This is a list of rivers in Bulgaria, organised geographically, taken clockwise from the westernmost tributaries of the Danube drainage. Tributaries are listed down the page in a downstream direction, i.e. the first tributary listed is closest to the source, and tributaries of tributaries are treated similarly. The main stem (or principal) river of a catchment is labelled as (MS), right-bank tributaries are indicated by (R), left-bank tributaries by (L). Note that in general usage, the 'right or left bank of a river' refers to the right or left hand bank, as seen when looking downstream. The list encompasses most of the main rivers of Bulgaria. It includes rivers shared with other countries. There is also a list of rivers over 50 km. The Bulgarian word for river, ÃÂõúð (transliteration reka) is often a part of the river names in the country.
There are 540 rivers in Bulgaria. The longest river in Bulgaria is the Danube (2,888 km), which spans most of the country's northern border for a length of 470 km. The longest one to run through the country (and also the deepest) is the Maritsa (480 km), while the longest river that runs solely in Bulgaria is the Iskar (368 km).
The density of rivers is highest in the mountain areas and lowest in Dobrudzha, the Danubian Plain and the Upper Thracian Plain. There are two catchment basins: the Black Sea (57% of the territory and 42% of the rivers) and the Aegean Sea (43% of the territory and 58% of the rivers) basins. The territory of Bulgaria is 110,994 km<sup>2</sup>, of which 63,270 km<sup>2</sup> fall within the Black Sea catchment bason, divided between the Danube (49,950 km<sup>2</sup>) and Black Sea Coast drainage (13,320 km<sup>2</sup>), and 47,730 km<sup>2</sup> fall within the Aegean Sea basin, divided between the Maritsa (34,166 km<sup>2</sup>), Mesta (2,767 km<sup>2</sup>) and Struma (10,797 km<sup>2</sup>) drainage.
The Balkan Mountains divide Bulgaria into two nearly equal drainage systems. The larger system drains northward to the Black Sea, mainly by way of the Danube. This system includes the entire Danubian Plain and a stretch of land running 48âÂÂ80 km inland from the coastline in the south. The Danube gets slightly more than 4% of its total volume from its Bulgarian tributaries. As it flows along the northern border, the Danube averages 1.6 to 2.4 km in width. The river's highest water levels usually occur during the May floods; it is frozen over an average of 40 days per year. The longest river located entirely in Bulgarian territory, the Iskar, with a length of 368 km and a catchment area of 8,640 km<sup>2</sup>, is the only Bulgarian Danubian tributary that does not rise in the Balkan Mountains. Instead, the Iskar has its origin in the Rila Mountains. It passes through Sofia's eastern suburbs and crosses the Balkan Mountains through a spectacular 65 kmâÂÂlong gorge. Other important tributaries of the Danube include the rivers Lom, Ogosta, Vit, Osam and Yantra. The longest river flowing directly to the Black Sea is the Kamchiya (254 km), while other rivers from north to south include the Batova, Dvoynitsa, Aheloy, Sredetska reka, Ropotamo, Veleka, Rezovo, etc.
The Aegean Sea catchment basin drains the Thracian Plain and most of the higher lands to the south and southwest. Several major rivers flow directly to the Aegean Sea. Many of their tributaries fall swiftly from the mountains and have cut deep, scenic gorges. The 480 kmâÂÂlong Maritsa (of them 321 km in Bulgaria) and its tributaries drain all of the western Thracian Plain, most of Sredna Gora, the southern slopes of the Balkan Mountains, and the northern slopes of the eastern Rhodopes. After it leaves Bulgaria, the Maritsa forms most of the Greco-Turkish border. Maritsa's major tributaries are the Tundzha, Arda, Topolnitsa, Vacha, Stryama, etc. The other Bulgarian rivers flowing directly to the Aegean are the Struma and the Mesta.