The Slipnir River is a river located on the border of modern-day (as of The Fifth Elephant) Istanzia. There were some issues with Istanzian troops launching incursions across it to attack whatever nation was on the other side, and the diplomatic corps of Ankh-Morpork was called upon to resolve the situation. Istanzia is bordered on The Discworld Mapp by two rivers un-identified on this map. However, it can be gleaned from The Compleat Discworld Atlas that the Slipnir is the river border between Istanzia and Omnia. (possibly also with Betrek). The Atlas also notes that Istanzia has a long-standing grievance with Omnia regarding the way its natural resources were denuded during the centuries of Omnian occupation, leaving them with nothing after independence and with no material compensation having been offered.
The Duke of Ankh, chosen representative of the city, warned the Istanzian ambassador that if these transgressions did not stop, the ambassador would be "going home in an ambulance". Since when not only have attacks across the river stopped, but the Istanzian army has withdrawn so far back that they are nearly in the next country. Which, according to the new and more accurate maps in The Compleat Discworld Atlas, is the unspecified land in between upper Klatchistan and the Morpork Mountains.