Impact anthropique et metamorphose fluviale : effets de la régulation du débit sur l'hydrologie, la morphologie et la température de l'eau de la Sauce Grande, Argentine = Impacto humano y transformación fluvial : efectos de la regulación de caudal sobre la hidrología, la morfología y la temperatura del agua del río Sauce Grande, Argentina

Abstract

La presente investigación evalúa el grado en que el dique-embalse Paso de las Piedras ha interrumpido la continuidad hidrológica del río Sauce Grande y cuantifica los impactos de la regulación hidrológica sobre el régimen de caudal, la morfología y la temperatura del agua del río aguas abajo del embalse. El trabajo de tesis presenta una evaluación integral y sistémica de los impactos del dique-embalse basada en el entendimiento de los procesos fluviales que ocurren aguas arriba del mismo. Además de proveer información sobre los impactos hidrológicos, morfológicos y térmicos de la construcción y operación del dique-embalse, este estudio genera nuevas series de datos climáticos e hidrológicos que proporcionan una importante plataforma sobre la cual basar futuras investigaciones de la cuenca del Sauce Grande. Por otra parte, este estudio implementa un marco metodológico aplicado al análisis hidro-geomorfológico de cuencas no aforadas que tiene un gran potencial de aplicación en otras cuencas de la región y en otras regiones semiáridas del mundo.

Despite the regional importance of the Sauce Grande River as main source for water supply and the large capacity of the Paso de las Piedras Reservoir, both the hydrology of the river basin and the effects of the impoundment on the river environment remain poorly evaluated. This study provides the very first assessment of the degree of flow regulation induced by the Paso de las Piedras Dam on the middle section of the Sauce Grande River, and quantifies its impacts on the hydrology, morphology and patterns of water temperature of the river downstream from the impoundment. In addition to providing new information on the response of regulated rivers to upstream impoundment and on the effects of impoundment on the Sauce Grande River specifically, this study generates spatial, climatic and hydrologic data and implements a methodological framework to hydrological assessment of ungauged basins. The methods include a set of procedures performed to generate data and a set of procedures performed to hydrological and morphological assessment. Methods used to generate data include (i) terrain modelling of the river basin, (ii) filling and modelling of climatic data, (iii) hydrological modelling of the upper river basin, and (iv) flow simulation and calculation. Methods used for hydrological and morphological assessment include (i) inspection of the water balance of the dam reservoir and quantification of the reservoir size-yield-performance relationships, (ii) quantification of the degree of hydrological alteration of the river downstream, (iii) classification of diurnal regimes of water temperature and quantification of their climatic sensitivity, and (iv) GIS-based diachronic analysis of change in the fluvial forms and the fluvial landscape. Results revealed that reservoir inflows are driven by varying climate conditions, whereas reservoir outflows are constrained by the dam purpose for water supply. Large volumes of water are yielded annually (73 % of the mean annual runoff) and the remaining volume is conserved within the reservoir for periods of drought. These patterns of flow regulation have seriously affected the hydrology and water quality of the river downstream. Floods were fully eliminated and the magnitude and frequency of high and low flows were reduced dramatically, with substantial increase in the duration of similar flow conditions. Furthermore, summer temperatures were depressed, winter temperatures were increased, and diurnal fluctuations were altered in both magnitude and timing. Analysis of the geomorphic response to altered hydrology revealed fluvial stability. Lateral and vertical channel activity was related to human intervention to facilitate the evacuation of dam outlets and spills more than to natural adjustments to reduced flow discharge and sediment load. The reduced flow, even if sediment-free, is incompetent to work its channel and at present, the downstream channel morphology is a relic of a fluvial environment created in the past under conditions of higher energy. The new climatic, hydrologic and morphologic information yielded herein has triple applicability: (i) it informs dam managers about the complex behaviour of the river system, (ii) it contributes to improve reservoir operation procedures while maintaining the overall integrity of the river system, and (iii) it provides a consistent scientific platform on which to base further research efforts conducting to an interdisciplinary framework of river restoration. Furthermore, the methods implemented in this research are widely transferable to hydrological assessment of ungauged basins worldwide, with special applicability to semiarid regions.