I also have taken advantage of low water and built a bridge across one spot on Possum creek...workin on a dock also...l had available to me some old elec poles (about a foot or more in diameter) that I cut and used for posts. I think that 4x4's might tend to sink once all the weight is added due to the small surface area. Adding concrete around the base might give them a bigger "footprint"...I would like to know also about how long the treated wood will last as I used it for the framing and deck on the bridge