  We use the Hubble Space Telescope ACS camera to obtain the first spatially
resolved, nebular imaging in the light of C IV 1548,1551 by using the F150LP
and F165LP filters. These observations of the local starburst Mrk 71 in NGC
2366 show emission apparently originating within the interior cavity around the
dominant super star cluster (SSC), Knot A. Together with imaging in He II 4686
and supporting STIS FUV spectroscopy, the morphology and intensity of the C IV
nebular surface brightness and the C IV / He II ratio map provide direct
evidence that the mechanical feedback is likely dominated by catastrophic
radiative cooling, which strongly disrupts adiabatic superbubble evolution. The
implied extreme mass loading and low kinetic efficiency of the cluster wind are
reasonably consistent with the wind energy budget, which is probably enhanced
by radiation pressure. In contrast, the Knot B SSC lies within a well-defined
superbubble with associated soft X-rays and He II 1640 emission, which are
signatures of adiabatic, energy-driven feedback from a supernova-driven
outflow. This system lacks clear evidence of C IV from the limb-brightened
shell, as expected for this model, but the observations may not be deep enough
to confirm its presence. We also detect a small C IV-emitting object that is
likely an embedded compact H II region. Its C IV emission may indicate the
presence of very massive stars (> 100 M_sun) or strongly pressure-confined
stellar feedback.
