Minecraft wiki entry for Man-made_lake, associated text: 1) Make a line of blocks at the water level and reach the current. When that is done, place a spring on the side to stop the current. Unfortunately, you will need to replace every spring you've eliminated by placing blocks. If you don't want to go through this hassle, you can place the line of blocks one level deeper and reach the current from below. A new spring will be created automatically above the new base you just created. Destroy the line and you're done.