  We present an experimental study of the mixing processes in a gravity current
flowing on an inclined plane. The turbulent transport of momentum and density
can be described in a very direct and compact form by a Prandtl mixing length
model: the turbulent vertical fluxes of momentum and density are found to scale
quadratically with the vertical mean gradients of velocity and density. The
scaling coefficient, the square of the mixing length, is approximately constant
over the mixing zone of the stratified shear layer. We show how, in different
flow configurations, this length can be related to the shear length of the flow
($\epsilon$/$\partial$ z u^3)^1/2. We also study the fluctuations of the
momentum and density turbulent fluxes, showing how they relate to mixing and to
the entrainment/detrainment balance. We suggest a quantitative measure of local
entrainment and detrainment derived from observed conditional correlations of
density flux and density or vertical velocity fluctuations.
