A Global Sensitivity Analysis (GSA) is carried out on the Mascaret model of the Odet river (France, Brittany) to identify and rank the major sources of uncertainty at observing stations on the network for the simulated water level, considering the upstream and downstream boundary conditions and the area distributed friction coefficients values. Upstream, ensemble hydrologic forcings are forecasted with the rainfall-runoff distributed model MORDOR-TS, using uncertain hydrologic model parameters drawn from uniform distributions. The downstream maritime boundary condition is perturbed taking into account the temporal correlation of the errors in storm surge. The Sobol' indices are computed at Kervir, Moulin-Vert and Justice stations given hypothesis on the statistical distribution of the aleatory variables. The study focuses on the 23 to the 26 December 2013 event. GSA highlights that the simulated water level at the three stations is mainly controlled by the immediate downstream friction coefficient when the boundary conditions are not perturbed. The flood plain friction coefficients only become important around the peak of the event. However, when the boundary conditions are also taken into account, they become predominant for the simulated water level and the value of the friction coefficients has less influence.