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Although England’s estuaries have been extensively changed by human activities, they still remain productive habitats, with a diverse range of wildlife. The challenge now is to keep the pressures manageable, avoid new pressures, and seek to restore some estuaries where damage has occurred. The most important features of an estuary are the change from fresh to saltwater, the presence of a range of brackish/freshwater interfaces in the adjacent marshlands, and the low wave energy, which allows the development of extensive and productive mudflats and saltmarshes. At the top of a natural estuary, most of which have now been truncated by land-claim or sluices, the water is mostly riverine. Plants and animals find this area extremely difficult to inhabit - at one time they may be living in freshwater, six hours later they may be in saltwater. Nearer the sea, estuaries normally become wider, with a shallow central channel, with more extensive mudflats on either side. These mudflats are highly productive, with large numbers of worms, shrimps and molluscs (invertebrates) providing abundant food for wading birds. On particularly rich mudflats the food available to wading birds in a square metre of mud is estimated to provide the same energy as contained in a Mars bar. When the tide is in, and the mudflats are covered by seawater, they form an important food source for fish, which are able to feed on the invertebrates that emerge from the mud to feed. Many fish pass through estuaries on their way between river and sea, including rare species such as shad. Estuaries are also important nursery grounds for many commercial fish. At their mouth, estuaries typically broaden, and the influence of the sea is greatest, so that only fully marine plants and animals can survive. In England, estuaries may be divided into four physiographic types: rias, coastal plain estuaries, bar-built estuaries, and complex estuaries. Of all estuary types, bar-built and coastal plain estuaries are the most common, making up around 74% of the number of estuaries in England. The shape and dimensions of an estuary evolve over a long time in equilibrium with the amount of water entering the estuary at high tide. All estuaries are constantly changing. Estuarine equilibrium has been likened to a water bed: push down on one end, and the water rises in the other. In the case of estuaries, building on the estuary in one place will make it try to expand in another, resulting in erosion of mudflats or salt marsh and leading to pressure for coastal defences.
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Estuaries are the largest coastal habitat resource in
England, covering
nearly 400,000 ha. For many, estuaries represent the last wilderness
areas around our coast - tranquil, mysterious places where the cries of
wildfowl penetrate early morning mist rising off the mudflats exposed
by the low tide. In reality estuaries, and the wildlife they support, are
amongst our most threatened maritime habitats. Where they are
relatively undeveloped, they are seen as the last remaining source of
development land for extensive port developments, storage areas and industry. It is estimated that around one third of the estuarine habitat in
Britain has been land-claimed since Roman times, and the loss has continued, due to large
developments, and the cumulative effects of
numerous small scale projects. Some estuaries, such as the Tees
Estuary in north-east England, have lost up to 90% of their intertidal mudflat, saltmarsh and
brackish/freshwater marsh habitats. This is
made worse by changes in sea level, causing further loss of intertidal
