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. Climate For so small an area, the climate varies widely and is unusually complex. Depending upon the depth to which they study the area, climatologists list six or more climatic subzones. The country lies on the line of transition between the strongly contrasting Eastern European continental and the Mediterranean climatic zones, and its mountains and valleys are local factors that act as barriers or channels to the air masses, contributing to sharp contrasts in weather over relatively short distances. The Black Sea, although too small to be a primary influence over much of the country's weather, also affects the immediate area along its coastline. In general, continental systems prevail in the north. They are characterized by hot summers, cold winters, and precipitation well distributed throughout the year, a major portion of it in early summer thunderstorms. The Mediterranean climate that is influential most of the time in the south has mild, damp winters but hot, dry, rain-free summers. The Stara Planina marks the lower limits of the area in which continental air masses circulate freely in typical circumstances. In the area between them and the Danube River there is an extension of the climate that is common to east-central Europe and adjoining regions of the Soviet Union. In the same fashion, the Rodopi marks the northern limits of domination by Mediterranean weather systems. The southern slopes of these mountains are sufficiently mild to merit the region's being called the Green Greece or Bulgarian California.