South Africa's newest Ramsar site is situated in the upper reaches of the Nyl River near the town of Naboomspruit in the Northern Province, and includes part of the largest floodplain vlei in the country. The reserve is well known for its exceptional avifaunal diversity, with 370 species recorded, of which 102 are waterfowl. During good rainy seasons the floodplain becomes a hype of activity, with the best estimate for water bird numbers being approximately 80 000. The system also plays an important role for frogs and fish, which gather in large numbers to breed when the floodplain becomes flooded.

Of the 102 waterfowl species recorded on the floodplain, 58 are known to breed in the wetland, more than on any other South African wetland. Twenty three of the waterfowl species are Red Data listed and eight of these are known to breed here. The number of bird species recorded on the entire floodplain is 412, almost half the total number of bird species in South Africa. The striped crake Aenigmatolimnas marginalis and streakybreasted flufftail Sarothrura boehmi have not been recorded anywhere else in the country, and this is the only site in South Africa where rufousbellied heron Butorides rufiventris has been known to breed. The floodplain also supports more than 1% of the national populations of great white egret Egretta alba, squacco heron Ardeola ralloides, blackheaded heron Ardea melanocephala and blackcrowned night heron Nycticorax nycticorax.

Breeding of endangered Roan antelope Hippotragus equinus, which use the floodplain extensively, and rare Tsessebe Damaliscus lunatus is another of the conservation priorities and appealing features of the reserve. Also of particular interest are the stands of wild rice, Oryza longistaminata. The reserve and parts of the floodplain are the only recorded localities for this plant species in South Africa.

Nylsvley Nature Reserve is regarded as one of the premier birdwatching localities in the country. Most of the 10000 visitors who visit the reserve each year are birdwatchers. Scientific research is also an important component of the reserve, making it one of the most intensively studied sites in the world. Various international scientists still conduct scientific research in the reserve and surrounding areas of the floodplain.