The river rises in Snowdonia, Wales, flows east via Chester, England, and discharges to the sea in an estuary between Wales and the Wirral Peninsula in England. It has a total length of 70 miles (110 km).
The River Dee was the traditional boundary of the Kingdom of Gwynedd in Wales for centuries, possibly since its founding in the 5th century. It was recorded in the 13th century as flumen Dubr Duiu; the name appears to derive from the Brythonic dēvā: "River of the Goddess" or "Holy River".
The total catchment area of the River Dee down to Chester Weir is 1,816.8 km2 (701.5 sq mi). The estimated average annual rainfall over the catchment area is 640 mm (25 in), yielding an average flow of 37 m3/s. 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. Between its source and Bala Lake the river is known by its Welsh name, Afon Dyfrdwy. Legend tells that the waters of the river pass through Bala Lake and emerge undiluted and unmixed at the outflow. On leaving Bala the river meets its confluence with Afon Tryweryn and passes through the Bala sluice gates, part of the Dee Regulation System protecting communities further downstream from severe flooding. Skirting the village of Llanfor, the path of the river takes it past Llandderfel and under the Grade II listed Pont Fawr bridge. The river trends generally east-southeast through the Vale of Edeyrnion, shadowed by the B4401 Bala to Cynwyd road. 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. Through its forested valley the course takes it through Carrog, Glyndyfrdwy and Llantysilio, with the Llangollen Railway following the river on its route between Llangollen and Corwen.
At Berwyn the river passes over the manmade Horseshoe Falls, before picking up speed on a downhill gradient past the Chain Bridge Hotel and its historic pedestrian bridge. First built in 1814, and later refurbished by Henry Robertson in 1870, it was considered a marvel of early suspension bridge design. In 1928 the original bridge was destroyed by severe flooding and was rebuilt in its current form from original parts in 1929. The course of the river then takes it through Llangollen and under its 16th-century, Grade I listed bridge. The bridge is also a Scheduled Ancient Monument and considered one of the Seven Wonders of Wales. On leaving Llangollen the river continues east, generally skirting the outcropping Karstic limestone exposures of Eglwyseg Rocks (Welsh: Creigiau Eglwyseg). 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.