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Most of us understand the difference between fresh water, such as those in rivers and streams and the salt water of the ocean. But, since these rivers of fresh water run into the sea of salt water, they must meet somewhere; that meeting place is known as an estuary. Typically, a fresh water source such as a river runs into a body of water near the coast that is some what contained similar to a lake but just as it has a source running into it, an estuary also has an exit with a open route to the ocean.
The water that mingles in an estuary is known as brackish because it is neither truly fresh water, since it has salt content, but it is also not consider salt water because it has a lower level of salt than the ocean. Typically, fresh water has a salinity of less that 0.5 ppt (parts per thousand) where as salt water has a salinity rate that falls between 30 to 50 ppt. Anything above 50 ppt is considered brine and generally not able to support a marine ecosystem (such as the dead sea). The brackish water found in estuaries around the world typically have a salinity level of between 0.5 to 30 ppt.
As you can see the salinity of an estuary can vary greatly. One reason for this is that due to the upstream, every estuary is unique and may contain more or less sediment, sludge, pollution, or fresh water than another estuary. Also the size of the estuary itself plays a role in the level of salinity based on how much fresh water is flowing into the area and how wide the salt water access point is into the estuary. For example, an estuary in which the amount of fresh water is considerably less in comparison to the inflow of sea water results in higher salinity levels and is a type of estuary known as a vertically mixed estuary. On the other hand, when there is a large stream of fresh water and little sea water, the heavier salt water settles to a bottom layer and creates what is known as a salt wedge.
Estuaries are essential to the ocean and is generally populated with a variety of marine life. They also play a vital role in the life cycle of certain migratory species such as salmon. Unfortunately, this also means that estuaries become a critical fighting grounds for keeping pollution out of the fragile ocean ecosystem. All of the debris and pollution that occurred up stream meets the ocean here along with the water so it is yet another area in which we must be diligent about living responsibly to insure a healthy world for us all.