Cow Green Reservoir lies equally within Cumbria and County Durham where the boundary follows an equidistant line down the length of the reservoir between the inflow and outflow of the River Tees. It lies wholly within the North Pennines AONB and separates the two distinct sections of the Moor House – Upper Teesdale National Nature Reserve. The reservoir itself is approximately 3.5 km long, 1 km wide and 26 m deep with a surface area of 3.2 km2 supplied from a catchment area of ~59 km2.
It was built to supply water to service the growing industrial demand from Teesside, the last of three designated for that purpose the others being Selset (1960) and Balderhead (1965). Because of a range of sensitive environmental issues its construction had been delayed for several years before beginning – at least ceremonially – in October 1967. When it was completed in 1971 it superseded Chew Reservoir in the Peak District as the highest reservoir in England. Since then demand from industry has dwindled but it maintains a regulatory role supplying water to top up levels for extraction downstream.
The dam wall is 25 m high and 550 m long with the eastern end being built on the hard, volcanic rock of the Whin Sill which is at its thickest in this part of Upper Teesdale (around 70 m) while the western end is an earth bank built on top of a pre-glacial (>115,000 years ago) channel in which the Tees once ran. During the period the channel filled with boulder clay as a result of glacial activity forcing the re-established river to carve out a new course over the Whin Sill. The original channel can still be traced to the western side of the foot of Cauldron Snout.
Its location takes advantage of a natural lake previously known as The Weel which was described in 1833 by the mining engineer Thomas Sopwith as follows: “The Weel is a sullen lake, in breadth about half a mile and three of four times that length, formed by the interception of the Tees by basaltic rocks over which the water flows into a deep and rapidly descending gorge and forms the waterfall of Cauldron Snout”. After extended periods of heavy rain the volume of water flowing over Cauldron Snout could increase to catastrophic levels. The resulting deluge known as the ‘Tees Roll’ was powerful enough to damage bridges and property as well as taking lives far down the dale.
Location NY 814290 (Tees Outfall)
Darlington & Stockton Times. “50th anniversary of reservoir work is a blast from the past.” Darlington & Stockton Times, 13 October 2017, https://www.darlingtonandstocktontimes.co.uk/lookingback/15595230.october-13-50th-anniversary-of-reservoir-work-is-a-blast-from-the-past/. Accessed 13 October 2020.
Institution of Civil Engineers. “Cow Green Reservoir.” Institution of Civil Engineers (ICE), 2020, https://www.ice.org.uk/what-is-civil-engineering/what-do-civil-engineers-do/cow-green-reservoir. Accessed 13 October 2020.
Natural England. Moor House – Upper Teesdale National Nature Reserve. Natural England, 2014. http://publications.naturalengland.org.uk/publication/30049. Accessed 13 October 2020.
North Pennines AONB Partnership. Cow Green Geotrail – Layers & Landscapes. North Pennines AONB Partnership, 2019. https://www.northpennines.org.uk/wp-content/uploads/2019/09/Cow-Green-Geotrail.pdf. Accessed 13 October 2020.
Sopwith, Thomas. An Account of the Mining Districts of Alston Moor, Teesdale and Weardale. W. Davison, 1833. Google Books. Accessed 06 October 2020.
“Cow Green Reservoir” Wikipedia, Wikimedia Foundation, 02 10 2020, https://en.wikipedia.org/wiki/Cow_Green_Reservoir. Accessed 13 October 2020.