flowing in a roughly southern direction. The ranges separating the main rivers are generally steep, high and continuous. Towards the east, as well as in the Wang and Yom drainage basins, they are lower. The Pai River in the northwest flows westwards into the Salween and the northeastern part is drained by rivers of the Mekong basin, like the Kok and Ing. Geology Geologically in the southern subranges of the Shan Hills layers of alluvium are superimposed on hard rock. The ranges closer to Laos consist of Permo-Carboniferous limestone, which makes for a more jagged and steep relief, despite the more moderate height. Most of the Thai highlands are part of the Shan-Thai Terrane, a tectonic plate. Environment and human impact The natural environment of the hills used to be dense montane rain forest. Swidden agricultural practices and logging have much reduced the old-growth forest areas which have been replaced by secondary forest. For centuries the Thai highlands have been inhabited with hill tribes mostly from Chinese or Tibeto-Burman descent, such as the Akha, Yao, Lahu, Khmu, Hmong and Lisu. These human groups immigrated into this relatively empty region fleeing persecution or harsh central rule in their respective environments, as well as seeking new land for their shifting agricultural productions system. For the past decades these groups have been undergoing a process of integration into the Thai mainstream. Owing to the unrest in Burma, some refugee camps have been established for cross-border refugees in the Thai highlands. Certain Kayah and Karen communities, like the "long-necked Karen", are regularly visited by organized tourist groups. At higher elevations, above , one of the main crops was opium until the 1990s, when the combined effects of development became evident—from the construction of roads into the remote area, increasingly efficient policing, and opium replacement programs. Yearly wildfires are started by local farmers during the dry season in different areas of northern Thailand. Often speculators also hire people to set forests on fire in order to claim land title deeds for the areas that, post-fire, become "degraded forest". The smoke produced by these fires is the main cause of the intense seasonal air pollution in the Thai highlands, also known as the "northern haze". Fires also contribute to the floods in the country by denuding forest undergrowth and the dry forest soil leads to lower water intake for the trees to extract when the rains