Estuaries, where rivers meet the sea, are not only suitable for salt marsh development but also ideal places for human habitation. The ocean-river connection is a navigational link, flat land is easy to build upon, the river provides drinking water, the salt marsh and coastal fisheries provide food, outgoing tides facilitate wastewater disposal, and seawater provides an essential preservative and universal seasoning, NaCl. Thus, many cities, such as Venice, Boston, Amsterdam, London, Buenos Aires, Washington, DC, and Los Angeles, were built on or rapidly grew to displace salt marsh ecosystems. Major ports within smaller natural bays, such as San Diego, have displaced nearly all the natural salt marsh, while others, such as San Francisco, sustain large salt marshes despite extensive conversion.
The process of converting salt marsh into nontidal land was historically called reclamation. The practice of building embankments to exclude tidal flows eliminated thousands of hectares of European salt marshes. In the Netherlands, embankments reclaimed substantial land as polders for agriculture. In USA, reclamation reduced salt marsh area by 25% between 1932 and 1954. While the trend is to halt or reverse this practice, estuaries are being dammed in Korea to create tillable fields from mudflats. In Vietnam, Mexico, and other coastal nations, salt marshes are yielding to fish and shrimp impoundments.
In such cases, people who use mudflats for fishing and crabbing are displaced by farmers.
Although salt marshes are highly valued, they are increasingly threatened by human population growth. It is estimated that 75% of the global population will live within 60 km of the coast. Thus, coastal ecosystems are particularly at risk.
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