Minecraft wiki entry for Fluid, associated text: The shape of the ground around a flow is considered when evaluating its spread, giving preference to the creation of water or lava falls, for aesthetic purposes. During the evaluation of horizontal spread, the 5-block area around source and flowing blocks is checked for air one block below the fluid block. These air blocks, and the blocks preceding them, are all converted to fluid blocks with depth level of 1 greater than the current block to establish a simple flow, but are not added to the collection for later consideration.