The River Avon System is a 475.9-hectare (1,176 acres) biological Site of Special Scientific Interest in Hampshire and Wiltshire, southern England.^[1] It is part of the Avon Valley and New Forest RamsarInternational treaty for the conservation and sustainable use of wetlands, signed by 18 countries in 1971. The UK signed in 1976, and as at 2025 has 176 Ramsar sites. sites,^[2][3] and of two Special Protection Areas: Solent and Southampton Water, and the New Forest.^[4][5]

The system consists of stretches of the River Avon and its tributaries, described by Natural England as “of national and international importance for their wildlife communities”. It contains more than 180 species of aquatic plants, and one of the most diverse fish species in the country. There is also a rich invertebrate fauna, along with mammals including water voles (Arvicola amphibious), water shrews (Neomys fodiens) and evidence of otters (Lutra lutra).^[6][a]The water vole’s scientific name was changed from Arvicola terrestris to Arvicola amphibious after the report was produced.^[7]