The average depth of the sea is about 1,000 meters (3,300 ft). The northern and eastern parts are shallower than 180 meters (600 ft) due to the silt deposited by the Irrawaddy River. This major river flows into the sea from the north through Burma. The western and central areas are 900–3,000 meters deep (3,000–10,000 ft). Less than 5% of the sea is deeper than 3,000 meters (10,000 ft). In a system of submarine valleys east of the Andaman-Nicobar Ridge, the depth is more than 4,000 meters (13,200 ft). The sea floor is covered with pebbles, gravel and sand.