  Analysis of the saturation of the Kelvin-Helmholtz (KH) instability is
undertaken to determine the extent to which the conjugate linearly stable mode
plays a role. For a piecewise-continuous mean flow profile with constant shear
in a fixed layer, it is shown that the stable mode is nonlinearly excited,
providing an injection-scale sink of the fluctuation energy similar to what has
been found for gyroradius-scale drift-wave turbulence. Quantitative evaluation
of the contribution of the stable mode to the energy balance at the onset of
saturation shows that nonlinear energy transfer to the stable mode is as
significant as energy transfer to small scales in balancing energy injected
into the spectrum by the instability. The effect of the stable mode on momentum
transport is quantified by expressing the Reynolds stress in terms of stable
and unstable mode amplitudes at saturation, from which it is found that the
stable mode can produce a sizable reduction in the momentum flux.
