| 100% | RHYMNEY | 7 | River in Wales forming the boundary between the historic counties of Glamorgan and Monmouthshire |
| 46% | URAL | 4 | River forming part of the boundary between Europe and Asia |
| 46% | ORANGE | 6 | River rising in the Drakensberg mountains that forms the boundary between South Africa and Namibia |
| 42% | AMUR | 4 | River that forms the boundary between Manchuria and Russia |
| 41% | DRINA | 5 | River forming the boundary between Bosnia and Herzegovina and Montenegro |
| 41% | DRINA | 5 | River that forms the boundary between Bosnia and Herzegovina and Montenegro |
| 39% | TERMINATOR | 10 | Boundary between the illuminated and darkened part of the moon, in astronomy |
| 38% | TWRCH | 5 | River that formed the ancient boundary between Brecknockshire and Carmarthenshire |
| 38% | ARAS | 4 | River forming part of the border between Azerbaijan and Iran |
| 38% | MOSELLE | 7 | River forming part of the border between Germany and Luxembourg |
| 38% | ODER | 4 | River that forms part of the border between Germany and Poland |
| 38% | TAMAR | 5 | River that forms most of the border between Cornwall and Devon |
| 38% | ORINOCO | 7 | River that forms part of the border between Colombia and Venezuela |
| 38% | MOSELLE | 7 | River that forms part of the border between Germany and Luxembourg |
| 38% | DOURO | 5 | River that forms part of the border between Spain and Portugal |
| 38% | RIONEGRO | 8 | River that forms part of the border between Colombia and Venezuela |
| 37% | YAZOO | 5 | River in Mississippi, US, formed by the confluence of the Tallahatchie and Yalobusha Rivers |
| 37% | ELBE | 4 | River that formed part of the border between East and West Germany |
| 37% | SNAKE | 5 | — River, US watercourse forming part of the border between Idaho and Oregon |
| 37% | KASAI | 5 | Tributary of the Congo River that forms part of the border between the Democratic Republic of the Congo and Angola |
| 37% | SHATTALARAB | 11 | River in SE Iraq formed by the confluence of the Tigris and Euphrates Rivers |
| 37% | YALU | 4 | River that forms a large part of the border between North Korea and China |
| 36% | GIRONDE | 7 | Estuary in France formed by the confluence of the Rivers Dordogne and Garonne |
| 36% | WIRRAL | 6 | Peninsula in NW England between the estuaries of the Rivers Dee and Mersey |
| 36% | PARANA | 6 | River of South America formed at the confluence of the Paranaiba and Grande Rivers in Brazil |
| 36% | DOURO | 5 | River that forms part of the border between Spain and Portugal before flowing west to the Atlantic |
| 36% | GASPE | 5 | — Peninsula, area of land in E Canada between the St Lawrence River and Chaleur Bay |
| 35% | POLTAVA | 7 | City in Ukraine on the Vorskla River; site of a 1709 battle between Russia and Sweden |
| 35% | ABERGAVENNY | 11 | Market town in Monmouthshire, Wales, on the north bank of the River Usk housing an 11th-century castle |
| 34% | MONMOUTH | 8 | Town in SE Wales near the border with England at the confluence of the Rivers Wye and Monnow |