[2] Northumbrian Water's operations cover an area of 9,400 km2 and extend from the urban conurbations of Tyneside, Wearside and Teesside to the sparsely populated rural districts of Durham and Northumberland.

[4][6] Water is abstracted from the Wear at Chester-le-Street to supply a treatment works at Great Lumley, the minimum flow being maintained through discharges from Burnhope or Tunstall.

[6] Coastal parts of the Central zone, including Sunderland, are supplied with drinking water from boreholes and shafts that abstract groundwater from aquifers in the underlying Magnesian limestone.

[4] The largest reservoir in the Southern zone is Cow Green, in upper Teesdale, which is used solely to regulate flow in the River Tees.

There are two chains of reservoirs on the Lune and the Balder, tributaries of the Tees, which in combination supply a water treatment works at Lartington, just south of Cotherstone.

[6] Two further reservoirs, at Lockwood Beck and Scaling Dam, on the North Yorkshire Moors, are no longer used for water supply and serve purely as recreational facilities.

The tunnel is fed by a pipeline, nearly 4 miles long and 6.5 ft (2 metres) in diameter, between Riding Mill, on the Tyne, and Letch House, the high point in the system.

Transfers to the Central zone are usually required for a period of 12 weeks in the summer, when the reservoirs supplying Honey Hill are unable to meet the full demand.