Biodiversity & Conservation

MarLIN Glossary

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Estuary
1) A semi-enclosed coastal body of water which has a free connection with the open sea, and within which sea water is measurably diluted by fresh water derived from land drainage (Pritchard 1967). This is the definition used in the context of the MNCR. 2) An inlet of the sea reaching into a river valley as far as the upper limit of tidal rise (Fairbridge, 1980). (This definition includes freshwater tidal areas, excluded by Pritchard (1967).) (See Davidson et al., 1991 and McLusky, 1993 for discussion of definitions.) Three main types of estuary are recognised: a) Coastal plain estuaries, formed by the flooding of pre-existing river valleys. Maximum depth in these inlets is usually less than 30 m, with a large width-to-depth ratio; b) Bar-built estuaries: also partially-drowned river valleys, with a characteristic sediment bar across their mouth; c) Complex estuaries: river estuaries formed as a result of a range of influences such as geological constraints from hard rock outcrops, glaciation, erosion, and sea-level change. See also 'marine inlet'. For the purposes of the EC Habitats Directive, 'estuaries' are defined as: "Downstream part of a river valley, subject to the tide and extending from the limit of brackish waters. River estuaries are coastal inlets where, unlike 'large shallow inlets and bays' there is generally a substantial freshwater influence. The mixing of fresh water and sea water and the reduced current flows in the shelter of estuaries leads to deposition of fine sediments carried in from the sea and down rivers, often forming extensive intertidal sand and mud flats. Where the tidal currents are faster than flood tides, most sediments deposit to form a delta at the mouth of the estuary" (European Commission 1995).

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Glossary references