River Dee Geology

The River Dee (Welsh: Afon Dyfrdwy) flows 110 miles from it's source to Hilbre Island. Travelling through Wales and England and also forming part of the international border between them, the river rises in Snowdonia, flows north via Chester and discharges into an estuary between Wales and The Wirral. The lower reaches of the river are unusual in that comparatively little water occupies so large a basin. One theory of a contributory factor to the large basin is that once the River Mersey and/or the River Severn flowed into the Dee. A more recent theory, however, is that the estuary was not formed by water, but by ice being pushed southwards by the pressure of an icecap over the Irish Sea. The total catchment area of the River Dee up to Chester Weir is 1,816.8 square kilometres (701.5 sq mi). The average rainfall over the catchment is estimated to be 640 millimetres (25 in) yielding an average flow of 37 m³/s.



The history of the Dee has many interwoven layers. There is the geological record, from the Ordovician rocks at its source to the modern deposits in the estuary. There is a historical record starting with it's use as a trade route in pre-Roman times. A path of myth which places the young Arthur at it's source. And an industrial tale bringing gold, stone, wool and water downstream as the railways and canals crept ever upstream towards it's head. We make no apology for mashing-up these strands for the Dee is a river to be explored in its many moods on many levels.

River Dee geology through time


The River Dee rises on Ordovician volcanic rocks which formed when North Wales was a sea dotted with volcanic isalands near the south pole over 400 million years ago. These mineralised rocks include deposits of much mined gold and silver with copper, lead and zinc sulphides. It flows northwards along the Bala fault over Silurian rocks laid down as sediments in an ancient ocean, which were raised into the massive Caledonian mountain range when Wales collided with North America. Over 20,000 feet of these mountains have since been eroded away.

Near Langollen the Dee has cut through Carboniferous limestones, grits and coal measures which fueled the Industrial Revolution in the region and it emerges from the Vale of Llangollen, crossing a fault line onto a range of 200-300 million year old sandstones from the Permian and Trassic. The coal measures and later the sandstones seem to have been formed in the vicinity of a river flowing northwards from what is now France, wearing down the long-vanished Variscan mountains. All more recent rocks have been eroded away, and in the last few million years glaciation has left a layer of boulder clay on the Cheshire plain.

The Dee in its "modern" form has existed over these few million years, either as a river or a glacial flow in the ice sheets which covered North Wales and Cheshire. The river continues to erode in the Upper Reaches, and transport material through its Middle Reaches to deposit silt in the Lower Reaches. Geology had influenced history, with the Bala corridor and the Upper Reaches of the River Dee being used for settlement, cattle raising and communication since Neolithic times and the fertile Cheshire plain supporting a dairy industry that exported its surplus as Cheshire Cheese.

Nowadays, the Dee is one of the most regulated rivers in Europe, it supplies more water for public supply than the whole of the English Lake District and two-thirds of the river water is abstracted before the River Dee reaches the weir at Chester. The natural flow of the River Dee during most summers is insufficient to sustain this rate of abstraction, so a series of reservoirs have been constructed to store excess water available in wintertime and release it back into the River Dee during drier months. This system of low-flow regulation was used by Thomas Telford at the beginning of the 19th Century in order to guarantee a supply of water to the Ellesmere Canal. Telford constructed sluices at the outlet of Bala Lake to control the flow of the Dee downstream so that there was always sufficient water to supply the canal where it started at Horseshoe Falls.

The eventual silting of the Dee Estuary led to a downstream migration of Chester's port and the eventual canalisation of the River from Chester to Connahs Quay with "Sealand" reclaimed - on the English side so that land prices would be better, and the river channel cut so shallow that Liverpool became the dominant port in the region.

sources and links

 * Cheshire Trove on the Geology of the area;
 * Environmental Change and Mineral Formation in Wales;
 * Mineral Resource Maps of Wales;