How:
The Laird was returning from a shooting trip up the glen and the ford that they'd crossed going had become a lot deeper and faster due to the fact it had been raining constantly all day.
The Range Rover had a moment as the force of the water drove it down stream slightly but it made it out however, the Argocat on the trailer was not so lucky. It was swept down stream by the force of the water, grounding a couple of hundred yards down.
The Laird and party drove down to the farm for the tractor with the lifting bucket on the front to retrieve it.
I should point out that the ground on both sides of the burn and peat bog and are only drivable in the winter due to the frost in the ground.
With the tractor attached to the Argo, they started to lift, the Argo came off the rocks and the force of the water started her going again, toppling the tractor over on to its side, but at least the Argo was attached to something!
By the time the team had been back to the farm for 2 more tractors (its 5 miles each way over rough tracks)
the light had gone and the recovery of the 1st tractor was in the dark during which the rope securing the Argocat snapped and it was swept further downstream and over a 10-15' waterfall where it came to rest and where we came in.
Unfortunately I do not have any photographs from the above.

The Story:

Arriving on site early on a Saturday morning, we were guided to the river and told "It's down there!"

As you can seen from the photo, there's not a lot to see. The pool it was in is at the bottom of the waterfall and about 25' below the surrounding ground with vehicle access on one side only. All the rocks about the pool were covered with ice so salt was spread and hammers used to make safe a working area and access route.
The reason that our "Splash Test Dummy" had been standing in the snow in his shorts was that he was kited out in a WET suit, under a set of waterproof overalls so if anyone was going to get wet........

The first problem was the Argo was stuck under a rock ledge on the same side of the pool as us, so a lot of time was wasted trying to find an anchor point on the opposite bank for the winch cables. Ground anchors just pulled out of the peat so in the end some large rocks were used involving every strop and winch extension cable carried (about 250') just to hold the snatch block.
The other major problem was that, in case you don't know, Argocats are made of plastic/fiberglass so where to attach the cables too had been causing some concern, fortunately (for us)  it was stuck nose in so the tow bar was accessible and this was used in conjunction with the roll cage.

With a mixture of lifting, pulling and swearing it slowly started to break the surface. For a "plastic" tub the weight was incredible! This was investigated and our "man in the water" spent about 1/2 an hour removing stones from the hull. Because it was at the bottom of the waterfall, all the silt had been collecting in it, everything from small pebbles to stones 3' long. The small stones had got every where including into the chain box's jamming all the drive mechanism which was going to cause us problems later. The Argo had been in the water for 1 week and we reckon that we removed about 1/4 ton of stones, I'm glad we did not have to wait for summer.

Due to the nature of the location, the easiest was out of the hole was up the rock face above where it had been stuck. In the photo, you can see the lock off rope on the cage whilst yet more stones were removed, and the cables onto the tow bar.

After several reriggs of the winch gear, it made it to the top and the full extent of the damage could be seen.

Out of 8 wheels, there was only one tyre still on the rim, the others were full of silt and the rims battered and bent. Most of the axles were damaged, either bent, smashed bearings or snapped. The chain box's inside had not fared much better, both were bent and full of silt. 

The body had not fared much better, having been battered by the rocks during its final voyage downstream and with several panels missing of which there was no sign. 

Conclusion:

Was it worth it?
Yes for several reasons, if left it would have caused pollution to the burn, various parts are salvageable for spares and what else do you do on a Saturday.