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estuary Estuaries are coastal embayments influenced by tides and waves that also have significant freshwater influence derived from a river system that drains into the head of the bay, representing transitional environments between rivers and the sea. Most were formed when sea levels were lower, and rivers carved out deep valleys now flooded by water. They are typically bordered by tidal wetlands and are sensitive ecological zones prone to disturbances by pollution, storms, and overuse by humans. Estuaries accumulate sediment from the river systems from the mainland as well as from the coast, and tend therefore gradually to fill in over time. Each estuary is a unique environment that also preserves a range of water chemistry, reflecting a gradual mixing of the fresh river water from the land and saltwater from the ocean. Saltwater is denser than freshwater, however (having 3.5 percent dissolved salt), and tends in many cases to form a lens that underlies a freshwater cap across the surface of the estuary. The exact nature of the mixing depends on seasonal changes in freshwater influx, basin shape, depth, wave energy, tidal range, and climate. some estuaries preserve stratified water with saltwater below and freshwater on top, whereas others show complex mixing between the different water types. The biota of estuaries are diverse, reflecting the large range in environments available for different species.
Different parts of estuaries are dominated by river and others by tidal processes. Rivers that enter large estuaries tend to form bayhead deltas that prograde into and may eventually fill the estuary. In other examples, such as Chesapeake Bay, many small rivers may enter the estuary and few have any significant delta, since the river valleys in this system trap most of the stream sediment. Estuaries bordered seaward by barrier islands have much less tidal influence than those with mouths open to the sea, although the tidal range and size and the shape of the estuary also play large roles in determining the strength of tidal versus riverine processes. Tidal-
Satellite image of Betsiboka estuary in Madagascar: Waters of estuary run yellow and red from soil washed down by heavy rains. (M-Sat Ltd./Photo Researchers, Inc.)
dominated estuaries tend to lack barrier islands at their mouths and exhibit funnel-shaped shorelines that amplify the tides by forcing the incoming tides into progressively more confined spaces. Estuaries that are tidal-dominated with strong tidal currents tend to have well-mixed waters and sandy bottoms, whereas river-dominated estuaries often have stratified water columns and muddy bottoms. Most estuaries exhibit a range of different conditions in different parts—river conditions predominate at the head of the bay, tidal processes dominate at the mouth, and a mixed zone occurs in the middle. An extremely diverse biota inhabits estuaries, with organisms that prefer salty high-energy conditions situated at the mouth of the estuary and freshwater species found near the bayhead. Organisms that tolerate brackish conditions and can adapt to changing salinities and energy are typically found in between in the zone of mixing.
See also beaches and shorelines; continental margin; ocean basin; oceanography.
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