Estuaries are formed when freshwater rivers meet the ocean. Tidal pressure (at high tide) forces ocean water up into the river, forming a huge "mixing bowl" of freshwater and saltwater; of inland nutrients and ocean life. Three types of rivers contribute to estuaries: alluvial, blackwater, and tidewater rivers. The alluvial rivers have the longest paths to travel, comming down from head waters in the Georgia mountains and the Piedmont. The Savannah and Altamaha rivers are two examples. These waters carry large amounts of sediments which contribute to the sand deposits on the islands and nutrients which feed the life in the river. Blackwater rivers get their name from the tannic acid that comes from decomposing leaves and other organic matter. They are shorter and offer a smaller nutrient and sediment contribution to the estuary. Tidal rivers are very short and are almost entirely influenced by the tidal push.
During high tide, marine plankton, fish, and crustaceans are pushed into the river. Bottom nutrients are stirred up and released. Dead organic matter is flushed out of the marsh. Salt is mixed in various concentrations with the fresh water of the river. As the tide ebbs, the fortified nature's soup is released back into the ocean. It is estimated that 70% of the commerically important fish and shrimp are nurtured from the estuaries. In addition, phytoplankton, which can't live more than a few inches below the muddy surface because of the lack of sunlight, are washed out and dispursed into the ocean, where they can remain viable at greater depths.