An ongoing supporter of the area, Sir Oliver, MP for North East Hertfordshire, has previously supported a successful campaign to stop water abstraction from the Mimram River.
The MP was invited to visit Tewinbury Nature Reserve, a diverse reserve lying in the Mimram Valley, managed by the Herts and Middlesex Wildlife Trust.
Tom Day, Head of Herts and Middlesex Living Landscapes, said: "We were pleased to welcome Sir Oliver to our Tewinbury Nature Reserve. His continued interest in the rare chalk stream is valued and we were delighted to show him what else the reserve has to offer. We demonstrated riverfly monitoring, kick surveying and discussed mink and water voles among other elements of the reserve during his visit.
"Visitors to Tewinbury at this time of year will be able to see the attractive pink marsh orchids in full bloom and can keep their eyes peeled for water voles swimming in the reedbed channels as well as explore a variety of habitats including wet meadow, tall fen and swamp, alder carr, willow scrub, willow pollards and chalk stream."
Sir Oliver Heald QC MP, Member of Parliament for North East Hertfordshire, said: "I pay tribute to the work of the Herts and Middlesex Wildlife Trust. Chalk streams are rare internationally and have a precious environment with special plants and creatures. It is important to look after them for future generations. I have been involved in successful campaigns to improve river quality and it was great to have the chance to see this thriving Nature Reserve on the River Mimram at Tewinbury."