So the Rhine river flooded damaging my aquaduct. and interrupting my dam construction in the process. I repaired the flood damage, but during dam construction Niter had appeared on the tile the dam was being constructed on. But now that all the flood damage is repaired I am unable to resume the dam because the niter is blocking it. Once I finish Gunpowder, I will update this thread to indicate whether or not I am able to clear the niter.