The River Nar is a tributary of the River Great Ouse. It rises near Litcham in Norfolk and flows 15 miles west through Castle Acre and Narborough, joining the Ouse at King's Lynn.
The river rises from springs on the chalk uplands close to the 200-foot (60 m) contour to the south-west of Tittleshall, it provides habitat for sea trout, which are quite rare in East Anglia, while its banks are frequented by water voles and otters. 78 different types of river plants have been identified growing in or along it, which includes the Southern Marsh Orchid, while insects found include 12 different species of dragonfly. Grey wagtails, kingfishers, reed warblers and willow and marsh tits are some of the constituents of the bird population.
The geology of the upper river consists of chalk covered by a layer of boulder clay, making it one of only a few remaining fenland chalk streams. Rainfall is purified as it passes through the chalk, and the spring water, which is crystal clear, alkaline, and always cool, gives the river its chalk stream characteristic. For this reason, the whole of the river is a designated Site of Special Scientific Interest, one of only ten chalk streams which have been designated in this way in the United Kingdom.
[From Wikipedia (abridged)]