The year 641, shortly before EL's first trip to the Dread Dimensions.

The party are Equestrian journalists and their escorts (mercenaries or explorers) following rumours from a traveler that a unicorn wizard is brutalizing the indigenous population far upriver, and the press wonders if this is true and to what end.

A month ago, you landed in Port Hoofcourt after a long sea voyage from Equestria.

Eldritch Lore, under the alias "Elixir Lotus" and Colonel Curse, a unicorn mage and mercenary commander, were working to excavate an ancient Elephant civilization. Nearing the completion of their work, Curse realized that Eldritch was a dangerous partner who was probably likely to kill him to ensure a monopoly on Elephantine soul magic, and tried to kill her, failing (and being injured in the process) but managing to drive her off. The joint operation is reliant on the brutal exploitation of Warthog slave labor, including mind-control alchemy. (Naturally, no sooner had EL mastered zebra alchemy than she set about using it for evil.)

EL made the difficult journey downriver toward Port Hoofcourt alone, having been deprived of her money and stuff by the circumstances of her escape. She was saved from death (when magically exhausted) by some zebra traders. There, she wrote a letter to the Canterlot Colonial Benevolent Society exposing Colonel Curse's wicked doings. (Saying nothing obviously about her own part in the enterprise.)

The CCBS sent Angel Gaze, who recruited some help in Port Hoofcourt, rented a boat, and headed upriver with EL...

EL will split with the party when they get to the Headwaters of Seven-Tusks, take her research notes and some artifacts, and steal the party's boat (destroying any others) to ensure she is gone from Port Hoofcourt before they get back.

Kalamu was one of EL's tutors in zebra alchemy. He is protective of, and loyal to, her because she found his missing children with crystallomancy and helped to recover them from the bandits who had abducted them.

"Sayyid" "Sayid-Eli" (i.e. Mistress Elly).

"Afendi" for others.

upriver:

It's been a week since you began your journey up the Sabapembe river. A formidable obstacle lies before you: the Elephantine Cataract, a huge waterfall. For centuries, it blocked the way of outlanders who would journey up from the coast, but twenty years ago, an Equestrian company built a case of locks to allow boats to be moved upriver. Quite profitable, the locks have allowed Equestrian commerce to penetrate deeper and deeper into the continent. 

At the bottom of the locks is the creatively named Fort Lock, a trading post, and surrounding allied native villages.

If the zebra sailor is with the party, she might note that there are unusually few boats coming downriver, and maybe the smoke thing as well.

Someone might smell smoke on the breeze.

You hear the sounds of the waterfall long before you see it. You round a bend in the river, and see the spectacular cataract, water cascading down a drop of at least forty meters, branching over rocky outcrops and creating a beautiful rainbow in the afternoon sunlight.

For several seconds, the lovely scene distracts you from the horrific devastation. Fort Lock is burning. Corpses bob in the river. A few riverboats are on fire, some beached, some floating free. One of them seems at first to have escaped successfully, but you see its helmsman impaled with a spear, and it circles aimlessly in the plunge basin.

The attack must have been swift, and not too long ago, or surely someone escaping downriver would have warned you.

If the party had warning, they are more likely to avoid the initial volley of spears form boars and their trained monkey lurking in the jungle.

There are few other boars - mostly just looters, and they are of local tribes primarily. The raiders are Curse's boars from upriver, trying to ensure privacy. It should be clear enough on examination that the local warthogs have been killed along with zebras and ponies - a warthog killed by spears for example.

There are enormous circular tracks in front of the fort's gates, which were battered down. Most of the corpses are in here; the civilians sheltered in the fort during the attack, but were massacred, pony, zebra and allied-warthog alike. Some have been killed with native weapons, some have been slain with some sort of much larger stabbing weapon. The sole survivor is one of the warthogs, who is driven mad and keeps repeating the name of the warthog river goddess, a mercurial being made of water.

In the factory of the trading post, which is barricaded, the manager of the fort has killed himself by taking poison, leaving a note that says:

The fort is lost. A thousand warthogs, at least, out of the northeast. I don't know what they want. They had poor old chief Ngiri's head on a spike when they showed up, and they killed our translator without saying a word. They fight like fiends. No mercy.

The pigs have some kind of monster with them -- what sort of great evil this is, I cannot say, but none can stand against it. 
I can't listen to the screaming anymore. I don't want to die like that. Call me a coward if you want but I won't die like that.

Locks puzzle:

There are four locks controlled from a central operator's position. The operator is, of course, dead, so the party will have to figure out how to open the locks. If the party screws up very badly, there is another riverboat, intact but with a dead crew, on the upstream side of the locks just around a bend.

The locks are hydraulically powered. The water only goes downhill. The gates are operated by waterwheels in a race leading from the upper river level to the lower, which has a manually operated GATE VALVE, which is presently closed; a clutch engages each water wheel forward or reverse to open or shut each gate by means of a screw arrangement that prevents back-driving when the gate is retaining water. The clutches are operated by a pulley and cable system, ten handles, one "OPEN" (in Zebra) and one "CLOSE" (also in Zebra) for each gate, these cables pull the water wheel to engage one way or the other. (It reverts to a neutral position when neither handle is pulled.)

There are also five handles each labeled "MAIN". When the "MAIN" handle is opened, the boat-passing gate itself moves, otherwise the small gate to allow water movement only is actuated.

A note in Zebra reminds the operator "Sluice for the gate-moving wheels to be closed when the lock is not in use. Wear on the machine has become excessive."

https://docs.google.com/drawings/d/1Ko8NO0Fwb-XLJOPGDOHJg8rQKv36SsITmvFB2zM81f4/edit

Deadhorse Station.