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 . The
River Trent/Humber system would top the list with their combined length of . The
Thames tributary, the
River Churn, sourced at
Seven Springs, adds to the length of the Thames, from its traditional source at
Thames Head. The
Churn/Thames' length at is therefore greater than the
Severn's length of . 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 , but would be around 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 further, and the
Port of London's authority stretches now to Margate, further. Other countries have different conventions, making comparisons of limited value. Those rivers which empty into other (non-tidal sections of) rivers are indicated in the table thus*. In Yorkshire, the Aire, Derwent, Don, and Wharfe all empty into a tidal section of the Yorkshire Ouse. The calculation of
mean flow presents its own challenges, relying on sampling at a limited number of
gauging stations. The data presented in the table are derived from those offered up by the National River Flow Archive, in particular the flow measured at the lowermost gauging stations on each named watercourse. Sometimes the figures of further downstream tributaries are combined with those of the
main stem river to provide a more realistic flow figure for the lowermost non-tidal stretch of a watercourse. ==See also==