Topolnitsa River in the context of "Arda (Maritsa tributary)"

Sredna Gora (Bulgarian: Средна гора [ˈsrɛdnɐ ɡoˈra]) is a mountain range in central Bulgaria, situated south of and parallel to the Balkan Mountains and extending from the river Iskar to the west and the elbow of river Tundzha north of the city of Yambol to the east. Sredna Gora is 285 km long, reaching 50 km at its greatest width. Its highest peak is Golyam Bogdan at 1,604 m (5,262 ft). It is part of the Srednogorie mountain chain system, which extends longitudinally across the most country from west to east, between the Balkan Mountains and the Sub-Balkan valleys to the north and the Kraishte, Rila and the Upper Thracian Plain to the south.

The mountain is divided into three parts by the rivers Topolnitsa and Stryama — Ihtimanska Sredna Gora to the west, Sashtinska Sredna Gora in the center, and Sarnena Sredna Gora to the east. Compared to most other mountain ranges in Bulgaria, Sredna Gora has lower average altitude, which determines higher temperatures and lower precipitation. There is an abundance of mineral springs all over Sredna Gora with many spa resorts, such as Hisarya, Starozagorski bani, Pavel Banya, etc. Sredna Gora is rich in mineral resources and especially copper. The economy is well developed. The region is an important national hub of copper extraction and processing industry, as well as of optical industry, both centered in and around the town of Panagyurishte. The largest city in Sredna Gora is Stara Zagora, one of the biggest economic centers of Bulgaria.

42°15′N 26°0′E / 42.250°N 26.000°E / 42.250; 26.000

The Upper Thracian Plain (Bulgarian: Горнотракийска низина, Gornotrakiyska nizina) constitutes the northern part of the historical region of Thrace. It is located in southern Bulgaria, between Sredna Gora mountains to the north and west, a secondary mountain chain parallel to the main Balkan Mountains; the Rhodopes, Sakar and Strandzha to the south; and the Black Sea to the east. A fertile agricultural region, the Upper Thracian Plain proper has an area of 16,032 square kilometres (6,190 sq mi) and an average elevation of 168 metres (551 ft). The plain is part of Northern Thrace. The climate is transitional continental. The highest temperature recorded in Bulgaria occurred here: it was 45.2 °C (113.4 °F) at Sadovo in 1916. The precipitation is 550 millimetres (22 in) a year. Important rivers are the Maritsa and its tributaries, Arda, Tundzha, Stryama, Topolnitsa, and Vacha. Important cities include Plovdiv, Burgas, Stara Zagora, Pazardzhik, Asenovgrad, Haskovo, Yambol and Sliven.