The Great Basin
Several great rivers run through the rainforest basin, though none are named. In several places, there are spectacular falls and whitewater rapids, though for the most part, the rivers are fairly slow and meandering. The lutrai often fish in these rivers, though most of this fishing it done with long spears to avoid the dangerous alligators and pirhanas. The koba are more at home in these waters, inhabiting the marshy regions that flank the river at its widest points. They feed off of soft aquatic plantlife near the banks, and while they can in fact eat just about anything, this particular fare is a delicacy. Unlike the lutrai, most koba tend to avoid strangers, and will submerge and conceal themselves from anything unfamiliar, such as a boat or a pack of noisy adventurers.
The great basin is also the domain of several small, bipedal dinosaurian species, both herbivores and carnivores. Though many of the larger reptiles died out during the endless snowfalls that followed the ancient war, some managed to survive and adapt in the warmer regions, such as the basin rainforests, and now have largely mammallian cadiovascular systems. Many are also devilishly clever, and have learned to steal things from both the lutrai and the koba just as their ancestors once snatched eggs from nests. Any humanoid is, in fact, at risk of having something shiny or edible snatched away by one of these agile, ostritch-like creatures. Some will even work in teams. Certain larger varieties don't both with snatching, however, and go right for the jugular, but are more likely to attack more familiar prey such as deer and wild boars. These, too, tend to hunt in small packs.
The Earthgates
These are the ancient druid stones raised by the koba in places of significant natural power. They are places of worship and ritual, capable of opening gateways between far-away lands with the proper timing and invocation. These runed obelisks are about as complex as koba structures get, and most are so ancient and overgrown that they can only be spotted with careful observation. When activated, however, the stones flare with significant visible power, and hum ominously. These gateways are largely unused, however, the remnants of the koba exodus from their ancestral homes in Slank. Most simply connect to various other islands in the chain, precluding the need for boats.
Essentially, this is just a big astral zip-line. Stepping through the gateway catapults the traveller in a linear vector along the druidic pathways and releases them through an opening at the opposite end, allowing instantanous travel between numerous points within the basin and the rest of the island chain. Most are actually simple enough for anyone to use if they're given the proper key, usually a combination of enchanted beads arranged in a certain order granted by the koba shamans who maintain the ancient pathways. Every year, they walk these paths to maintain their definition, dusting them with salt to keep the jungle from quickly overwhelming them. Many animals also use these paths, everything from wild deer to legions of leafcutter ants.