The usual cure is to not bother. Just waste manpower cleaning up every flood season, since the floods rejuvenate the fields and gives them an effect comparable to fertilizers with natural phenomenon alone.
That's actually what saved some semblance of civilisatoon in Egypt after the Bronze Collapse. The Nile was still restoring the soil while what used to be Summer was a bunch of crusted salt. Not sure if Brushcrest's river works the same way though.
Thing is, we might be somewhat pop capped now bar a few chair rearanging and incremental progress, the point that I was trying to make earlier is that that's not nessesarily a bad thing.
As long as we keep a decently pro army and we keep turning the valley into a Fortress we are effectively invulnerable from outside. Siege trains aren't a thing in the Stone Age, any force to march up Greenvalley would have to storm the walls with ladders and take
axes to the gatehouse (maybe a pine serving as a battering ram if the dice gods are angry). It's either that or starve because they cant keep their forces supplied. They'd need ten times our numbers, and even a little raiding by the White Clans could see such a humongous force starve because their foraging got cut by a third.
Sure we'd have to spend a turn or two rebuilding stuff outside the walls, but its not much in the grand scheme of things. We should keep spamming culture and tech, let the superior numbers of the Sowing People live in a miasma of plague and turmoil in the free for all that is the low lands.
Edit: Besides, spreading our religion and eventually culture throughout the mountains is in a way a form of upping our pop. The White Clans are a great asset that could be harnessed in a vareity of purposes down the line, the least of which is having a shitload of auxiliaries on reserve.