these large basins is the Negro river, which has its headwaters at the junction of two other basins, the Limay and the Neuquén rivers, which together make up the Negro River Fluvial System (SFRN). Despite its importance, represented by a drainage area of more than 26,000 km2, very little is known about the morphometric and geochemical characteristics of the Negro river basin and the three segments into which the main collector is divided: Upper Valley, Middle Valley, and Lower Valley of the Negro river. The general hypothesis of the present thesis states that the evolution of the Negro river along the territory has been significantly influenced by anthropic factors referred to the construction of dams and the contribution of chemical elements due to urban development and different industries. Based on this, a spatio- temporal study of the morphometry and dynamics of the Negro River was carried out, analyzing both anthropic and natural processes occurring along the Negro River system. More specifically, the regimes of the most significant climatic and hydrological parameters on the Negro River were analyzed, together with the analysis of the relationship of the most significant climatic and hydrological parameters on the Negro river with the climatic forcing influencing the region. In addition, the Negro river was described from the morphometric point of view through the analysis of remote sensors to provide a basis for the interpretation of the natural dynamic processes. Finally, a physicochemical and elemental classification of the sediments is added for the observation of possible direct human influences on the river systems. The project started by studying data from twenty meteorological stations distributed along the Negro river basin. We sought to identify the relationship between the parameter’s precipitation and temperature with ENSO, and the values found were statistically analyzed. An analysis of the morphometric parameters that define the SFRN and the Negro river was carried out employing remote sensors and geographic information systems (SAGA GIS). A relationship of parameters, interdependence, and consequence was generated between the two. Parameters of shape, the hierarchy of the SFRN, and the Negro river were studied as part of it and as an isolated system as well. A hypsometric and relief analysis of the SFRN and the Negro river was obtained. External parameters or factors such as the topographic factor (LS) about the erosion of the system, topographic index of humidity, and exposure to prevailing winds were added. An analysis of elemental chemical structure, texture, and parameters such as pH and conductivity of the sediments was carried out. Statistically, annual accumulated rainfall data were obtained, and the statistical analysis of the climate database showed that the summer months are those with the highest proportion of rainfall in the Alto Valle. ENSO has a significant influence as a climatic force in Northern Patagonia. In the winter months, mostly June, the correlation between rainfall and ENSO is significantly positive, while there is a negative correlation for the spring months. There is a clear interdependence between the parameters of flow, runoff, and spill, which in turn show a large decrease in value after the dams were put in place in the 1970s onwards, with the dams and anthropogenic control proving to be the main source of the current flow of the Río Negro. This generates the effect of clear water, increased conveyance, and erosion of the Upper Valley. The SFRN and the Negro river were defined as "very large" basins of more than 5,000 km2, and lengths of 1,317 and 719 km, respectively. The Gravelius Compactness Index (Kc), Caquot Mean Elongation Index (Ca), Horton Shape Index (Kf), and Elongation Ratio (Re) define the catchments as elongated, low concentration with low risk of high-impact floods. As for the order of the Negro River, when isolated from the system of tributaries that give rise to it, it is defined as order 1 since, along its 719 km, from its source in Confluence to its outflow into the Atlantic Sea, it has no contributions; but when integrated into the SFRN it takes value of 5, and is defined as a transition from a system of high torrential flow to a plain, with a low drainage network. Using the hypsometric curve, the SFRN was defined as a mature basin, with a peniplanized relief, and a significant development of the ritral and potamal sections. The Hypsometric Integral (HI) shows an initial senile system, and the kurtosis and asymmetries show erosion in the headwaters and areas of steeper slopes on the channel bottom and slopes. Relief (R) showed for the SFRN predominance of maximum heights and reliefs, large runoff and high velocity flows. For the Negro river, R reaches values of 500 m at the confluence and decreases in value towards the estuary giving moderate to low flows, with the formation of meandering systems, higher infiltration, and lower runoff. All of this is then demonstrated by the Relief Ratio Index (Rr) and the Roughness Value (Rn). Through the Melton Roughness Number (MNR) a higher percentage of debris input in the SFRN is exposed over the percentage of debris input in the Negro river. Then with the Terrain Roughness Index (TRI), we highlight the great flatness that characterizes the Negro River, with level or almost level heights and the high correlation of this index with the slope. Turning to the aspect and slope analysis, it is shown that the plain represents 33% of the SFRN, and 25% is observed as moderate north- facing slopes and 39% as steep north-facing slopes. The physicochemical studies of the sediments of the Río Negro show the highest proportions or dominance of loamy textures, with conductivities ranging from mean values of 4,857 μS/cm to maximum values of 44,600 μS/cm in old, abandoned meanders without anthropic influence on the north coast of the Middle Valley to minimum values of 263 μS/cm in an old, abandoned meander, but with the presence of current anthropic activity on the north coast of the Middle Valley. In terms of pH values, mean values of 7.9 and maximum values of 10 are found in an abandoned meander on the south coast of the Middle Valley, and a minimum of 4.5 on the north coast of the estuary. The basic chemical elements of the sediments along the Río Negro show a main composition of elements such as C, O, and Si, together with minor percentages of Na, Mg, Cl, Fe, S, K, and Ca. Then, variations in low concentrations, but the presence of elements such as Ti, Ag, P, Mn, As, Ba, Mo, Ta, Ga, Zn, Cu, and Ge were detected in sediments from several points. Some of these elements are of natural origin, from sediments of formations or geological units. Others are from anthropogenic activity in the valleys, such as agronomy and mining and oil extraction operations. It is proposed to continue this analysis from the point of view of the point origin of the elements, due to their importance in understanding the origin, polluting characteristics, consequences of their availability in the valley, and their impact on the environment. The present thesis provides an approach that contributes both basic and applied knowledge for similar studies of sediment dynamics, which allows an integrated approach to fluvial management of rivers such as the Negro River as an independent river and as part of a river system. The aim was to generate a contribution that allows the use of the soil and water resources of the Negro River, for management, land use planning, and a knowledge base for risk studies and different disciplines