Bulgaria divides into four major physical regions, each running east and west. In the north is the Danubian Upland, a rolling to hilly plateau region cut by deep river valleys. The Balkan Mountains, which consist mainly of rounded summits 3,000 to 7,000 feet (900 to 2,100 m) in elevation, span north-central Bulgaria. South of the range lies the Thracian Plain, a fertile lowland formed by the valleys of the Maritsa River and its tributaries. The Rhodope Mountains, the highest on the Balkan Peninsula, occupy the south. Maximum elevations occur in the west, where Musala Peak attains 9,596 feet (2,925 m).
The Danube River forms most of the Bulgarian-Romanian border and drains the area north of the Balkan Mountains. Its chief Bulgarian tributary is the Iskur River. Southern Bulgaria is drained primarily by the Maritsa River and its tributaries, chief of which are the Tundzha and Arda. Other major rivers include the Struma and Mesta, which begin in the Rhodope Mountains and flow into Greece. Only the Danube is navigable. Many rivers have been dammed to provide hydroelectric power and water for irrigation.
Most of Bulgaria has a continental climate, with cold winters and warm summers. Average temperatures vary from 25° to 30° F. (-4° to -1° C.) in January and from 70° to 75° F. (21° to 24° C.) in July. Precipitation generally averages 20 to 25 inches (510 to 640 mm) a year, coming mainly in summer. On the Thracian Plain, where the climate is influenced by the Mediterranean Sea, temperatures are slightly higher and precipitation somewhat less than elsewhere in the country.