For many the white-clawed crayfish (Ustropotamobius pallipes) may not be the first species they associate with rivers but this globally endangered freshwater crustacean is the main reason that the River Wensum has the Special Area of Conservation E.U. status.
White-clawed crayfish require high levels of calcium, so are found mainly in chalk rivers such as the River Wensum, however they are not very tolerant of pollution or changes in water quality or river morphology. The main issue facing the white-clawed crafish is the introduced and invasive signal crayfish (Pacifastacus lenisculus), these were introduced in the 1970's from North America, escaping from farms. The bigger and more aggressive signal crayfish out-competes the white-clawed and also carries a fungus which can cause high mortality rates for our only native freshwater crayfish.
Water crowfoot and starwort create floating mats which are very important habitat for invertebrates and young fish. These plants are very sensitive to changes in flow and water levels as well as pollution.
The River Wensum is recognised as an important location for ranunculus and the River Wensum Restoration & Floodplain Enhancement Project aims to restore conditions so that this key habitat can flourish.
A splash in the water as you walk along a river bank may be the only evidence of water voles (Arvicola terrestris), occasionally you will have a glimpses as they head back to their burrow. Water vole numbers are in decline due mainly to loss of habitat and predation by mink.
Water vole surveys carried out at Pensthorpe have shown that there is a healthy populations particularly near the restored Meander Loop section of the River Wensum. Future work on the river aims to restore the required habitats so that we hear those splashes more often.