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It added "the river was running fast and the weir was in an extremely hazardous condition", and three of the participants became trapped after being pulled over the top of the Haverfordwest Town Weir.

The study found the highest levels of genetic isolation in rivers that were both polluted with metal and affected by physical barriers such as dams and weirs.

This showed how it is possible to maintain the full tidal range within existing dams or weirs.

The restoration of riverside forests, natural meanders in waterways and weirs in the mountains would all help diminish flooding, he said.

The city controls several weirs where water is diverted.