Demidovsky District in the context of Administrative divisions of Smolensk Oblast

The Kasplya (Belarusian and Russian: Каспля) is a river in Smolensky, Demidovsky, and Rudnyansky Districts of Smolensk Oblast of Russia and in Vitebsk Region of northern Belarus. It is a major left-bank tributary of the Daugava. Of its total length of 136 kilometres (85 mi), the first 116 kilometres (72 mi) are in Russia, and the rest in Belarus. It joins the Daugava in the urban-type settlement of Surazh. The town of Demidov is situated on the Kasplya.

The Kasplya originates from Lake Kasplya in Smolensky District. It flows north, crosses the border with Demidovsky District and turns northwest. It flows through Demidov, and next to the selo of Boroda turns west, crosses Rudnyansky District and enters Belarus. There, it turns northwest again and enters the Daugava.

Smolensky District (Russian: Смоле́нский райо́н) is an administrative and municipal district (raion), one of the twenty-five in Smolensk Oblast, Russia. It is located in the west of the oblast and borders with Demidovsky District in the north, Dukhovshchinsky District in the northeast, Kardymovsky District in the east, Pochinkovsky District in the south, Monastyrshchinsky District in the southwest, Krasninsky District in the west, and with Rudnyansky District in the northwest. The area of the district is 2,494.98 square kilometers (963.32 sq mi). Its administrative center is the city of Smolensk (which is not administratively a part of the district). Population: 44,964 (2010 Census); 47,281 (2002 census); 50,620 (1989 Soviet census).

Rudnyansky District (Russian: Рудня́нский райо́н) is an administrative and municipal district (raion), one of the twenty-five in Smolensk Oblast, Russia. It is located in the west and northwest of the oblast and borders with Velizhsky District in the north, Demidovsky District in the northeast, Smolensky District in the east, Krasnensky District in the south, and with Vitebsk, Liozna, and Dubrowna Districts of Vitebsk Region of Belarus in the west. The area of the district is 2,111.41 square kilometers (815.22 sq mi). Its administrative center is the town of Rudnya. Population: 25,244 (2010 Census); 28,037 (2002 census); 34,724 (1989 Soviet census). The population of Rudnya accounts for 39.7% of the district's total population.

Demidov (Russian: Деми́дов) is a town and the administrative center of Demidovsky District in Smolensk Oblast, Russia, located on the Kasplya River at its confluence with the Gobza River. Population: 7,333 (2010 census); 8,786 (2002 census); 10,198 (1989 Soviet census). It was previously known as Porechye (until 1918).