Iskar (river) in the context of "Endemism"

Rila (Bulgarian: Рила, pronounced [ˈriɫɐ]) is the highest mountain range of Bulgaria, the Balkan Peninsula, and of Southeast Europe. It is situated in southwestern Bulgaria and forms part of the Rila–Rhodope Massif. The highest summit is Musala at an elevation of 2,925 m which makes Rila the sixth highest mountain range in Europe after the Caucasus, the Alps, Sierra Nevada, the Pyrenees and Mount Etna, and the highest one between the Alps and the Caucasus. It spans a territory of 2,629 km with an average elevation of 1487 m. The mountain is believed to have been named after the river of the same name, which comes from the Old Bulgarian verb "рыти" meaning "to grub".

Rila has abundant water resources. Some of the Balkans' longest and deepest rivers originate from Rila, including the Maritsa, Iskar and Mesta rivers. Bulgaria's main water divide separating the Black Sea and the Aegean Sea drainage systems follows the main ridge of Rila. The mountain range is dotted with almost 200 glacial lakes such as the renowned Seven Rila Lakes, and is rich in hot springs in the fault areas at the foothills, including the hottest spring in South-eastern Europe in Sapareva Banya.

Sofia is the capital and largest city of Bulgaria. It is situated in the Sofia Valley at the foot of the Vitosha mountain, in the western part of the country. The city is built west of the Iskar river and has many mineral springs, such as the Sofia Central Mineral Baths. It has a humid continental climate.

Known as Serdica in antiquity, Sofia has been an area of human habitation since at least 7000 BC. The recorded history of the city begins with the attestation of the conquest of Serdica by the Roman Republic in 29 BC from the Celtic tribe Serdi. During the decline of the Roman Empire, the city was raided by Huns, Visigoths, Avars, and Slavs. In 809, Serdica was incorporated into the First Bulgarian Empire by Khan Krum and became known as Sredets. In 1018, the Byzantines ended Bulgarian rule until 1194, when it was reincorporated by the Second Bulgarian Empire. Sredets became a major administrative, economic, cultural and literary hub until its conquest by the Ottomans in 1382. From 1530 to 1836, Sofia was the regional capital of Rumelia Eyalet, the Ottoman Empire's largest and most important province. Bulgarian rule was restored in 1878. Sofia was selected as the capital of the Third Bulgarian State in the next year, ushering a period of intense demographic and economic growth.

The Danubian Plain (Bulgarian: Дунавска равнина, romanized: Dunavska ravnina) constitutes the northern part of Bulgaria, situated north of the Balkan Mountains and south of the Danube. Its western border is the Timok River, and to the east it borders the Black Sea. The plain has an area of 31,523 square kilometres (12,171 sq mi). It is about 500 kilometres (310 mi) long and 20 to 120 kilometres (12 to 75 mi) wide.

The Danubian Plain is contiguous with the Wallachian Plain (forming the Lower Danubian Plain), but its elevation is slightly higher and the relief is more hilly and rolling, featuring numerous plateaux and river valleys. The climate is markedly temperate continental with a weak Black Sea influence in the east. Precipitation is on average 450–650 mm a year. Important rivers include the Danube, the Iskar, the Yantra, the Osam, the Vit, the Rusenski Lom, the Ogosta, and the Lom.

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.

The Zlatna Panega (Bulgarian: Златна Панега, "golden Panega", also: Panega, old: Paneg, Altǎn Paneg) is a river in western Bulgaria, a right tributary of the river Iskar, itself a right tributary of the Danube, belonging to the Black Sea drainage. Its length is 50 km. Panega Glacier on Livingston Island in the South Shetland Islands, Antarctica is named after the river.