Dneiper Bank-Kiyev-Ltava-Polissya

There, finished the corner.  Moving southeast now, towards the Black Sea.

The "swamp" areas are more appropriately large sloughs or fens, where the ground is generally too wet to plant (not today, of course; measures have been taken), and where flooding makes dwelling impractical.  This last is particularly true of the large areas adjacent to the Dneiper, the large blue line left of centre.  Incidentally, this land is still too soft for tanks, especially in late winter-spring, which is why the present Russian push is through Kopol and Kherson to the south.  The country isn't great there for heavy mech, but it's better.

Kiyev became the centre it is because the Dneiper provided one good crossing point, which is excellent for east-west trade... particularly by water.  Below Kiyev, the old Dneiper, prior to the building of many dams, was filled with treacherous drops and water courses impractical for large barges, but above Kiyev, the Dneiper is a dream.  Plus, there are numerous other large rivers that flow into it, so that the whole area above Kiyev is excellent for water-trade; this made Kiyev a powerful collection point for everything coming south in the western heart of Russia, a natural centre for wealth and thus culture and religion.  So it remained until the Mongols destroyed everything in the 13th century, and Kiyev's defensive weaknesses became evident.