Southern Thailand is on the Malay Peninsula, with an area around 70,713 km², bounded to the north by Kra Isthmus, the narrowest part of the peninsula. The western part has steeper coasts, while on the east side river plains dominate. The largest river of the south is the Tapi in Surat Thani, which together with the Phum Duang in Surat Thani drains more than 8,000 km², more than 10% of the total area of Southern Thailand. Smaller rivers include the Pattani, Saiburi, Krabi, and the Trang. The biggest lake of the south is Songkhla Lake (1,040 km² altogether). The largest artificial lake is the Chiao Lan (Ratchaprapha Dam), occupying 165 km² of Khao Sok National Park in Surat Thani.