Hydrology
There is a gauging station at Marham, which records river data by measuring the flow over a flume. The catchment area of the river above this point is 59.2 square miles (153.3 km2) and the area receives an average of 27.3 inches (693 mm) of rain each year. This results in an average daily flow of 22 million gallons (100 Megalitres (Ml)), although flows reached nearly twice this value in February 1977. 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. At its source, the river is formed by surface runoff from the clay soils, but this is soon supplemented by springs in the chalk aquifer to the east of Litcham, which create a rapidly-flowing chalk river. 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.
In 2011 the Nar was recognised by the Environment Agency as one of the top ten most improved rivers in England and Wales, and was ranked seventh in the list. Historically, the engineering of the channel, to make it straighter, deeper and wider, first for navigation and later for land drainage, took no account of the destruction of habitat which such action causes. Some improvement has been made by setting the flood banks further back from the channel, and creating artificial meanders within the resultant flood plain. Structures which inhibit the passage of animal life, such as weirs, have also been removed where possible. Other improvements have been achieved through the Catchment Sensitive Farming (CSF) scheme, which helps farmers to reduce the runoff of perticides and fertiliser into watercourses. The river 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.
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