Known in the archaeological record as Boarland Rock 16, this modest structure sits in the middle of the estuary, north of a scattering of low-lying islands, and was only identified as recently as September 2009. That such a feature could go unrecorded until the twenty-first century says something about the nature of intertidal archaeology, a field that depends entirely on tide, season, and visibility conditions to reveal what ordinarily lies submerged.

The weir is what is known as an ebb weir, a type of fish trap designed to exploit the outgoing tide. As the water recedes, fish are funnelled along two converging fences into a narrow trap at the apex, where they can be collected. At Boarland Rock 16, the shore fence runs roughly east to west and extends eighteen metres, while the flood fence, running north to south, measures eight metres. The two meet at a funnel fitted with a basket of small dimensions, just 0.2 metres by 0.1 metres, where the catch would have been held. Fragments of horizontal timber have survived at this funnel end, a rare physical remnant in a landscape where organic materials seldom last. A separate line of posts, eight metres long, extends southward from the easternmost end of the shore fence, though researchers have not been able to confirm whether this feature belongs to the same structure or represents something else entirely.