Originally Posted by
DuncanDoenitz
Probably Volga-Don Canal, and the lower stretches of the namesake rivers. Google the maximum draught/beam etc to determine which of their (surviving) fleet can navigate the waterway.
Maximum length 141 m (463 ft)
Maximum beam 16.8 m (55 ft)
Maximum draft 3.6 m (12 ft)
https://en.wikipedia.org/wiki/Volga%E2%80%93Don_Canal