The most significant environmental problems in Turkmenistan include salinization of the soil and water pollution. The nation's water supply is threatened by chemical contaminants from farming activity. The problem is complicated by a lack of adequate sewage treatment facilities. A large share of the Amu Darya River's flow is diverted for irrigation, decreasing its contribution to the water supply from the Aral Sea. Water cycles have also affected the Garabogazol Aylagy, a lagoon-like appendage in the northwest that adjoins the Caspian Sea. It became fully enclosed because of a drop in the volume of the Caspian Sea, but is starting to rise again as the sea returns to previous levels. As of 2001, 4.1% of the country's total land area was protected. Eleven mammal species, 12 bird species, and 13 plant species are threatened. Threatened species include the cheetah, tiger, Aral salmon, slender-billed curlew, and whiteheaded duck.