Cow Green Reservoir is a 2-mile (3 km) long water reservoir forming part of the border between the historic counties of Westmorland and County Durham in northern England, built between 1967 and 1971 to supply the industries of Teesside.

These regulated flows allow six further reservoir levels to be maintained and these in turn provide water for industry and domestic use.

Their main concern was the protection of the rich flora and fauna of the district and especially rare alpine plants like the unique Teesdale violet.

The reservoir lies within the North Pennines Area of Outstanding Natural Beauty and European Geopark.

Cow Green Reservoir is also the largest reserve of wild brown trout in the North East.