I've tried in vain to track down (in print and on-line) a healthy discussion about the interchangeability of the prepositions "from" and "of" in sentences like: The boat dock is downstream of/from the dam.

I realize of can indicate possession and that from can indicate distance or location. However, the boatdock is downstream (distance and location) but is also on the dam's downstream side (possessive).

Help me sleep at night!

Brandon