Well, I think the culvert idea is a non-starter.

Even if you didn't have to worry about environmental rules around a stream,
putting any kind of structure "in" the stream seems like a bad idea.

I'm not sure I'd be able to really get away with a 16' span - at least
not without some really major effort to prevent the banks from
collapsing where I'd have to put the pier. I don't think I'd be able
to pour a pier at all - I think at best, I would just get some
pre-cast concrete footers like they make for decks.

For some reason I was thinking a concrete beam/block on each end, something
like 8"x12"x36". That's about five 60# bags of concrete per beam. Not too
bad if you can get the bags to the site. I would add a couple pieces of
rebar in each block for extra strength before pouring the concrete.

getting the 20' length is difficult.

I had no trouble getting 24' 2x12's for the ceiling joists in our garage or
the rafters in our house. Of course, those were standard framing lumber,
not pressure treated. That might make a difference.

You could always use regular framing lumber and apply a good coat of deck
stain on all surfaces. Then add flashing to the top surfaces before
installing the decking. Just make sure the ends rest on pressure treated
lumber and not directly on the ground/blocks.

I'll want to figure out my plan before getting a lumber yard to
procure this for me. Would they really bother helping me with this
for just two or 3 boards?

The lumber yards I've dealt have been happy to sell me a single board or
box of screws.

Getting 20' boards home will be the real challenge. Mine were delivered as
part of a much larger order. Maybe you could rent a U-haul truck or
something.

dpb suggested a 3' flat plate to hold two 10' pieces together at the
middle. I'm not sure exactly what you mean by that - searching HD and
Lowes I only saw long narrow metal bars. What do others think of this
solution?

I don't know if a 3' plate would be sufficient over that span, especially
if the joint was in the middle.

I mentioned a box beam earlier, but you could probably build a laminated
beam of sorts. Sister a couple of 2x joists together, staggering the seams,
with a good dose of construction adhesive between them. Then drive in a
couple rows of lag bolts every 12" to hold everything together.

Having full length joists would be safer, cheaper, and easier.

Anthony Watson
www.mountainsoftware.com
www.watsondiy.com