We consider the general transport properties of a guided beam of Bose-Einstein condensed particles relative to a scattering potential (an "atom laser"). In the particular case of a disordered potential, we identify two regions of stationary flow. One is subsonic and corresponds to superfluid motion. The other one is supersonic and dissipative. The latter contains a regime of Anderson localization, for which we compute analytically the interaction-dependent localization length. We also explain the disappearence of the stationary flow for large extensions of the disordered potential.