The northern and eastern parts are shallower than 180 meters  due to the silt deposited by the Irrawaddy River. This major river flows into the sea from the north through Myanmar. The western and central areas are 900-3,000 meters  deep. Less than five percent of the sea is deeper than 3,000 meters , and in a system of submarine valleys east of the Andaman-Nicobar Ridge, the depth exceeds 4,000 meters . The sea floor is covered with pebbles, gravel, and sand. The western boundary of Andaman Sea is marked by volcanic islands and sea mounts, with straits or passages of variable depths that control the entry and exit of water to Bay of Bengal. There occurs a drastic change in depth of water over a small distance of 200 km, as one moves from Bay of Bengal  to the vicinity of islands  and further into the Andaman Sea. The exchange of water between the Andaman Sea and the Bay of Bengal occurs through the straits formed between the Andaman and Nicobar Islands. Out of these, the most important straits  are: Preparis Channel , Ten Degree Channel , and Great Channel . PC is the widest but shallowest  of the three and separates south Myanmar from north Andaman. TDC is 600 m deep and lies between Little Andaman and Car Nicobar. GC is 1,500 m deep and separates Great Nicobar from Banda Aceh.. Of the three most inportant straits, which is the widest?