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Newsletter and Technical Publications

Lakes and Reservoirs vol. 2

The Watershed: Water from the Mountains into the Sea

Estuaries, Deltas and Coastal Areas: Water Gateways to the Sea

Photo 32: Portion of the Parana River Delta entering the La Plata River, Argentina and Uruguay.

Typically, estuaries are semi-enclosed areas along the coastal shoreline where fresh water enters the oceans (Photo 32), at the end of its journey over the land surface via rivers, lakes and/or wetlands. Fresh water can also enter the oceans via underground water flows (groundwater) although it is difficult to determine the extent of this phenomenon in a given situation. As a result, the salinity or saltiness of estuaries typically is intermediate between that of the freshwater inflows and the ocean waters into which they drain, dependent on the relative amounts of both. Accordingly, the quality of estuarine waters limits their beneficial human uses, particularly compared to freshwater resources.

Photo 33: Milford Sound Fjord, New Zealand.

There are several physical types of estuaries. The sinking or drowning of rivermouths, for example, can form coastal- plain estuaries. These estuaries can result from a lowering of the land surface and/or a rising sea-level, resulting in an elongated indenture along the coastline. Coastal-plain estuaries are typically shallow, with a river flowing into the upstream end. A second type of estuary is formed when an offshore bar develops along a relatively flat shoreline. In contrast to the shoreline indenture structure of coastal-plain estuaries, these estuaries are usually elongated embayments parallel to the shoreline, with a relatively narrow connection to the open sea. They often develop between offshore barrier island chains and the main coastline, an example being the barrier island system along the coastline of Texas in the United States. Deep-basin estuaries are usually significantly elongated coastline indentures, with a relatively deep basin and a shallow rivermouth area. The fjords of Norway, Canada and New Zealand (Photo 33), are prominent examples of this latter type of estuary.

Estuaries can sometimes be separated from the open ocean by islands or other land barriers. The hydrologic boundary of an estuary is usually controlled by the interplay of outflowing river water and incoming tidally influenced coastal waters. The upper limits of an estuary often move up and down the rivermouth as a function of the advancing and retreating tides. Winds can also influence the upper limit of the estuarine water, resulting in complex estuarine water mixing patterns. Estuaries also represent chemical and biological buffer zones between fresh water and the ocean. Shallower estuaries typically have a more uniform salinity from the surface to the bottom than do deeper estuaries. When the river outflow is large compared to the tidal flow, seawater can move a few kilometres inland as a saltwater “plume ” along the bottom of the river channel. In contrast, the freshwater discharge from some large rivers can extend many tens of kilometres into the ocean. Because the density of the freshwater is less than that of ocean water, this can result in a surface fresh water layer extending over the more dense saline water for a considerable distance offshore of the rivermouth.

Photo 34: Mississippi River delta discharging large amounts of sediments into the Gulf of Mexico, USA. Statellite image.

Because estuaries are the hydrologic connection between freshwater inputs and the open oceans, they often receive large pollutant loads, with accompanying water-quality impacts. Estuarine fishery nursery grounds are especially sensitive to upstream point and non- point pollutant sources. Because large rivers can carry huge volumes of soils and suspended sediments, estuaries are typically turbid. Further, when the inflowing sediment load becomes large, rivermouth deltas can form as the sediment drops out of the water column with the reduction in water velocity as the river enters the coastal waters. Prominent examples of this phenomenon are the major deltas located at the mouth of the Mississippi River (Photo 34), in the Gulf of Mexico and the mouth of the Nile River in the Mediterranean Sea.

Estuaries are highly variable and complex aquatic ecosystems. They are also highly productive ecosystems, containing abundant plant and animal life. As an example, estuaries are major spawning or nursery grounds for many commercially important fisheries, as well as being a magnet for sport- fishers of all types.

In addition to being major nursery systems for many fisheries, estuaries are also a physical buffer zone between inflowing rivers and the open ocean. Because they are located along coastal areas, estuarine components can include coastal wetlands, marshes and mangrove swamps. Unfortunately, continued human development and exploitation of coastal areas is seriously threatening the physical as well as the ecological integrity of many estuaries. Consequently, their ability to function as a buffer against the effects of such phenomenon as typhoons, cyclones, wave surges, etc., is being seriously jeopardized in many places.

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