Parallel drainage pattern. A parallel drainage system is a pattern of rivers caused by steep slopes with some relief. Because of the steep slopes, the streams are swift and straight, with very few tributaries, and all flow in the same direction.
1 In Creative, the player can use spawn eggs to spawn most mobs. 2  When mobs are spawned this way, all normal spawning requirements, such as light level and block type, are ignored. 3  Any entity can be spawned using the /summon command.
Make sure you are standing far enough away from the chunk for mobs to spawn. Mobs WILL NOT spawn within 24 blocks of the player. As pointed out above Slimes also spawn very slowly. You might need to wait quite a while for slimes to spawn.