Cow Green reservoir

Year:1971

Duration:4 years

Cost:£2.2m (£31m today)

Country: Teesdale, UK

What did this project achieve?

Build a big reservoir to supply water to the manufacturing industries of Teesside

Cow Green is a 3km (2 miles) reservoir in County Durham. At 480m it's one of the highest placed in England.

The structure is 25.8m deep with a surface area of 3,219,000m². It serves a catchment area of 59km².

Cow Green is a regulatory reservoir. This means it releases water into the river Tees during dry conditions so that it can be removed further downstream.

Plans for the reservoir faced nearly a decade of protests as this part of Upper Teesdale was considered environmentally important. Campaigners were concerned that rare plants such as the Teesdale violet would be threatened by the project.

About a tenth of the Teesdale violet's habitat was destroyed by the reservoir's construction. The rest of the area was designated the Moor House-Upper Teesdale National Nature Reserve – England's largest reserve of its type.

Cow Green was the last of 3 reservoirs built in the Tees Valley to meet the growing needs of local industry at the time.

The others were Selset and Balderhead reservoirs. Selset was completed in 1960, Balderhead in 1965.

Difference the reservoir has made

Cow Green continues to supply water to thousands of homes and industries in Teesside to this day.

The expensive and protracted disputes in the years before the reservoir was built were viewed as contributing to a growing mistrust between the water industry and environmentalists.

How the reservoir was built

Cow Green reservoir was created by building a dam at the point where a flat area of the river Tees crossed land near an old mine. The mine was previously dug for barium sulphate – a mineral used for making paint.

The first day's work on the project started with a bang when Sir Charles Allison, chairman of the local water board, hit the plunger on a detonator. This set off an explosion which blew a crater on the reservoir site.

Sir Charles may have chosen this violent start to the project to make a point. He was infuriated that environmental campaigners had delayed the reservoir for nearly 10 years.

The next 4 years saw engineers build the 25m high, 550m long dam to hold back 40,000m litres of water.

The project team paid close attention to the design of Cow Green's core following erosion problems with the clay core of recently completed Balderhead reservoir.

Engineers decided on sand drains for Cow Green as they thought this would cut down seepage from the reservoir. Sand drains use small particles to filter impurities from water.

Around 300 people made up the construction team.

"​‌

We as a nation seemed to be so obsessed in preserving the past that we had become blinded to the need for future prosperity.

Sir Charles Allison

Chairman of the Tees Valley and Cleveland water board, speaking on the first day of Cow Green's construction in 1967.

Fascinating facts

Cow Green's high altitude means conditions are too extreme for most birds as the water often freezes over in the winter. Species that do visit include teal, a small breed of duck, and goosander, a breed of diving duck.

Water from the Tees falls over a series of cataracts at the eastern end of Cow Green, beyond the dam. The cataracts form a rocky 'stairway' called Cauldron Snout. The 'stairway' is 200ft (61m) high – making this the highest waterfall in England.

A drought in 1984 made Cow Green's water levels drop so low that a Bronze Age farmstead appeared. Archaeologists were able to study it before the water rose and covered it again.

People who made it happen

  • Chief engineer: MF Kennard
  • Contractor: Mitchell Construction

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