Minecraft wiki entry for Man-made_lake, associated text: What you need to understand about water is that it can't create springs if there is no block under the would-be spring. That's the reason why lakes don't fill themselves when one infinite spring is placed up-stream. However, water floats as of now in Beta, and if there's a block between two floating springs, a new spring will be created. That is the basis of this technique. Both versions are viable, but this version is the most efficient block wise.