It is 3.5 miles (5.6 km) long) and covers an area of 4 km2 and has a maximum depth of 100 ft (30m) and when full, holds 11,000 million gallons (50,000,000m³).

It is the principal water source for the Tyne and Wear metropolitan area, supplying 28,000,000 imperial gallons (130,000,000 L; 34,000,000 US gal) per day.

Building work began on the reservoir in 1960, and it was opened in July 1967 by Princess Alexandra.

Sediment within the reservoir has been found to contain elevated levels of zinc, lead and cadmium.

These have drained into the reservoir from areas of historic mining activity around Blanchland.