Estuaries

An estuary is where the ocean meets a river in a semi-enclosed area. This causes a mixing of fresh and salt water. These areas are home to an immense amount of biological productivity and diversity. Estuaries tend to be rich in nutrients and productivity due to the trapping of nutrients and sediments.

Estuaries can be classified into 4 types depending on their origins:

1. Coastal Plains/Drowned River Mouth:
In the past 18,000 years sea levels have risen some 125m. This has caused ocean water to rise into the mouths of rivers. (River mouths are the “exits” of a river into a lake, ocean, etc).

2. Fjords:
Fjords are steep, caused by glacial eroding. They are U-shaped. Fjords are often very deep (300-400m) but end in a shallow lip (a sill) due to glacial deposits.  Shallow sills mean little vertical mixing below sill depth which causes stagnant water. Deeper sill allow for slow bottom water mixing.

3. Bar-Built:
Bar-built estuaries form when a barrier is built parallel to the coast above sea level. These estuarine habitats are shallow and have limited tidal action due to a narrow inlet (connection to the ocean). Mixing is primarily due to the wind.

4. Tectonic:
Tectonic estuaries are coastal indentations due to faulting and subsidence. Both fresh and salt water flow into the depression resulting in an estuary.

Estuaries can also be classified by the circulation patterns:

Salt Wedge: This is the simplest circulation pattern where a large, fast flowing river enters the ocean in an area where the tidal range is low to moderate. The rapidly entering freshwater holds back a wedge of salt water. The density difference allow the freshwater to flow over the salt water. The wedge of seawater will rise and fall with the tide. Example: Mississippi River Well-Mixed: When the river flows more slowly into an area with a moderate to high tidal range, a well-mixed estuary is formed. These contain differing mixtures of fresh and salt water through most of their length. Disturbances caused by the tide will stir the waters together. Example: Columbia River Partially Mixed: Partially mixed estuaries share properties of both the salt wedge and well-mixed estuaries. These are typically deeper estuaries where the greater tidal range allows for more mixing than the salt wedge. Example: San Fransisco Bay Fjord: Fjord circulation patterns are found in fjord estuaries. These areas have small surface areas, high river input and little tidal mixing. River water tends to flow at the surface with little contact with the sea water below. With little mixing, the bottom layer can become stagnant (little oxygen and few nutrients)