Where a river meets the sea there may be an area of muddy plain called an estuary. The sea washes over the plain, creating mudflats, creeks and salty marshes. Seabirds like to nest here.
In hot tropical places the coastal marshes are often swamps filled with mangrove trees.
They can survive in salty water because and can absorb oxygen from the air.
Some live and some die ture salty water under discovery.
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From @jazzman123