The Nedern Brook Wetland is a flood meadow habitat near Caldicot, Monmouthshire, Wales, designated since 1988 as a Site of Special Scientific Interest (SSSI).

[2] The site comprises flat low-lying flood-meadow in the shallow valley of the Nedern Brook, which flows from the north-west near Caerwent to the Severn estuary between Caldicot and Portskewett.

The temporary lake that is formed by the flooding, similar in some respects to a turlough, can be up to 1.5 kilometres (0.93 mi) long, up to 30 hectares (74 acres) in extent, and up to 2 metres (6 ft 7 in) deep.

[1][3] A hydrological study by the British Geological Survey, published in 2016,[4] noted that the Nedern Brook had in the past been deepened and canalised, though the paleochannels of the old river meanders can still be seen.

The study found that "as water levels rise in the Carboniferous limestone aquifer below the site, eventually reaching the surface, springs and seepages result in the formation of the temporary (ephemeral) freshwater lake.

This may have been the result of natural processes, possibly accompanied by active land reclamation in the mediaeval period, and certainly exacerbated by work relating to the excavation of the Severn rail tunnel in the 1870s.