It rises at Mileham near Litcham in Norfolk and flows 15 miles west through Castle Acre and Narborough (the latter giving the Nar its name[2]), joining the Ouse at King's Lynn.

The final section of the river near its mouth was diverted northwards when the Great Ouse was re-routed to a new outfall at King's Lynn after the thirteenth century.

Turning to the west, it passes through the south of Litcham and enters a series of lakes at Lexham Hall, where it is joined by Broad Water.

[18] 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.

[19] 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.

[21] Other improvements have been achieved through the Catchment Sensitive Farming (CSF) scheme, which helps farmers to reduce the runoff of pesticides 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.

Commissioners were appointed, any nine of whom could agree to improve the river by constructing locks, removing annoyances, and creating towing paths.

The carriage of guns and nets to catch wildfowl and fish was prohibited, and the commissioners could borrow money to finance the work, with the toll revenue acting as security.

[26] The Commissioners considered applying for a new Act of Parliament in 1760, to create a new cut linking the river at Setchey to the Great Ouse by the shortest route.

He loaned the navigation £1,345 to cover the cost of the work and of obtaining the act, to be added to his existing debt, all of which would accrue interest at five per cent.

The sluices were a little over 11 feet (3.4 m) wide, and although no evidence of any construction above Narborough has been found, it appears likely that it reached West Acre bridge.

[29] By the middle of the nineteenth century, the Marriott brothers, who owned a wharf at Narborough, and whose business included malting and selling coal and corn, also controlled the navigation.

Their cause was not successful, and the line opened from King's Lynn to Narborough on 1846, and onwards to Dereham in 1848, after which trade on the river declined.

An Act of Parliament for the Nar Valley Drainage scheme, obtained in 1881, included a clause to remove the need to maintain the river for navigation, and navigation to Narborough ceased in 1884,[30] when construction of the Old Nar Tidal Sluice prevented boats from accessing the river at King's Lynn.

This would have been enlarged, and the two bridges, located on Thiefgate Lane and the bank of the Flood Relief Channel, would have been rebuilt to provide more headroom.

[35] However, the non-tidal link was eventually abandoned, as there were concerns over environmental issues, the channel was thought to be too narrow to accommodate boats travelling in opposite directions, and some of the bridges would not have provided sufficient headroom.

As the area is low-lying, a network of drainage ditches feeds into larger drains, and the IDB maintains seven pumping stations to discharge water into the main rivers.

The Puny Drain formerly ran to a gravity outfall sluice in King's Lynn,[37][38] but has been re-routed to discharge into the Ouse Relief Channel.

The Polver Drain runs along the south side of the river, and continues to the Ouse Relief Channel, where there is a pumping station.

[37] As part of the redevelopment of the area to the south of King's Lynn, the drain was re-routed to a new outfall some 2.5 miles (4 km) further upstream.

[40] The scheme cost £5.5 million, and was achieved over an 18-month period starting in September 2006, after two years of negotiation with Railtrack to get the design for the tunnel under the railway approved.

[33] To allow for the drainage of surface water from the Nar Ouse Regeneration Area, the channel below the bridge on Wisbech Road has been enlarged.

[44] West Acre mill was a four-storey building, with a brick-built ground floor and the upper storeys built in weatherboarded timber.

The ground floor of the mill dated from the 15th century, and appeared to be built from material obtained from West Acre Abbey.

The four-level stone driving frame, which allowed all four wheels to be operated at once, was noted as an outstanding example of milling machinery in the 1950s, when the building was surveyed, but it was demolished in 1959, as it was deemed to be unsafe.

Charles Tyssen added an extension in 1845, but failed to provide adequate foundations, and the addition gradually sank until the roof collapsed in 1980.

There are several components that are used to determine this, including biological status, which looks at the quantity and varieties of invertebrates, angiosperms and fish.

The reasons for the quality being less than good include physical modification of the channels, and in the case of Middleton Stop Drain, naturally occurring nickel and its compounds.

Like most rivers in the UK, the chemical status changed from good to fail in 2019, due to the presence of polybrominated diphenyl ethers (PBDE) and mercury compounds, neither of which had previously been included in the assessment.