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.