Proof-of-concept test successful today.

A small payload wrapped with, but not sealed inside, a zip-lock bag was tied to the upstream end of two 10 foot long branched. The branches were overlapped 3 feet and lashed in three places along the overlap.

The downstream end of the lashed branches was tethered to a small bush. When the upstream end and payload were pushed into the current of a 12 foot ditch filled with fast flowing water, the current relatively rapidly swung the floating "pole" across to the other side, as the downstream end stayed in place and "hinged" on its tether to allow the door-closing effect.

It appears to me this is a relatively low energy, low risk strategy compared to swimming or rafting.

The lashing was probably disproportionately heavy compared to paracord and small trees. If I can find the right place to try this full-size in warm water I expect the strength of paracord lashings to become a problem.