Minecraft wiki entry for Fluid, associated text: fluid blocks have a depth value that measures how "empty" it is. A source block has a depth value of 0, so it is full. Flowing fluids have a depth value equal to their source's depth + 1, with a maximum possible "emptiness" value of 7 where the flow stops. Thus, a flowing block next to a source has depth value 1, the next further away 2, and so on till the flow stops.