bittsen: It would appear that the cache evolved and either grew legs or gills.
There was substantial evidence that the flood of 2011 was, indeed, high enough to affect the cache. Though it was considered too heavy to be carried or to float, it's not where it should be and neither is all of the cover it once had (and a tree is missing too).
Either way it seems to have gone further West.
I shall archive this cache and remember it well.