It's a brave adventurer that delves into the lair of a dragon, much less a cunning and vicious green dragon - it's a very dumb adventurer who thinks the first or second entrance is the real entrance. A twisting honeycomb of tunnels and currents, the only thing to be found here is frustration, failure, and death.
I have been wanting to make a waterfall map for a while now. There's something incredibly appealing for a dungeon in a waterfall: the extreme heights, the powerful water, the nooks and crannies... Visiting a hydroelectric dam opens up more possibilities in terms of large. grinding. inexorable machinery. Which plays well with the idea of a dragon's false lair!
What's important to keep in mind is that there's still a large river rushing overhead, so the places where the water makes its way down forms currents that can be hard to push against. In addition, while the roaring water can be deafening to newcomers, any creatures a dragon places here as another line of defense would grow accustomed to the noise. It would be quite easy for them to distinguish the noises of intruders splashing about in the more static pools.
Now, this is a False Dragon's Lair; where's the real one? A green dragon would use every tool in its arsenal to keep itself safe, and this one is no exception: the only entrances are underwater, either directly underneath the punishing waterfall or on the cliff above, deep under fifty feet of heavy river current. There's an additional section of open cavern which lets the dragon use its ability to fly to keep distance from its foes. And, of course, plenty of peepholes and cracks to keep an eye on rats wandering its maze.