Why is there a boundary between clear water and muddy water in the sea?

An area where the ocean and land interact. That is, the transition zone from the ocean to the land. The modern coastal zone includes the upper limit of modern seawater movement in the coast and its adjacent land, and the influence range of seawater on the erosion and deposition changes of the bank slope profile in the subtidal zone. It consists of three basic units: ① coast. The coastal land part above the mean high tide line is usually called tidal zone; ② Intertidal zone. Between the average high tide line and the average low tide line; ③ Underwater bank slope. The shallow water below the average low tide line is usually called the subtidal line. The ancient coastal zone is a coastal land part that has been separated from the influence of wave activities. In addition, the coastal zone also includes estuaries and harbors. In the coastal zone survey in China, the survey scope is defined as: the isobath extending from the coastline to the land direction of about 10 km and going out to sea to the water depth of 10 ~ 15 m; In the estuary area, it extends to the tidal boundary on land and to the muddy water line or fresh water tongue on the sea. Coastal zone is a valuable land resource for coastal countries, a base for marine development and economic development, and a link for foreign trade and cultural exchanges, and its position is very important. \ r \ n \ r \ The total length of the world coastline is 44× 104km, which is the dividing line between land and sea. Due to the change of tidal level and the increase and decrease of water quantity caused by wind, the coastline is changeable, and the narrow strip exposed when the water level rises is the coastal zone.