  We investigate the stability and critical velocity of a weakly interacting
Bose gas flowing in a random potential. By applying the Bogoliubov theory to a
disordered Bose system with a steady flow, the condensate density and the
superfluid density are determined as functions of the disorder strength, flow
velocity, and temperature. The critical velocity, at which the steady flow
becomes unstable, is calculated from the spectrum of hydrodynamic excitation.
We also show that in two dimensions the critical velocity strongly depends on
the system size.
