Plans to build a vast reservoir by the side of the Thames in Oxfordshire are under threat - from a small, rare freshwater mussel that has made the river its international stronghold.

Plans to build a vast reservoir by the side of the Thames in Oxfordshire are under threat - from a small, rare freshwater mussel that has made the river its international stronghold.

According to Thames Water, a five-square-mile reservoir may have to be built on farmland near Abingdon to meet the extra water demand if the Government accepts official planning advice to build 1.1 million new houses in south-east England by 2016.

But a survey of river life that it commissioned last year has revealed that the Thames may contain one of the world's largest populations of the depressed river mussel, a threatened species listed in the Government's "Biodiversity Action Plan" as being of "particular conservation concern", at risk from water pollution and disturbance of its habitat.

The survey, by Dr David Aldridge, found five species of mussel in the Thames near the proposed reservoir site, at densities of up to 200 individuals per square metre. Of the 1,674 mussels collected in the survey, 2 per cent were depressed river mussels, which is enough to make the Thames one of the most abundant known sites for the species.

The report adds that the mussels are "keystone fauna" in the river because, as filter feeders, they reduce turbidity in the water, allowing light to penetrate and stimulate the growth of aquatic plants, thus creating habitats for fish and invertebrates. Mussels are also a food source for other species, such as otters.

Jean Saunders, co-ordinator of Oxfordshire Friends of the Earth, says the discovery of the depressed river mussel should make Thames Water think twice about its reservoir plans. "The quality of the water will be affected by the building of the reservoir, and that could affect the mussels and other wildlife in ways that are hard to predict," she warns.

The Environment Agency, whose permission is needed for the new reservoir to go ahead, is investigating the possible impacts of the reservoir.

"It is too early to say if the reservoir would be bad for the mussel," said its water resources planning manager for the Thames region, Brian Arkell. "But we have spent a lot of time talking to the company about the species in the river and the risks they might face under alternative flow regimes. All these matters need to be addressed before any promotion of these reservoir plans goes ahead."

A spokesman for Thames Water said: "The reservoir is only one of many options that we are assessing, and we have yet to decide how to proceed."