From a drainage standpoint, the country is divided into two nearly equal parts. The slightly larger one drains to the Black Sea, the other to the Aegean. The northern watershed of the Stara Planina, all of the Danubian plateau, and the thirty to fifty miles inland from the coastline drain to the Black Sea. The Thracian Plain and most of the higher lands of the south and southwest drain to the Aegean Sea. Although only the Danube is navigable, many of the other rivers and streams have a high potential for the production of hydroelectric power and are sources of irrigation water. Many are already being exploited.

Insignificant when compared with the watersheds that drain to the seas, about 125 square miles of the country drain into a few small salt lakes that have no outflowing water. The largest such lake has a surface area of 2.5 square miles.

By far the greater part of the country that drains to the Black Sea does so through the Danube. Most of its major tributaries in the country (from west to east, the Ogosta, Iskur, Vit, Osum, Yantra, and Lom) carry more water than do the combination of the Provadiyska, Kamchiya, Fakiyska, and Veleka rivers, all of which flow directly into the Black Sea. Of the Danube's Bulgarian tributaries, all but the Iskur rise in the Stara Planina. The Iskur rises in the Rila and flows northward through a narrow basin. Territory not far from the river on both sides of it drains in the opposite direction, to the south. The Iskur passes through Sofia's eastern suburbs and cuts a valley through the Stara Planina on its way to join the Danube.

The Iskur and the other of the Danube's north-flowing tributaries have cut deep valleys through the Danubian plateau. The eastern banks tend to rise sharply from the rivers; the western parts of the valleys may have broad fields with alluvial soils. The peculiar, though consistent, pattern is caused by forces resulting from the earth's rotation; these forces give the water a motion that tends to undercut the right banks of the streams. Some of these rivers are sizable streams, but the Danube gets only a little more than 4 percent of its total volume from its Bulgarian tributaries. As it flows along the northern border, the Danube averages one to 1.5 miles in width. Its highest water levels are usually reached during June floods, and in normal seasons it is frozen over for about forty days.

Several major rivers flow directly to the Aegean Sea, although the Maritsa with its tributaries is by far the largest. The Maritsa drains all of the western Thracian Plain, all of the Sredna Gora, the southern slopes of the Stara Planina, and the northern slopes of the eastern Rodopi. Other than the Maritsa, the Struma in the west and the Mesta (which separates the Pirin from the main Rodopi ranges) are the two largest of the rivers that rise in Bulgaria and flow to the Aegean. Most of these streams fall swiftly from the mountains and have cut deep, scenic gorges. The Struma and Mesta reach the sea through Greece. The Maritsa forms most of the Greek-Turkish border after it leaves Bulgaria.

About 3,750 square miles of agricultural land have access to irrigation waters. Dams provide the water for about one-half of the acreage; diversions from rivers and streams serve about one-third; and water pumped from the ground and from streams accounts for the remainder.

Of the dams, ninety-two are termed large state dams. Their combined capacity is three times that of some 2,000 smaller dams. The sources of four large rivers—the Maritsa, Iskur, Mesta, and Rilska (a major tributary of the Struma)—are within a few miles of each other in the high Rila. Water from the upper courses of these and several other streams supplies the Sofia area with both water and electricity, and they have a potential for further development. There are major dams on the Tundzha, Iskur, Rositsa, and Struma rivers. The Danube is too massive a stream to harness, and damming the Maritsa along most of its course would flood too much valuable land. The rivers flowing north across the Danubian plateau also tend to be overly difficult to use in the areas where they are most needed.

The Vucha River, flowing from the Rodopi into the Maritsa River, is often used to illustrate how rivers have been effectively harnessed to provide a variety of benefits. Its cascade system of hydroelectric development employs six dams having the capacity to generate over 600,000 kilowatts of electricity. The water they back up serves the municipal water systems in Plovdiv and a number of other towns in its vicinity, and the dams provide irrigation water for nearly 250,000 acres of cropland. The reservoirs themselves are being developed as recreational areas and mountain resorts.

Where a stream is difficult to dam or to divert, water is pumped from it. This has been feasible only since about 1950, when low-cost diesel engines and sufficient hydroelectric power became available from newly constructed dams on other streams. About eighty-five huge pumping stations have been set up along the Danube River, which furnishes about three-quarters of the water acquired by this method; and in 1970 there were about 1,200 lesser stations operating on smaller streams, most of them on the Thracian Plain.

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