The environment is a rain forest basin, similar in many ways to both the Amazon and the Congo. There is a mighty river and its many tributaries that flood every year, raising and lowering the water level 10-15 meters each year. In this basin, a river civilization develops to a Bronze Age level.
Edit: Editing to add more about the nature of the floodplains. This environment is like a varzea, a flooded rainforest environment. The flood levels are much higher than and not comparable to any river other than the Amazon. There is almost no topographic variation (hills) and in the rainy season dense jungle covers all the available dry land. The people are restricted to the river, and the grassy parts of the flood plain, all of which are under 10m or more of water in the wet season.
I imagine that this civilization will have to go to extraordinary measures to build and maintain cities. In order to keep the city high and dry all year, the city could be built on a 20m platform of limestone blocks. For example, the bases of eight Great Pyramids would provide a limestone surface of about half a square kilometer, perhaps enough for a city of 5,000.
You can assume that limestone can be quarried near the headwaters of one of the river's tributaries and is then easily transported by water to anywhere on the river network.
Is it feasible to build and maintain a city of half a square kilometer in these conditions (i.e. with the erosion of a massive regularly flooding river)? How big of a city could be reasonably made? What sort of innovations would these city dwellers need to make to keep their city livable?