Italy-Slovenia - River Judrio/Idrija
Since Slovenia-Italy border and a lot of its particularities are posted pretty much weekly here, here is something different!
For about half of its 50km length small river Judrio(italian)/Idrija(slovene) acts as a border separating the two countries in the sparsely populated prealpine world of Benetia/Benečija. The river really only really gets alive in the summer when locals use its clean water to refresh in one of the many natural pools. Right bank (left on the picture) is Italy and the left bank is Slovenia.
While most of the present Slovenia-Italy-Croatia border changed multiple times in 20th century, this river was set as a border between then Austro-Hungarian empire and Italy back in 1866 with the Treaty of Vienna and remained so ever since.