It flows through Chester then re-enters Wales; the final section is canalised and discharges to the Irish Sea via an estuary 23 km (14 miles) long.

The larger reservoirs in the catchment area are: The River Dee has its source on the slopes of Dduallt above Llanuwchllyn in the mountains of Snowdonia in Meirionydd, Gwynedd, Wales.

Skirting the village of Llanfor, the path of the river takes it past Llandderfel and under the Grade II listed Pont Fawr bridge.

Leaving Gwynedd and entering Denbighshire the Dee flows beneath other historic bridges at Llandrillo and Cynwyd before arriving at the town of Corwen.

From here the river passes the Iron Age hillfort of Caer Drewyn and enters the Clwydian Range and Dee Valley AONB.

Overlooking the river here is the medieval Castell Dinas Brân, a ruined fortress abandoned by John de Warenne, 6th Earl of Surrey in 1282.

The river then enters Wrexham County Borough, passing south of Trevor and under Thomas Telford's Pontcysyllte Aqueduct, of 1805, which carries the Llangollen Canal 120 feet (37 m) overhead.

Beyond this point the river forms the boundary between Wrexham County Borough in Wales and Shropshire in the West Midlands of England.

The course continues past Erbistock on the Welsh side, and the 5th-century earthwork of Wat's Dyke on the English, before passing wholly into Wales at Overton bridge.

One of the major tributaries of the Dee, the River Alyn (Afon Alun), crosses the Carboniferous Limestone from Mynydd Helygain (a.k.a.

Throughout the length of the Alyn there are numerous swallow holes and caverns and during the summer months long stretches of the river bed run dry.

Once the main River Dee approaches the Cheshire border and the Carboniferous Coal Measures, it turns sharply northwards before meandering up to Chester.

The rich adjoining farmland has many remnants of abandoned coal workings and deep clay pits used to make bricks and tiles.

Approaching Churton and Aldford, the river crosses entirely into England, and passes the grand country house of Eaton Hall, seat of the Duke of Westminster.

The same weir was used as part of a hydroelectric scheme in 1911 with the help of a small generator building which is still visible today, used as a pumping station for water since 1951.

It was destroyed in the Civil War but an octagonal tower built in 1690 for the same purpose lasted until the gate was replaced with an arch in the mid-18th century.

This used to be the site of Chester's Roman harbour until, aided by the building of the weir, the River Dee silted up to become the size it is today.

The only curiously remaining reminder of this site's maritime past is a stone cross which stands in the middle of the Roodee which exhibits the marks of water ripples.

After four years' work, the river was diverted from its meandering natural course which passed Blacon, Saughall, Shotwick Castle, Burton and Parkgate and up the west shore of Wirral.

South of Bagillt and Parkgate the Dee Estuary forms the boundary between the local authority areas of Flintshire and Cheshire West and Chester.

The waters of the Dee then converge with those of the River Mersey and Ribble, producing a notable drop in salinity and increase in sediment which can be tracked a considerable distance along the Sefton and Lancashire coasts.

Its value lies in the huge expanses of mud which are exposed between tides and the extensive saltmarsh developed on both sides but principally on the right bank north and south of Neston.

The channel continues inland south of Chester but its higher reaches have long since been infilled with sand, gravel and mud.

The process of infilling by mud continues to the present day as the rapid growth of the saltmarsh in the last century testifies, pushing the high tide line further out into the estuary.

From Chirk downstream, the river valley has supported a wide range of industries that were initially drawn to the area by the presence of coal mines and later by the deep deposits of Carboniferous clays used to make bricks and tiles.

Canoeing used to be allowed on about twelve weekends per year, and tens of thousands of canoeists descended on Llangollen for recreational paddling (several Dee tours were held every winter), slalom competitions, and wild water races.

[13] The river has been designated as a Special Area of Conservation because of its role as a habitat for Atlantic salmon and floating water plantain.