It's a known bug, that was dubbed the "instant flood bug" ages ago, by Darkstar IIRC.
It happens when you start with a map from scratch (ie. all ocean). The first time a river is created, land is elevated by formers, when the map is saved and reloaded, and possibly other triggers, the oceans will immediately rise.
The only way I know around this, since it occurs only once on a given map, is to rise a bit of land, place a river on it to trigger the bug and finish editing it.
There are no other map editors that I know about.
Aredhran
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