For a simple two-dimensional nonlinear model of the collisional driftinstability the relationship between mixing length and strong turbulence fluxesinvestigated. Calculation of the turbulent wavenumber spectrum within astrong turbulence model shows that the transport is dominated by an inertialrange contribution; this yields a similar result for the fluxes to that from animproved mixing length theory. The nature of the spectrum in the dissipationregion, where collisional viscosity dominates, provides an explanation of anapparent paradox that arises when transport fluxes are cast in a formproportional to viscosity (or the diffusion coefficient).