The banks of the Moskva river have collapsed and flooded, creating new wetlands. Much of the subway system is also flooded, but some are known to have survived in sealed sections. In summer, Moscow swarms with the activity a new ecosystem, and dozens of species of stinging insects. In winter the river freezes, and the survivors emerge to scavenge and hunt hibernating beasts.
The USSR will not abandon its former capital without a fight. There have been numerous attempts at reclamation over the years, all of which have failed, and many of which have left behind pockets of soldiers. Most die, some are assimilated by bands of survivors, passing on their skills.