More after the fold...
Yours is a uniform world, devoid of relief, with the exception of the levees that have formed alongside rivers thanks to periodic over-bank excursions. Yours is also a fantastic world, with gigantic, hairy, two- and four-leggéd creatures, some with enormous curving teeth protruding from their maw, huge cats, bears, rhinoceroses, wolves, and deer. The river is a fluid cornucopia of estuarine and freshwater species, and your grandfather tells you stories about how you could almost walk across the stream when, every year, the migrating fish would return in greater numbers than before.
But the world around you is in flux, and there seems to be no natural explanation. Your grandfather, in his lifetime, has seen the plain disappear at the seaside, as if it were sinking beneath the waves. A mud flat now appears at low tide where Grandfather says his father's house once stood. Everyone is aware of the inexorable transformation, and it's not simply about the disappearing sea shore. Where once there were endless beds of clams and other mollusks, there are many fewer now, and in places near the river mouth, none. Nowadays it seems as if the land on either side of the river has no time to recover from one year's floods before the next inundation occurs. It's great for mosquitos, and for the fast-growing plants. But it means slow death for species that don't multiply rapidly and easily take root like the brush cover and salt-loving plants that are replacing them.
The river itself seems about to flood all through the year, and each summer fewer migrating fish are caught in the nets, their once inexhaustible numbers are fading into memory for the old people. This is also a time of terrible storms, with wind and sleet in place of cooling breezes and the nurturing rain of days past. You've never known anything but the world as it is, so it may well seem to you as if live goes on as it always has. But the old ones believe that there's some terrible genius at work making the world less welcoming, less predictable. A witch, perhaps, or a deity. And the elders suspect that everyone will need to work harder as time goes on, just to get by. There is also a widespread feeling that the land animals are affected. The herbivorous herds seem less coherent, and the predators are taking advantage of the aimlessness. So, the meat-eaters are flourishing. They used to keep their distance from the people, but now they roam dangerously close to your settlements, and when there isn't enough meat on the hoof, they'll settle for meat on the foot. These are trying times.
The people who live upstream, never very friendly in the past, are having the worst of it, since the river's once-teeming animal life can no longer sustain you and them year round. They're compelled occasionally to raid their neighbours, your own cousins who live upriver. Raids bring reprisals, and reprisals brings reprisals in return. Tensions are high all the time between the two river-dependent groups. Soon your people will displace the ones up river. Perhaps it will happen passively as their numbers diminish, leaving territory unclaimed; perhaps the displacement will be more purposeful, and foul deeds born of perceived hardship will live on, in memory, long after the time of dearth.
All of the people in your world will be forced to deviate from old habits and ways as the life-giving plain on which they lived for millennia literally disappears beneath their feet. The resultant chaos means that future generations will need to find a way to exist in an ever-changing and ever-more difficult-to-exploit environment. The magnificent creatures of the plain will be gone, as will the plain iteself. On what's left of the world a bewildering array of different animals and plants will demand constant, novel human responses, taking human ingenuity to the wall.
Really, there's no need to imagine any of this, at least not in the sense that it's a fiction. This world was once your world. And most of the Earth's inhabitants would have experienced something similar. People everywhere at the continental margins suffered cataclysmic change during the terminal Pleistocene marine transgression. Those who were left were starting from scratch, just like Noah in the "Book of Genesis." The virtual extinction of life-ways everywhere gave rise to a global zeitgeist that echoed through the generations that came after. That's undoubtedly what's recorded in the histories (oral and written) of dozens of the world's contemporary people. Myriad cultures, the world over, conserved, for thousands of years, stories of a great flood. The Bible's Noachian flood was only one of dozens of similar accounts of which anthropologists are aware.
Development of writing in the Fertile Crescent 6,000 (give or take) years ago contributed greatly to the memory's longevity among the descendants of Abraham, today's Judaeo-Christian cultures. And even though the flooding of earth's continental shelves would have been accomplished by 8,000 to 10,000 years ago, it's no great wonder that the memory persisted long enough to be set down, especially when you consider that at the time of the first written texts around 6,000 years ago, the events recounted in Noah's story were no deeper in the past for the people of that time than were the events that gave rise to Christianity for those living today.