This paper applies the exergetic method to a renewable resource: the water of a river. Its application can be used to compare, diagnose and evaluate the river water along its different stages, while the rivew flows from its source to its mouth. River water availability (defined as the sum of the exergy components) will help to characterize the pysical and chemical properties of the river. The different stages of the river can be characterized by it mass flow and the measure of five parameters pressure, temperture, altitude, velocity and composition. As is well known, the exergy method can associate each parameter with its exergetic component: mechanical , thermal, potential, kinetic and chemical, respectively. These componentes will help to quantify some quality and quantity aspects of the river. The information provided by the exergy method will allow to understand concepts related to the water river availability. Furthermore, by comparing these availabilities at different states, it will be possible to obtain a broad understanding of the degradation process that the river undergoes. An example of application has been developed to a particular river: the Ebro, the most abundant Spanish river. From eight sampling stations [1], which are ordered following the going down of the river (stages), it is possible to obtain field data measuremetn needed to calculate the exergy components. The information provided will help to describe and compare the degradation process of the Ebro, from its source to its mouth: stage by stage, and during the year. The exergetic approach fulfils these aims by unifying in the same exergy units. An interpretation of the results has been made in terms of the geographic and hydrologic aspects.