This is a list of the longest rivers of the United Kingdom.
|Rank||River||Length (miles)||Length (km)||Mean Flow (m3/s)||Country|
|5||River Great Ouse||143||230||15.6||England|
|6||River Ure/River Ouse, Yorkshire||129||208||69.8||England|
|11||River Bann / Lough Neagh||99||159||92.2||Northern Ireland|
|13||River Avon, Warwickshire||96||154||17.3||England|
|14||River Eden, Cumbria||90||145||53.7||England|
|15||River Dee, Aberdeenshire||87||140||47.8||Scotland|
|18=||River Don, Aberdeenshire||80||129||21.3||Scotland|
|18=||River Foyle||80||129||58.8||Northern Ireland/Republic of Ireland|
|22=||River Avon, Bristol||75||120||22.2||England|
|26||River Derwent, Yorkshire||72||115||17.4||England|
|32=||River Dee, Wales||70||112||34.1||Wales/England|
|32=||River Don, South Yorkshire||70||112||16.3||England|
There seems to be little consensus in published sources as to the lengths of rivers, nor much agreement as to what constitutes a river. Thus the River Ure and River Ouse can be counted as one river system or as two rivers. If it is counted as one, the River Aire/ River Ouse/Humber system would come fourth in the list, with a combined length of 161 miles (259 km); and the River Trent/Humber system would top the list with their combined length of 222 miles (357 km). Also, the Thames tributary, the River Churn, sourced at Seven Springs, adds 14 miles (23 km) to the length of the Thames (from its traditional source at Thames Head). The Churn/Thames' length at 229 miles (369 km) is therefore greater than the Severn's length of 220 miles (354 km). Thus, the combined Churn/Thames river would top the list. Sue Owen et al., in their book on rivers, generally restrict the length to the parts that bear the same name. Thus the River Nene is quoted at 100 miles (160 km), but would be around 5 miles (8 km) more if the variously named sources were included. Many of the above lengths are considerably different from Sue Owen's list, some longer and some shorter.
Where a river ends in an estuary the conventional British approach has been to treat the river as ending at the end of the administrative zone. Thus the Severn ends at the mouth of the Bristol Avon and the Thames at the Yantlet Line. The currently accepted end of the Severn Estuary is about 18.5 miles (29.8 km) further, and the Port of London's authority stretches now to Margate, 30 miles (48 km) further. Other countries have different conventions, making comparisons of limited value.