TheRiver Nadder is a tributary of theRiver Avon, flowing in southWiltshire, England.

The name is derived from aCeltic word *nootr, "flowing water", from the same root as the Latin natare, "to swim". (Hoare's account of Wiltshire gives a folk etymology which is still popular, deriving it fromadder.)^[1]

The river flows north fromLudwell to West End where it is joined by the Ferne Brook, close to theLower Coombe and Ferne Brook Meadows^[2] site of special scientific interest (SSSI).^[3] AtWardour it is joined by theRiver Sem. The river then flows east throughTisbury, where it is joined by theFonthill Brook, and then ontoBarford St Martin andBurcombe before reachingWilton.^[4][5][6] NearQuidhampton, theWylye joins from the north.^[7] After passingHarnham, the Nadder joins the Avon nearSalisbury Cathedral.^[8]

Including its headwaters, the river's length is about 55 km (34 mi).^{[9][10][11][12]}

The Environment Agency measures the water quality of the river systems in England. Each is given an overall ecological status, which may be one of five levels: high, good, moderate, poor and bad. There are several components that are used to determine this, including biological status, which looks at the quantity and varieties ofinvertebrates,angiosperms and fish. Chemical status, which compares the concentrations of various chemicals against known safe concentrations, is rated good or fail.^[13]

Water quality of the River Nadder in 2019:

• 
  Flowing under the Palladian Bridge at Wilton House
• 
  Salisbury Cathedral seen from the banks of the Nadder
• 
  The watermill at Harnham is near where the Nadder meets the Avon
• 
  Reed beds along the banks of the Nadder at Harnham

• Discover Nadder -Cranborne Chase AONB

51°03′N1°48′W / 51.050°N 1.800°W /51.050; -1.800

• Rivers of Wiltshire
• Stour catchment

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