Minecraft wiki entry for Man-made_lake, associated text: When your grid is perfect, you can start filling up. Start by placing water on the inside of the square (where the is on the schema above). When you're done with that part, replace every block in your grid by a spring of water. There's no particular way to do it that is best, but you must make sure to never, ever, isolate a block. You always want every block to be relied to another block so that you can put a springs in its place. If you goof, refer to the "there are current" section.