Yeah, this was exactly my problem. I had a lot of desert tiles next to lakes or oases, and couldn't Spring any of those fresh water desert tiles. There was already an explicit check for floodplains in the Spring python code, so maybe Magister was trying to prevent Spring on Oasis tiles by adding the fresh water check? I think an explicit check for Oasis would have worked a lot better, unless he really was trying to nerf the Spring spell. In the latter case, I have to say that it took me quite a while to figure out the pattern of which tiles allowed or didn't allow Spring. So, either a bug or a very confusing game mechanic.