On 10/15/2015 12:53 PM, J wrote:
A few years back (~5) I built a rustic bridge over our seasonal stream by
laying deck boards over a couple of tree trunks (5"-6" diameter) that I cut
down. The stream channel is about 16' from bank to bank and ~4' deep. I
cut the tree trunks to about 20', so about 2' of contact with the ground on
either end.

The bridge was a little bouncy, but otherwise seemed structurally sound -
that is, until now. I had hoped that by placing the ends of the tree trunks
on gravel and some pressure-treated 4x4s it would last for many years, but
the trunks have rotted in the middle and begun to crack, and the bridge is
no longer safe.

The reason I cut down a couple of trees in the first place, was that even
though it apparently exists, it's very hard to get ahold of 20' pressure
treated lumber. I had thought that a couple of 4"x6"x20' beams might work
well to support the bridge - though I never did any actual load
calculations.

I'm not feeling the love for the work involved of cutting another couple of
trees every 5 years, so I'm looking for a better - and hopefully simple
solution. I don't really have the energy or skill for something fancy. If
I could get the 4"x6"x20' pressure treated beams (and a friend to help move
them), and if they would work in this application, I would do that. But
failing that - is there another way to solve this problem? (like maybe
nailing together some shorter overlapping 2"x6" boards?).

Trying to keep it as simple as possible. Thanks in advance for any ideas.

What sort of *traffic* are you trying to accommodate? How accessible
is this area? What sort of maximum water flow do you have to accommodate
(obviously, "0" is your pictured low end! : )