In this note, we use both developing and fully developed buoyant flow in a 2-d channel as a simple flow model useful for approximating buoyancy driven interpenetrating mixing phenomenon, such as the Rayleigh-Taylor instability. Our approach towards understanding and quantifying mixing within this flow follows the development by Ottino (The kinematics of mixing: stretching, chaos and transport, Cambridge University Press, Cambridge, 1989) that mixing is the kinematically efficient stretching and folding of material lines. Like Ottino, we use simple flows as a prototype to understand more complex problems. Here we derive the stretch length i.e. the deformation of a unit filament subjected to both the fully developed and developing buoyant channel flow. We use the simpler fully developed flow to compute the specific rate of stretching for the fully developed flow. An early time contraction-expansion effect is noted for stretching. Asymptotically, the stretching rate decays by the expected t(-1) behavior. This note indicates that early time buoyancy induced inter-penetrative mixing is a typical 2-d, shear driven phenomenon, exhibiting neither strong re-orientation (with attendant enhance mixing) nor negligible filament deformation (indicating minimal disruption of material surfaces).