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There seems to be little consensus as to the lengths of rivers in published sources, nor much agreement as to what constitutes a river. Thus the River Ure / River Ouse can be counted as one river system or two rivers. If it is counted as one, the River Aire / Yorkshire Ouse / Humber system would come fourth in the list, with a combined length of 161 miles (259 km) and indeed, the River Trent / Humber system would top the list with their combined length of 222 mi (357 km). Also, the Thames tributary, the River Churn, sourced at Seven Springs adds 14 miles to the length of the Thames (from its traditional source at Thames head). The Churn/Thames' length at 229 mi (369 km) is therefore greater than the Severn’s length - 220 mi (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 correct name. Thus the River Nene is quoted at 100 miles (160 km), but would be around 5 miles (8.0 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 use the end of the administrative zone. Thus the Severn ends at the mouth of the Bristol Avon and the Thames to the Yantlet Line. The currently accepted end of the Severn Estuary is nearly about 18.5 miles (29.8 km) further and the PLA's authority stretches now to Margate, 30 miles (48 km) further. Other countries have different conventions, making comparisons of limited value.