Abstract
The stability of a steacty, laminar, immiscible, dense line plume falling through a fluid whose horizontal extent is much larger than the plume thickness is investigated. It is shown analytically that the primary flow is linearly unstable in the limit of large wavelength and, if surface tension is negligible, is also unstable in the limit of small Reynolds number. A rich variety of modes of perturbation exist, characterized by a range of balances on different lengthscales between diffusion, advection and propagation of vorticity.