Regions are sections of The River that you travel down. There are multiple Biomes throughout a Region. Each Region becomes progressively more difficult and more advanced, typically yielding more materials from the biomes and having greater enemies.
Region 1 (~0.9 Miles) - Deep Country. Seems to comprise mostly of Camp grounds, Wilderness and Farms with some Churches. Doesn't seem to have any Wolves. Stock up as much as you can here. It is a very short region.
Region 2 (~3.3 Miles) - Deep Country. You will see enemies pretty consistently and see a good variety of Biomes including Fishing Shack and Marina. This region does last longer than region one but is ultimately not so long.
Region 3 (~5.5 Miles) - Small Town. You will see enemies like Snakes, a packs of 3 wolves or possibly even the King Boar. You'll see a larger variety of Biomes including Filling Stations, Liquor Stores, Hardware Stores and Clinics. This region is pretty long compared the the first 2 regions and needs more exploration.
Region 4 11.2 Miles - Seems to be for story progression and an intermission. Its a short region With 1 Biome, no resources or enemies and Flimflam the shady trader.
Region 5 12.8 Miles - (currently exploring) Seems to have an abundance of wilderness and a variety of other Biomes. So far goes on for a bit much like Region 3 did.