igable, 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 stre