Dead Drop Falls

It is a cave which can only be reached by passing through Fort Dawnguard.

Description
The cave contains a waterfall and a hole. There is a small lake at the bottom which can be reached by jumping down the hole. There are two paths at the bottom, each partially submerged in water. One is an ascending spiral which will eventually lead to a lever and upwards to the beginning of the cave and thereby its exit/entrance. The other path will lead to a big open area where several traps are located.

Bridge Puzzle
There is a bridge puzzle in this room. To lower both bridges and to solve subsequent puzzles the Dragonborn needs a projectile weapon (such as a bow or crossbow), or any form of projectile magic (such as Firebolt, Magelight or even Shouts). First pull the lever, then shoot the target which hangs on the pillar behind the lever. Both bridges will be lowered.

The third and final bridge can be lowered by pulling an adjacent lever. This closes an iron gate at the platform the bridge leads to, while also closing another gate behind the first. Because of this alternation between the two gates, just as with the alternation of the two bridges earlier, it is impossible to pass through solely by operating the lever adjacent to the last bridge.

In addition, the lever also operates the last bridge which provides access to the platform. Walk up to the closed gate and then turn around to face another target. There is also a chest which can be seen at water level, though it can be found by traditional exploring of the pool of water in this area as well. Shooting the target will open the locked gate, but close the one behind it.

From inside the space between, the gates past the first opened gate, before the second closed gate, shoot the same target again and the next gate will open. The previous gate however will close and prevent backtracking. This area can be accessed again by completely passing through the next area, Dead Drop Mine (below), and retracing steps back to the bottom of the falls and into the cavern. Alternatively, from the right position, an arrow or bolt can be shot through the bars of the gate to reactive the target switch and open the door.

In addition to the intended method of going through this puzzle, if the Dragonborn pulls the lever and then runs up bridges before they are fully upright, the bridge section of the puzzle can be skipped.

Cavern with pressure plates
The gate must be opened by placing an object on the all three floor pressure plates, (there is already a book on the left pressure plate), then shooting the pressure plate above the locked door. The items can be picked up after the gate is open or a follower can stand on a pressure plate and the Dragonborn on the last one. After the gate is open it's safe to retrieve the items again. Facing this gate, a ledge can be seen to the right where a hidden chest can be found towards the back. It is possible to climb up by hopping on the steep rock slope directly to the right of the gate, then onto the ledge.

Note: It is actually not neccesary to place anything on the three pressure plates. When each plate is stepped on that plate remains active for approx. 10 seconds. This allows one to walk across all three floor plates and then quickly fire on the wall plate.

Around the corner is another locked gate. It can be opened by shooting at the pressure plate above the gate. It will close in a short time, so it requires either sprinting through the hallway and gate before it closes or using the Whirlwind Sprint shout. There are several useful items in the next room. There are some Exploding Steel Bolts of Fire, Exploding Steel Bolts of Ice and Exploding Steel Bolts of Shock as well as a chest with random loot. The book Confessions of a Khajiit Fur Trader can be found on the table. The next door will lead to Dead Drop Mine.

Dead Drop Mine
Dead Drop Mine contains several Corundum Ore Veins, two crossbows, the non-skill books The Aetherium Wars and The Sultry Argonian Bard, v1, and some other minor loot. It's only a passage back to the bottom of the Falls, and thus allows for an exit from the Falls by taking the ascending spiral path back to the exit/entrance to the cave described above.

Notable Loot

 * The Sultry Argonian Bard, v1
 * Exploding Steel Bolt of Fire
 * Exploding Steel Bolt of Ice
 * Exploding Steel Bolt of Shock

Bugs

 * Projectile magic such as Ice Spike may not work on all of the targets.
 * Its possible that the book on the left of the three pressure plates isn't counted as being on top of the plate, even when its placed in the middle. Using a weapon or pickaxe instead, will do the trick.
 * Uthgerd the Unbroken (and possibly other followers) might not follow the Dragonborn out of this area.
 * After entering dead drop mine there is no floor leading to the following platform