In the context of the dramatic decline of the catadromous European eel populations (Anguilla anguilla), a global reduction of all anthropogenic pressures is required, and particularly the impacts of obstacles such as hydroelectric facilities affecting both upstream (restricted free movement) and downstream migrations (increased mortality following turbine passage). From 2007 to 2010, 192 downstream migrating silver eels were monitored along 60km of the Gave de Pau river (South-West France) using radio and PIT telemetry. The objective was to quantify the effects of highly variable hydrological conditions on their migratory behaviour and especially on their choice of a passage route (i.e. water intake vs. spillway) at several hydroelectric power plants. Statistical operational tools were derived from those observations to estimate route choice probabilities according to hydrology. Although different between obstacles, passage routes were mainly influenced by (i) the ratio of spill flow to total river flow, (ii) the configurations of the obstacle and its water intake (position relative to river flow, intake trashrack bar spacing…) and (iii) the specific management of the hydropower plant. Passages over spillways were predominant at turbine flows less than ⅔ of the total river flow. Regarding the site configuration, dams with strong inclination may favour fish guidance towards the intake canal while intake trashracks with 2.5 or 3cm bar spacing limit turbine passage for most eels. Turbine shutdowns planned on certain hydroelectric plants at increasing river flow may also limit turbine passage and so favour escapement. The probability for eel escapement was predicted as a function of both spill flow to total river flow ratio and eel total length to intake trashrack bar spacing ratio, using P=exp(η)/(1+exp(η)) with: η=-6.89+4.28*(Qspill/Qriver)+0.273*(TL/s). The use of such tool in determining the cumulative impacts of hydroelectric plants on downstream migrating eels should help us to propose devices to mitigate those impacts.