[potlatch]

Wonder why they wouldn't just rust away? Don't know what all metals they used. How interesting to be able to search for things like that.

[devolve]

Metals and woods can start having problems as soon as you take it out of the water it has been submerged in.

Hard to imagine iron or steel not rusting away over the many centuries.

Better they dive than I -

[Drammach]

According to the National Geographic article I linked above, the river's bed is a very fine grain sediment that preserves the artifacts..
I would guess it is much like the peat bog mummies, etc., in that the items are protected from oxygen, therefore rust, oxidation does not occur...
Apparently this works well with not only metals but wood, fibers, etc..