One of the two datasets that make up the Priority River Habitat Map. Consists of rivers and streams that exhibit a high degree of naturalness. The naturalness classification used to map priority river habitat is based on recent work to review the river SSSI series. It evaluates four main components of habitat integrity: hydrological, physical, physico-chemical (water quality) and biological. An additional classification of the naturalness of headwaters (defined as streams with a catchment area of <10km2 to coincide with WFD typology boundaries) uses land cover data as a surrogate for direct information on river habitat condition (information which is generally lacking on headwaters). Streams and rivers operating under natural processes, free from anthropogenic impact and with a characteristic and dynamic mosaic of small-scale habitats that supports characteristic species assemblages (including priority species), are the best and most sustainable expression of river ecosystems. Key elements are: a natural flow regime; natural nutrient and sediment delivery regimes; minimal physical modifications to the channel, banks and riparian zone; natural longitudinal and lateral hydrological and biological connectivity; an absence of non-native species; low intensity fishery activities. These conditions provide the best defence against climate change, maximising the ability of riverine ecosystems to adapt to changing conditions. They also provide the most valuable and effective transitional links with other priority habitats, including lakes, mires and coastal habitats. In English rivers and streams, high levels of naturalness are rare.Full metadata can be viewed on data.gov.uk.