  A linearly unstable, sinusoidal $E \times B$ shear flow is examined in the
gyrokinetic framework in both the linear and nonlinear regimes. In the linear
regime, it is shown that the eigenmode spectrum is nearly identical to
hydrodynamic shear flows, with a conjugate stable mode found at every unstable
wavenumber. In the nonlinear regime, turbulent saturation of the instability is
examined with and without the inclusion of a driving term that prevents
nonlinear flattening of the mean flow, and a scale-independent radiative
damping term that suppresses the excitation of conjugate stable modes. A simple
fluid model for how momentum transport and partial flattening of the mean flow
scale with the driving term is constructed, from which it is shown that, except
at high radiative damping, stable modes play an important role in the turbulent
state and yield significantly improved quantitative predictions when compared
with corresponding models neglecting stable modes.
