So I embarked on a temperate region that had two springs and an additional river, everything was frozen at the beginning before the eventual thawing. Once that happened the lower end of the river didn't seem to be able to drain away the water sufficiently fast and now in mid summer I've got a steadily growing lake in what used to be a valley. The water level is still some 5 z-levels from the initial embarkement spot and the volume of the would be megalake being enourmous there's no way it will reach it for years to come. However, since I don't expect this to be convenient to my FPS in the long run I was wondering what could be done to get the river flowing neatly. I'm not sure what exactly is the problem here, but going out on a limb here I'd say there's more incoming water due to one of the springs entering the river than what can flow out (the other spring flows to a different exit point alltogether). So just to give a quick sketch of the map:
Noticeably the river enters from the northwest and drops as a waterfall to the level of the spring in the middle, then the combined water flows to the south flooding the southwest corner of the map.
So how to end this deluge? I guess I could try damming the river before it hits the waterfall or damm the spring before it gets to the river but I was kinda hoping for finding a way to set the drainage appropriate, probably when it's all frozen again - if there's a way to influence the drainage that is, the tiles at the edge don't seem to be different from any others in the exit point of the river, just the entry points are clearly labeled as water sources. Any ideas?