| 100% | TWRCH | 5 | River that formed the ancient boundary between Brecknockshire and Carmarthenshire |
| 62% | RUBICON | 7 | Stream that formed the ancient boundary between Italy and Cisalpine Gaul |
| 61% | AMUR | 4 | River that forms the boundary between Manchuria and Russia |
| 60% | DRINA | 5 | River that forms the boundary between Bosnia and Herzegovina and Montenegro |
| 53% | MORAVA | 6 | River that forms the border between Slovakia and Austria |
| 53% | MEKONG | 6 | River that forms the border between Laos and Myanmar |
| 50% | EBRO | 4 | River that was the ancient dividing line between Rome and Carthage |
| 50% | ODER | 4 | River that forms part of the border between Germany and Poland |
| 50% | TAMAR | 5 | River that forms most of the border between Cornwall and Devon |
| 50% | ORINOCO | 7 | River that forms part of the border between Colombia and Venezuela |
| 50% | MOSELLE | 7 | River that forms part of the border between Germany and Luxembourg |
| 50% | DOURO | 5 | River that forms part of the border between Spain and Portugal |
| 50% | RIONEGRO | 8 | River that forms part of the border between Colombia and Venezuela |
| 50% | ELBE | 4 | River that formed part of the border between East and West Germany |
| 50% | ORANGE | 6 | River rising in the Drakensberg mountains that forms the boundary between South Africa and Namibia |
| 49% | YALU | 4 | River that forms a large part of the border between North Korea and China |
| 48% | DOURO | 5 | River that forms part of the border between Spain and Portugal before flowing west to the Atlantic |
| 46% | DRINA | 5 | River forming the boundary between Bosnia and Herzegovina and Montenegro |
| 44% | URAL | 4 | River forming part of the boundary between Europe and Asia |
| 43% | DEE | 3 | River that forms part of the England/Wales boundary |
| 43% | RHYMNEY | 7 | River in Wales forming the boundary between the historic counties of Glamorgan and Monmouthshire |
| 40% | KASAI | 5 | Tributary of the Congo River that forms part of the border between the Democratic Republic of the Congo and Angola |
| 40% | LIMPOPO | 7 | River that serves as the border between Zimbabwe and South Africa |
| 40% | WAVENEY | 7 | River forming the eastern border between Norfolk and Suffolk |
| 39% | LIMPOPO | 7 | River forming the border between Zimbabwe and South Africa |
| 39% | ERIE | 4 | Canal that forms a link between the Atlantic and the Great Lakes |
| 38% | ETRURIA | 7 | Ancient region of central Italy between the Rivers Tiber and Arno |
| 38% | MESOPOTAMIA | 11 | Ancient region of SW Asia between the Rivers Euphrates and Tigris |
| 38% | STMARYS | 7 | River that forms the Michigan-Ontario border |
| 38% | UNA | 3 | 132 mile river that runs along the border between Bosnia and Herzegovina and Croatia |