What's a waterproof material to replace a wood pole with?

I use irrigation dams to flood a few of the fields at work, like the one in the picture. It's got a 3 inch diameter wood pole running through the end of the dam. Any thinner than that and the pole usually breaks. The poles range from 10 foot to 16 foot long. My problem is these poles tend to rot out after a... show more I use irrigation dams to flood a few of the fields at work, like the one in the picture.

It's got a 3 inch diameter wood pole running through the end of the dam. Any thinner than that and the pole usually breaks. The poles range from 10 foot to 16 foot long.

My problem is these poles tend to rot out after a couple years and break due to the mold and moisture.

I'm just looking for ideas of something else that's as strong as a wood pole, but is waterproof, and hopefully not too much heavier than the wood, but just as strong, if not stronger. Also, I don't think paint would make much of a difference, and be too expensive.
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