The ruined city of Ngála originally dates from the First Imperium, which flourished some fifteen millennium prior to the current time. It is located in what is now the Tsechélnu Flats. Did the flats exist back then? That is three thousand years before Éngsvan hla Gánga and almost thirteen millennium before the cataclysm that ended that Empire. Personally I think that the flats are a byproduct of that seismic event. I'm not sure if this is stated anywhere though.
The reason I bring this up is because it occurred to me today that the Mssúma River might not have always flowed into the Gulf of Perudáya. I know that the Indus river changed its path as the result of an earthquake. Why not the Mssúma River?
It seems reasonable to me that if Ngála is one of Queen Nayári's, "gleaming cities of the South" it is highly likely it would be built on a major watercourse. My speculation is that instead of curving east and then flowing into the gulf, the river instead flowed south west to Ngála and then south to enter the ocean to the west of Point Kuné in hex 2713.
I'm going to do some sketching tomorrow. I have the start of it blocked in, I think, as I rather fancy that Nisuél - seat of the Golden Sunburst Clan IIRC - also has a similar ancient heritage. This is from a period that pre-dates the sákbe roads if I'm not mistaken.
I need to look at the Indus river for inspiration as to what happens to the old riverbed, only in that example the region is much drier and so might not be entirely appropriate...