The site consists of areas of calcareous fen wetland and associated carr woodland and meadow along the Little Ouse river which marks the county boundary. It is notable for being an internationally important site for the rare black bog rushSchoenus nigricans and saw sedgeCladium mariscus plant species. The site supports a "very large number of plant species" on the wetter fen areas. Dominant species tend to be black bog rushSchoenus nigricans, saw sedge Cladium mariscus and purple moor grassMolinia caerulea, with species such as fen orchidDactylorchis praetermissa, devil’s bit scabious Succisa pratensis, long-stalked yellow sedge Carex lepidocarpa, quaking grassBriza media, parnassus Parnassia palustris and a number of rare mosses also found in these areas. Other areas of the fen become drier during periods of dry weather and support a different range of plant life. Taller vegetation is more common, particularly reed Phragmites australis and meadowsweet Filipendula ulmaria. Different plants are associated with this vegetation, increasing biodiversity to include species such as hemp agrimonyEupatorium cannabinum, purple loosestrifeLythrum salicaria and great hairy willowherbEpilobium hirsutum. Scrub and woodland vegetation has developed throughout the fens with sallow and alder predominant in these areas, although Blo' Norton Fen has some oak and ash woodland. Surrounding meadowlands and ditches support other species, including, for example, ragged robinLychnis flos-cuculi, marsh marigoldCaltha palustris. Bird species found on the site include snipe and grasshopper warbler and the protected Eurasian water shrew is found in the area.
Management
Management includes the managed cutting of fen vegetation on a four year rotation in order to maintain biodiversity and a range of habitats. The adjacent area of Hinderclay Fen was removed from the SSSI area in 1983 following drainage projects causing the area to dry out. The LOHP has begun to manage the area to return the area to a more semi-natural state, including cutting scrub vegetation and creating ponds on the site. The fen areas were traditionally used for sites for cutting peat for fuel and for fuelwood. Marked paths, including areas of boardwalk and a footbridge across the Little Ouse, provide access for visitors and the Angles Way path passes through the SSSI.
Access
There is access to Thelnetham Fen from Loggers Lane and Blo' Norton Fen from Fen Road. The fens are connected by a bridge across the River Little Ouse, and the Angles Way footpath runs through Thelnetham Fen.
