A new lake for Wales?

The low-lying valley in which the ephemeral lake forms

A recent hydrological study shed light on an unusual wetland in South Wales. During the autumn and winter a shallow lake, up to 2 m deep and 1.5 km in length, can form, only to dry up completely during the spring and summer. The lake is not marked on any maps, neither does it have a name. It is partially located within the Nedern Brook Wetland Site of Special Scientific Interest (SSSI), near the picturesque Caldicot Castle in Monmouthshire. The wetland is a popular site for overwintering birds, especially waders and waterfowl including Bewick's swans, redshank and widgeon.

Situated in a broad valley, the wetland is underlain by glaciofluvial sand and gravels and Carboniferous limestone bedrock. A small main river, the Nedern Brook, flows north–south through the wetland. The brook has a long history of alteration, being both over-deepened and canalised, resulting in a loss of morphological and ecological status. It was due to these negative impacts that Natural Resources Wales (NRW) were required by European Law to consider options for restoration of the water course.

Flood depth map for Tir-Llyn Nedern

Previous studies in the area highlighted a lack of hydrological evidence (in both the brook and associated wetland) as the main knowledge gap prohibiting restoration plans. To help fill in the knowledge gaps we undertook a year of groundwater and surface water monitoring, recording the levels and flow to better understand the hydrological regime.

Our study, funded by NRW, suggested 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. The process by which the temporary lake occurs is not due to 'fluvial' flooding caused by the over-topping of river banks, but rather is a result of rising groundwater levels. This process is entirely natural and is dominated by the seasonal changes in groundwater levels coupled with the topographically low setting of the Nedern: it is just 5 m or so above sea level. The lake dries up as groundwater levels in the underlying aquifer recede during the spring and summer.

Naturally fluctuating wetlands that form ephemeral open water bodies are rare in the UK. They are rarer still when groundwater controls the flooding. Currently the only recognised examples of this 'aquifer fed, naturally fluctuating wetlands' habitat in the UK are the Norfolk Breckland Meres, a single turlough in south Wales and three turloughs in Northern Ireland, covering just 31 hectares combined. The Nedern Brook Wetlands, when flooded, cover an area of nearly 40 hectares and could potentially be a significant addition to this rare UK habitat. It is tempting to suggest that the Nedern Brook Wetlands be referred to as a turlough, but in Wales we need a name to distinguish our fluctuating wetlands. Thus we propose a new name for this ephemeral lake: Tir-Llyn Nedern. (Tir-llyn is 'land lake' in Welsh).

'Tir-Llyn Nedern'

Acknowledgements

Natural Resources Wales for funding of the study and numerous staff at NRW including Catrin Grimstead, Liz Lawrie-Meddins, Rob Bacon, Alan Price and Julian Woodman.

Reference

Farr, G.  2015.  Nedern Brook Wetlands SSSI Phase 1 hydrological monitoring.  British Geological Survey Open Report OR/15/038 for Natural Resources Wales.

Contact

Please contact Gareth Farr for more information.