The instabilities which form due to globally unstable stratification of density in the shear layer which develops between two coincident streams of different velocity in a channel are described. A portion of the flow is contaminated with dye to visualize flow features. The channel employed is 1.59 cm thick, Reynolds numbers range from 10(0)-10(1) and Richardson number ranges from 10(-3) 2 x 10(1). A variety of flow features are evident in unstable, marginally stable and unstable flow domains, including streamwise vortices, hairpin vortices, spanwise structures, and localized turbulent motions. Spanwise and streamwise wave number variations in these structures are also described, along with the mixing onset boundary, and a scaling factor which accounts for the dependence of the mixing onset boundary on channel thickness.