The linear instability of a weakly non-zonal barotropic flow is investigated numerically using the spectral method and analytically via WKB theory. The basic steady-state is the consequence of the interaction of a constant mean zonal wind and a sinusoidal topography. The (numerically determined) slowest growing eigenmode is stationary, on the scale of the topography and depends crucially on a form drag mechanism. The fastest growing eigenmode, on the other hand, does not involve a form drag mechanism but depends rather on the presence of local shears inherent in the basic flow itself. A large part of the discussion involves an analysis of the structure and energetics of the fastest growing eigenmode.