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.

Photos of the current bridge he https://www.flickr.com/gp/7865232@N08/9104G5

-J