  The Cl1604 supercluster at $z \sim 0.9$ is one of the most extensively
studied high redshift large scale structures, with more than 500
spectroscopically confirmed members. It consists of 8 clusters and groups, with
members numbering from a dozen to nearly a hundred, providing a broad range of
environments for investigating the large scale environmental effects on galaxy
evolution. Here we examine the properties of 48 post-starburst galaxies in
Cl1604, comparing them to other galaxy populations in the same supercluster.
Incorporating photometry from ground-based optical and near-infrared imaging,
along with $Spitzer$ mid-infrared observations, we derive stellar masses for
all Cl1604 members. The colors and stellar masses of the K+A galaxies support
the idea that they are progenitors of red sequence galaxies. Their
morphologies, residual star-formation rates, and spatial distributions suggest
galaxy mergers may be the principal mechanism producing post-starburst
galaxies. Interaction between galaxies and the dense intra-cluster medium is
also effective, but only in the cores of dynamically evolved clusters. The
prevalence of post-starburst galaxies in clusters correlates with the dynamical
state of the host cluster, as both galaxy mergers and the dense intra-cluster
medium produce post-starburst galaxies. We also investigate the incompleteness
and contamination of K+A samples selected by means of H$\delta$ and [OII]
equivalent widths. K+A samples may be up to $\sim50\%$ incomplete due to the
presence of LINER/Seyferts and up to $\sim30\%$ of K+A galaxies could have
substantial star formation activity.
