The landlocked region of Northeast India includes the states of Assam, Arunachal Pradesh, Mizoram, Manipur, Meghalaya, Nagaland, Sikkim, and Tripura. It has land borders with China, Myanmar, Bhutan, and Bangladesh. The region is usually categorised into the Eastern Himalaya and the Brahmaputra and the Barak valley plains. Northeast India has a predominantly humid sub-tropical climate with hot, humid summers, severe monsoons, and mild winters. The region is covered by the mighty Brahmaputra–Barak river systems and their tributaries. Geographically, apart from the Brahmaputra, Barak, and Imphal valleys and some flat lands in between the hills of Meghalaya and Tripura, the remaining two-thirds of the area is hilly terrain interspersed with valleys and plains; the altitude varies from almost sea level to over above mean sea level. The region's high rainfall, averaging around and above, creates problems of ecosystem, high seismic activity, and floods. The states of Arunachal Pradesh and Sikkim have a montane climate with cold, snowy winters and mild summers.
