Estuaries are transition zones between saltwater and freshwater ecosystems. They are located at the mouth of rivers where freshwater meets the saline water of the nearshore. No two estuaries are alike and they are often associated with other coastal ecosystems such as salt marshes, mudflats, beaches and bogs.
Tides play a major role in the salinity of the water.
With each incoming tide there is an influx of saltwater. When saltwater mixes with freshwater it is termed brackish. Fjords are the deepest of all estuaries, with water properties that differ from the classic estuary. The circulation of water in the upper layers is similar to other estuaries, with a brackish outflowing surface layer and deeper inflowing saline layer. But the circulation of deep water in the fjord is dependent on the depth of the sill, the amount of tide water entering the fjord, and the amount and frequency of deep ocean water flushing behind the sill.