most important rivers in northern Tajikistan are the Syr Darya and the Zeravshan (Zarafshan). The former, the second longest river in Central Asia with a total length of , stretches across the Fergana Valley in far-northern Tajikistan. The Zeravshan River, with a total length of , runs for through the north-center of Tajikistan. Tajikistan's rivers reach high-water levels twice a year: in the spring, fed by the rainy season and melting mountain snow, and in the summer, fed by melting glaciers. The summer freshets are the more useful for irrigation, especially in the Fergana Valley and the valleys of southeastern Tajikistan. Most of Tajikistan's lakes are of glacial origin and are located in the Pamir region in the eastern half of the country. The largest, the Karakul (Qarokul) Lake, is a salt lake devoid of life, lying at an elevation of . Tajikistan's second largest water body is the Kayrakum Reservoir, a long artificial lake in the heart of the Fergana Valley, not far from the city of Khujand in Sughd Province. The lake is fed by the Syr Darya. Another well-known natural lake of glacial origin is Iskanderkul. It is smaller than the Kayrakum Reservoir and lies in the Fann Mountains in western Tajikistan. Climate Tajikistan's climate is continental, subtropical, and semiarid, with some desert areas. The climate changes drastically according to elevation, however. The Fergana Valley and other lowlands are shielded by mountains from Arctic air masses, but temperatures in that region still drop below freezing for more than 100 days a year. In the subtropical southwestern lowlands, which have the highest average temperatures, the climate is arid, although some sections now are irrigated for farming. At Tajikistan's lower elevations, the average temperature range is in July and in January. In the eastern Pamirs, the average July temperature is , and the average January temperature is . Tajikistan is the wettest of the Central Asian republics, with the average annual precipitation for the Kafernigan and Vakhsh valleys in the south being around , and up to in the mountains. At the Fedchenko Glacier, as much as of snow falls each year. Only in the northern Fergana Valley and in the rain shadow areas of the eastern Pamirs is precipitation as low as in other parts of Central Asia: in the eastern Pamirs less than falls per year. Most precipitation occurs in the winter and spring. Environmental problems Most