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Kasplya (Belarusian and Russian: Каспля) is a river in Russia and Belarus, a left tributary of the Daugava River. Its length is 224 km, the first 157 km in Russia (Smolensk Oblast), and the rest in Belarus (Vitebsk Voblast). It starts from Lake Kasplya at 54°59′N 31°38′E / 54.983°N 31.633°E, and joins the Daugava in the town of Surazh. The town of Demidov is situated on the Kasplya.
During the Viking Age, the river was an important part of the Dnieper trade route, as there was a portage from the Kasplya to the Dnieper River tributaries entering the Dnieper near Gnezdovo.