Here is that large lake that I showed off yesterday. I've given it a name, stolen from I don't know where. OK, I know where...do you? ;-)
The idea is that the lake is around all year but varies considerably in size. When the rains start the channels flowing from the Mssúma river fill and it starts to grow in size. Then the floods come - some years higher than others - and the lake overflows its banks until some or all of the shaded area is inundated. In many areas the water will not be that deep: waist deep and often less than that, but in the central portion the waters could be many meters deep. The white areas are those areas that generally do not get flooded though when those once-in-a-hundred-year rains come, even those areas may see some flooding.
This area will have small villages or clusters of clan houses dotted all across the floodplain. They will typically be up on mounds above the normal flood level, or will be on the shores where it rarely floods. When the water recedes they plant their crops and reap the benefit of the rich soil the rivers sediment provides.
What is on that island in the center of the lake? A monastery? A shrine? Or perhaps that is where the local fief-holder has his seat? Which makes me wonder again about fiefs? How big should they be?