An estuary is a water feature where fresh and salt water mix. Because an estuary requires free and open access to the sea, estuaries always occur along coastlines, although in some cases they can extend several miles inland. The unique natural environment of an estuary hosts numerous animal and plant species, and tends to be a profitable place for humans to settle as well, since an estuary can make an excellent location for a harbor. Some biologists specialize in studying the complex systems present in estuaries.
There are a number of different types of estuaries, usually broken up by how they are formed. Many, for example, are drowned river valleys, created when sea levels rose, causing the ocean to flood low-lying land. Others are formed through tectonic movements, when the plates of the Earth buckle or pull apart, creating deep folds in the land. A bar-built estuary is sheltered behind a sand bar or island, while fjords are estuaries carved out of the land during periods of glaciation.
The construction of an estuary ensures that it is at least partially sheltered from the more severe weather conditions found on the open sea. Many bays and coves are actually estuaries, for example. The more gentle conditions are ideal for juvenile species of marine animals, and they can shelter an assortment of waterfowl and plant species as well. In addition, an estuary tends to collect nutrients, providing an ample source of food for animals which call estuaries home.
The water in estuaries is classified as brackish, meaning that it is saltier than fresh water, but not as salty as true seawater. Different estuaries have unique levels of mixing; some are heavily stratified, meaning that more dense, cold salt water hugs the bottom while warmer freshwater floats on top, and others are more blended. Precise salinity and pH levels vary in estuaries around the world, depending on the levels of flow from the water sources which combine to make the estuary.
An estuary can be very sensitive to environmental degradation. For example, heavy damming upstream can result in a reduced flow of freshwater to the estuary, dramatically changing the environment. This can lead to a decline in biodiversity, as more delicate species die off or find more hospitable locations. Pollution can also severely impact an estuary, especially nutrient pollution such as runoff from farms, which can cause a major decline in water quality.