Why is most of the East China Sea considered shallow?

The East China Sea is largely shallow because its western edge is a continuation of a broad continental shelf that extends between the South China Sea and the Yellow Sea. Almost three-fourths of the sea is less than 650 feet (200 meters) deep, with an average depth of 1,145 feet (350 meters).12

This shallow shelf area is heavily influenced by geological and sedimentary processes. It is covered with significant deposits of sediment brought down by major rivers, such as the Yangtze River and the Huang He (Yellow River). These sediments have accumulated to thicknesses of up to one mile in areas between rock ridges, which act as barriers along the continental shelf and the Ryukyu Islands. While the sea is generally shallow, it does feature a deeper section known as the Okinawa Trough, which reaches depths of over 3,300 feet (1,000 meters) near the Ryukyu Island chain.12