Rhodope Mountains in the context of "Nestos (river)"

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.

Thrace (/θreɪs/, thrayss; Bulgarian: Тракия, romanised: Trakiya; Greek: Θράκη, romanised: Thráki; Turkish: Trakya) is a geographical and historical region in Southeast Europe roughly corresponding to the province of Thrace in the Roman Empire. Bounded by the Balkan Mountains to the north, the Aegean Sea to the south, and the Black Sea to the east, it comprises present-day southeastern Bulgaria (Northern Thrace), northeastern Greece (Western Thrace), and the European part of Turkey (East Thrace). Lands also inhabited by ancient Thracians extended in the north to modern-day Northern Bulgaria and Romania and to the west into Macedonia.

The Maritsa, known in Greek as the Hebros and in Turkish as the Meriç, is a river that runs through the Balkans in Southeast Europe. With a length of 480 km (300 mi), it is the longest river that runs solely in the interior of the Balkan peninsula, and one of the largest in Europe by discharge. It flows through Bulgaria in its upper and middle reaches, while its lower course forms part of the border between Greece and Turkey. Its drainage area is about 53,000 km (20,000 sq mi), of which 66.2% is in Bulgaria, 27.5% in Turkey, and 6.3% in Greece. It is the main river of the historical region of Thrace, most of which lies in its drainage basin.

It has its origin in the Rila Mountains in Western Bulgaria, its source being the Marichini Lakes. The Maritsa flows east-southeast between the Balkan and Rhodope Mountains, past Plovdiv and Dimitrovgrad in Bulgaria to Edirne in Turkey. East of Svilengrad, Bulgaria, the river flows eastwards, forming the border between Bulgaria (on the north bank) and Greece (on the south bank), and then between Turkey and Greece. At Edirne, the river meets its two chief tributaries Tundzha and Arda, and flows through Turkish territory on both banks. It then turns towards the south and forms the border between Greece on the west bank and Turkey on the east bank all the way to the Aegean Sea, which it enters near Enez, forming a river delta. The upper Maritsa valley is a principal east–west route in Bulgaria. The unnavigable river is used for hydroelectric power generation and for irrigation.

Boril (Bulgarian: Борил) was the emperor (tsar) of Bulgaria from 1207 to 1218. He was the son of an unnamed sister of his predecessor, Kaloyan and Kaloyan's brothers, Peter II and Ivan Asen I, who had restored the independent Bulgarian state. After Kaloyan died unexpectedly in October 1207, Boril married his widow, a Cuman princess and seized the throne. His cousin, Ivan Asen, fled from Bulgaria, enabling Boril to strengthen his position. His other kinsmen, Strez and Alexius Slav, refused to acknowledge him as the lawful monarch. Strez took possession of the land between the Struma and Vardar rivers with the support of Stefan Nemanjić of Serbia. Alexius Slav secured his rule in the Rhodope Mountains with the assistance of Henry, the Latin Emperor of Constantinople.

Boril launched unsuccessful military campaigns against the Latin Empire and the Kingdom of Thessalonica during the first years of his reign. He convoked the synod of the Bulgarian Church in early 1211. At the assembly, the bishops condemned the Bogomils for heresy. After an uprising broke out against him in Vidin between 1211 and 1214, he sought the assistance of Andrew II of Hungary, who sent reinforcements to suppress the rebellion. He made peace with the Latin Empire in late 1213 or early 1214. After Henry died in 1216 and Andrew II left Hungary for a crusade, Ivan Asen returned to Bulgaria. He captured and blinded Boril in Tarnovo in 1218.

Rhodope (Greek: Ροδόπη, Rodópi [roˈðopi]) is one of the regional units of Greece. It is part of the region of East Macedonia and Thrace. Its name is derived from the Rhodope Mountains, which cover the northern part of its territory. Together with the regional units Evros and Xanthi, it forms the geographical region of Western Thrace. The capital of the prefecture is the city of Komotini. The second largest town is Sapes. Most of the Muslims of Thrace, the only officially recognized minority in Greece, are settled in this area, where they form around half of the regional unit's population.

The Pirin Mountains (Bulgarian: Пирин [ˈpirin]) are a mountain range in southwestern Bulgaria, with the highest peak, Vihren, at an altitude of 2,914 m (9,560 ft).

The range extends about 80 km (50 mi) from the north-west to the south-east and is about 40 km (25 mi) wide, spanning a territory of 2,585 km (998 sq mi). To the north, Pirin is separated from Bulgaria's highest mountain range, the Rila Mountain, by the Predel saddle, while to the south it reaches the Slavyanka Mountain. To the west is located the valley of the river Struma and to the east the valley of the river Mesta separates it from the Rhodope Mountains. Pirin is dotted with more than a hundred glacial lakes and is also the home of Europe's southernmost glaciers, Snezhnika and Banski Suhodol.