| 100% | RUBICON | 7 | Stream that formed the ancient boundary between Italy and Cisalpine Gaul |
| 57% | TWRCH | 5 | River that formed the ancient boundary between Brecknockshire and Carmarthenshire |
| 46% | AMUR | 4 | River that forms the boundary between Manchuria and Russia |
| 45% | DRINA | 5 | River that forms the boundary between Bosnia and Herzegovina and Montenegro |
| 39% | MORAVA | 6 | River that forms the border between Slovakia and Austria |
| 39% | MEKONG | 6 | River that forms the border between Laos and Myanmar |
| 37% | ORANGE | 6 | River rising in the Drakensberg mountains that forms the boundary between South Africa and Namibia |
| 36% | ODER | 4 | River that forms part of the border between Germany and Poland |
| 36% | TAMAR | 5 | River that forms most of the border between Cornwall and Devon |
| 36% | ORINOCO | 7 | River that forms part of the border between Colombia and Venezuela |
| 36% | MOSELLE | 7 | River that forms part of the border between Germany and Luxembourg |
| 36% | DOURO | 5 | River that forms part of the border between Spain and Portugal |
| 36% | RIONEGRO | 8 | River that forms part of the border between Colombia and Venezuela |
| 36% | ELBE | 4 | River that formed part of the border between East and West Germany |
| 35% | ERIE | 4 | Canal that forms a link between the Atlantic and the Great Lakes |
| 35% | ADRIATIC | 8 | Sea that lies between Italy and the Balkans |
| 35% | ETRURIA | 7 | Ancient region of central Italy between the Rivers Tiber and Arno |
| 35% | EBRO | 4 | River that was the ancient dividing line between Rome and Carthage |
| 35% | RHYMNEY | 7 | River in Wales forming the boundary between the historic counties of Glamorgan and Monmouthshire |
| 35% | YALU | 4 | River that forms a large part of the border between North Korea and China |
| 34% | DRINA | 5 | River forming the boundary between Bosnia and Herzegovina and Montenegro |
| 33% | DOURO | 5 | River that forms part of the border between Spain and Portugal before flowing west to the Atlantic |
| 32% | URAL | 4 | River forming part of the boundary between Europe and Asia |
| 32% | RIOGRANDE | 9 | Boundary between the US and Mexico |
| 31% | CALABRIA | 8 | Admin region that forms the toe of Italy's boot |
| 30% | STYX | 4 | Boundary between the earth and the underworld, in myth |
| 30% | ADHESIONS | 9 | Fibers that form between tissue and organs |
| 30% | APENNINES | 9 | Mountain range that forms the "backbone" of Italy |
| 30% | ALPS | 4 | Natural boundary between France and Italy |
| 29% | DEE | 3 | River that forms part of the England/Wales boundary |