This paper presents measurements of mixing efficiency of the two counter-flowing gravity
currents created by symmetric lock exchange in a channel. The novel feature of this
work is that the buoyancy Reynolds number of the currents is higher than in previous
experiments, so that the mixing is not significantly affected by viscosity. We find that
the mixing efficiency asymptotes to 0.08 at high Reynolds numbers. We present a model
of the mixing based on the evolution of idealized mean profiles of velocity and density at
the interface between the two currents, the results of which are in good agreement with
the measurements of mixing efficiency.