Water is an important weathering factor on rock discontinuities and in rock mass mechanical behavior. The increase of rainfall in frequency or in intensity highlights some problems on the rock slope stability analysis. The aim of this paper is the multi scale analysis of the chemical impact of water on rock (surface roughness and matrix). In this study we show how water induces degradation and thus may decreases the stability of the discontinuous rock mass. Water is known to have significant erosion and dissolution effects on rock surface or rock matrix. The chemical features of water such as temperature, pH or salinity make it a 'good' candidate to rock degradation. This study has two main components. The first one is the study of water-solid chemical mechanisms and the other is the analysis of the mechanical response of the discontinuity modified by the water alteration. The stability of the rock mass is naturally a function of the type and the space distribution of discontinuities. The study aims also to evaluate the effect of water flow on the rock slope stability and it is performed at two space scales: laboratory (micro scale and macro scale) and in situ scales. The last one is still under investigation and will be presented.