The water vole is one of the UK’s fastest-declining mammals.
Nationally, since the 1960s, its population has fallen by 90% due to habitat loss and predation by invasive American mink, the trust added.
The Ayot Estate and the angling club worked with the trust to prepare the site, and created a “welcoming environment for this important native species”, it added.
Mr Kalms said: “Water voles eat a huge range of plants species and in doing so, they move seeds around, helping to maintain varied plant cover and creating lush and wild bankside vegetation.
“They are also a key food source for native species like stoats, predatory fish, herons and birds of prey.”
The water voles, bred by the Derek Gow Consultancy in Devon, were released into temporary riverside pens to acclimatise before establishing their own burrows.